summaryrefslogtreecommitdiff
path: root/doc/gnutls.texi
blob: 5efa562605e66080a98ab1ebb20eeef0ffa97d82 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
3405
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
3517
3518
3519
3520
3521
3522
3523
3524
3525
3526
3527
3528
3529
3530
3531
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
3557
3558
3559
3560
3561
3562
3563
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
3587
3588
3589
3590
3591
3592
3593
3594
3595
3596
3597
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
3612
3613
3614
3615
3616
3617
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
3646
3647
3648
3649
3650
3651
3652
3653
3654
3655
3656
3657
3658
3659
3660
3661
3662
3663
3664
3665
3666
3667
3668
3669
3670
3671
3672
3673
3674
3675
3676
3677
3678
3679
3680
3681
3682
3683
3684
3685
3686
3687
3688
3689
3690
3691
3692
3693
3694
3695
3696
3697
3698
3699
3700
3701
3702
3703
3704
3705
3706
3707
3708
3709
3710
3711
3712
3713
3714
3715
3716
3717
3718
3719
3720
3721
3722
3723
3724
3725
3726
3727
3728
3729
3730
3731
3732
3733
3734
3735
3736
3737
3738
3739
3740
3741
3742
3743
3744
3745
3746
3747
3748
3749
3750
3751
3752
3753
3754
3755
3756
3757
3758
3759
3760
3761
3762
3763
3764
3765
3766
3767
3768
3769
3770
3771
3772
3773
3774
3775
3776
3777
3778
3779
3780
3781
3782
3783
3784
3785
3786
3787
3788
3789
3790
3791
3792
3793
3794
3795
3796
3797
3798
3799
3800
3801
3802
3803
3804
3805
3806
3807
3808
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
3824
3825
3826
3827
3828
3829
3830
3831
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
3862
3863
3864
3865
3866
3867
3868
3869
3870
3871
3872
3873
3874
3875
3876
3877
3878
3879
3880
3881
3882
3883
3884
3885
3886
3887
3888
3889
3890
3891
3892
3893
3894
3895
3896
3897
3898
3899
3900
3901
3902
3903
3904
3905
3906
3907
3908
3909
3910
3911
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
3923
3924
3925
3926
3927
3928
3929
3930
3931
3932
3933
3934
3935
3936
3937
3938
3939
3940
3941
3942
3943
3944
3945
3946
3947
3948
3949
3950
3951
3952
3953
3954
3955
3956
3957
3958
3959
3960
3961
3962
3963
3964
3965
3966
3967
3968
3969
3970
3971
3972
3973
3974
3975
3976
3977
3978
3979
3980
3981
3982
3983
3984
3985
3986
3987
3988
3989
3990
3991
3992
3993
3994
3995
3996
3997
3998
3999
4000
4001
4002
4003
4004
4005
4006
4007
4008
4009
4010
4011
4012
4013
4014
4015
4016
4017
4018
4019
4020
4021
4022
4023
4024
4025
4026
4027
4028
4029
4030
4031
4032
4033
4034
4035
4036
4037
4038
4039
4040
4041
4042
4043
4044
4045
4046
4047
4048
4049
4050
4051
4052
4053
4054
4055
4056
4057
4058
4059
4060
4061
4062
4063
4064
4065
4066
4067
4068
4069
4070
4071
4072
4073
4074
4075
4076
4077
4078
4079
4080
4081
4082
4083
4084
4085
4086
4087
4088
4089
4090
4091
4092
4093
4094
4095
4096
4097
4098
4099
4100
4101
4102
4103
4104
4105
4106
4107
4108
4109
4110
4111
4112
4113
4114
4115
4116
4117
4118
4119
4120
4121
4122
4123
4124
4125
4126
4127
4128
4129
4130
4131
4132
4133
4134
4135
4136
4137
4138
4139
4140
4141
4142
4143
4144
4145
4146
4147
4148
4149
4150
4151
4152
4153
4154
4155
4156
4157
4158
4159
4160
4161
4162
4163
4164
4165
4166
4167
4168
4169
4170
4171
4172
4173
4174
4175
4176
4177
4178
4179
4180
4181
4182
4183
4184
4185
4186
4187
4188
4189
4190
4191
4192
4193
4194
4195
4196
4197
4198
4199
4200
4201
4202
4203
4204
4205
4206
4207
4208
4209
4210
4211
4212
4213
4214
4215
4216
4217
4218
4219
4220
4221
4222
4223
4224
4225
4226
4227
4228
4229
4230
4231
4232
4233
4234
4235
4236
4237
4238
4239
4240
4241
4242
4243
4244
4245
4246
4247
4248
4249
4250
4251
4252
4253
4254
4255
4256
4257
4258
4259
4260
4261
4262
4263
4264
4265
4266
4267
4268
4269
4270
4271
4272
4273
4274
4275
4276
4277
4278
4279
4280
4281
4282
4283
4284
4285
4286
4287
4288
4289
4290
4291
4292
4293
4294
4295
4296
4297
4298
4299
4300
4301
4302
4303
4304
4305
4306
4307
4308
4309
4310
4311
4312
4313
4314
4315
4316
4317
4318
4319
4320
4321
4322
4323
4324
4325
4326
4327
4328
4329
4330
4331
4332
4333
4334
4335
4336
4337
4338
4339
4340
4341
4342
4343
4344
4345
4346
4347
4348
4349
4350
\input texinfo   @c -*-texinfo-*-
@comment %**start of header
@setfilename gnutls.info
@include version.texi
@settitle GnuTLS @value{VERSION}

@c don't indent the paragraphs.
@paragraphindent 0

@c Unify some of the indices.
@syncodeindex tp fn
@syncodeindex pg cp

@comment %**end of header
@finalout
@copying
This manual is last updated @value{UPDATED} for version
@value{VERSION} of GnuTLS.

Copyright @copyright{} 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
copy of the license is included in the section entitled ``GNU Free
Documentation License''.
@end quotation
@end copying

@dircategory Software libraries
@direntry
* GnuTLS: (gnutls).		GNU Transport Layer Security Library.
@end direntry

@dircategory System Administration
@direntry
* certtool: (gnutls)Invoking certtool.	Manipulate certificates and keys.
* gnutls-serv: (gnutls)Invoking gnutls-serv.	GnuTLS test server.
* gnutls-cli: (gnutls)Invoking gnutls-cli.	GnuTLS test client.
* gnutls-cli-debug: (gnutls)Invoking gnutls-cli-debug.	GnuTLS debug client.
* psktool: (gnutls)Invoking psktool.	Simple TLS-Pre-Shared-Keys manager.
* srptool: (gnutls)Invoking srptool.	Simple SRP password tool.
@end direntry

@titlepage
@title GnuTLS
@subtitle Transport Layer Security Library for the GNU system
@subtitle for version @value{VERSION}, @value{UPDATED}
@sp 7
@image{gnutls-logo,6cm,6cm}
@author Nikos Mavrogiannopoulos
@author Simon Josefsson (@email{bug-gnutls@@gnu.org})
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@macro xcite{ref}
[\ref\] (@pxref{Bibliography})
@end macro

@contents

@ifnottex
@node Top
@top GnuTLS

@insertcopying
@end ifnottex

@menu
* Preface::
* The Library::
* Introduction to TLS::
* Authentication methods::
* More on certificate authentication::
* How to use TLS in application protocols::
* How to use GnuTLS in applications::
* Included programs::
* Function reference::
* All the supported ciphersuites in GnuTLS::
* Guile Bindings::
* Internal architecture of GnuTLS::
* Copying Information::
* Concept Index::
* Function and Data Index::
@c * @mybibnode{}::
* Bibliography::
@end menu


@node Preface
@chapter Preface

This document tries to demonstrate and explain the @acronym{GnuTLS}
library API.  A brief introduction to the protocols and the technology
involved, is also included so that an application programmer can
better understand the @acronym{GnuTLS} purpose and actual offerings.
Even if @acronym{GnuTLS} is a typical library software, it operates
over several security and cryptographic protocols, which require the
programmer to make careful and correct usage of them, otherwise he
risks to offer just a false sense of security. Security and the
network security terms are very general terms even for computer
software thus cannot be easily restricted to a single cryptographic
library.  For that reason, do not consider a program secure just
because it uses @acronym{GnuTLS}; there are several ways to compromise
a program or a communication line and @acronym{GnuTLS} only helps with
some of them.

Although this document tries to be self contained, basic network
programming and PKI knowlegde is assumed in most of it. A good
introduction to networking can be found in @xcite{STEVENS} and for
Public Key Infrastructure in @xcite{GUTPKI}.

@anchor{Availability}

Updated versions of the @acronym{GnuTLS} software and this document
will be available from @url{http://www.gnutls.org/} and
@url{http://www.gnu.org/software/gnutls/}.

@menu
* Getting help::
* Commercial Support::
* Downloading and Installing::
* Bug Reports::
* Contributing::
@end menu

@node Getting help
@section Getting Help

A mailing list where users may help each other exists, and you can
reach it by sending e-mail to @email{help-gnutls@@gnu.org}.  Archives
of the mailing list discussions, and an interface to manage
subscriptions, is available through the World Wide Web at
@url{http://lists.gnu.org/mailman/listinfo/help-gnutls}.

A mailing list for developers are also available, see
@url{http://www.gnu.org/software/gnutls/lists.html}.

Bug reports should be sent to @email{bug-gnutls@@gnu.org}, see
@xref{Bug Reports}.

@node Commercial Support
@section Commercial Support

Commercial support is available for users of GnuTLS.  The kind of
support that can be purchased may include:

@itemize

@item Implement new features.
Such as a new TLS extension.

@item Port GnuTLS to new platforms.
This could include porting to an embedded platforms that may need
memory or size optimization.

@item Integrating TLS as a security environment in your existing project.

@item System design of components related to TLS.

@end itemize

If you are interested, please write to:

@verbatim
Simon Josefsson Datakonsult
Hagagatan 24
113 47 Stockholm
Sweden

E-mail: simon@josefsson.org
@end verbatim

If your company provides support related to GnuTLS and would like to
be mentioned here, contact the author (@pxref{Bug Reports}).

@node Downloading and Installing
@section Downloading and Installing
@cindex Installation
@cindex Download

GnuTLS is available for download from the following URL:

@url{http://www.gnutls.org/download.html}

The latest version is stored in a file, e.g.,
@samp{gnutls-@value{VERSION}.tar.gz} where the @samp{@value{VERSION}}
value is the highest version number in the directory.

GnuTLS uses a Linux-like development cycle: even minor version numbers
indicate a stable release and a odd minor version number indicates a
development release.  For example, GnuTLS 1.6.3 denote a stable
release since 6 is even, and GnuTLS 1.7.11 denote a development
release since 7 is odd.

GnuTLS depends on Libgcrypt,
and you will need to install Libgcrypt
before installing GnuTLS.  Libgcrypt is available from
@url{ftp://ftp.gnupg.org/gcrypt/libgcrypt}.  Libgcrypt needs another
library, libgpg-error, and you need to install libgpg-error before
installing Libgcrypt.  Libgpg-error is available from
@url{ftp://ftp.gnupg.org/gcrypt/libgpg-error}.

Don't forget to verify the cryptographic signature after downloading
source code packages.

The package is then extracted, configured and built like many other
packages that use Autoconf.  For detailed information on configuring
and building it, refer to the @file{INSTALL} file that is part of the
distribution archive.  Typically you invoke @code{./configure} and
then @code{make check install}.  There are a number of compile-time
parameters, as discussed below.

The compression libraries (libz and lzo) are optional dependencies.
You can get libz from @url{http://www.zlib.net/}.  You can get lzo
from @url{http://www.oberhumer.com/opensource/lzo/}.

The X.509 part of GnuTLS needs ASN.1 functionality, from a library
called libtasn1.  A copy of libtasn1 is included in GnuTLS.  If you
want to install it separately (e.g., to make it possibly to use
libtasn1 in other programs), you can get it from
@url{http://www.gnu.org/software/gnutls/download.html}.

The OpenPGP part of GnuTLS uses a stripped down version of OpenCDK for
parsing OpenPGP packets.  It is included GnuTLS.  Use parameter
@code{--disable-openpgp-authentication} to disable the OpenPGP
functionality in GnuTLS.  Unfortunately, we didn't have resources to
maintain the code in a separate library.

Regarding the Guile bindings, there are additional installation
considerations, see @xref{Guile Preparations}.

A few @code{configure} options may be relevant, summarized in the
table.

@table @code

@item --disable-srp-authentication
@itemx --disable-psk-authentication
@itemx --disable-anon-authentication
@itemx --disable-extra-pki
@itemx --disable-openpgp-authentication
@itemx --disable-openssl-compatibility
Disable or enable particular features.  Generally not recommended.

@end table

For the complete list, refer to the output from @code{configure
--help}.

@node Bug Reports
@section Bug Reports
@cindex Reporting Bugs

If you think you have found a bug in GnuTLS, please investigate it and
report it.

@itemize @bullet

@item Please make sure that the bug is really in GnuTLS, and
preferably also check that it hasn't already been fixed in the latest
version.

@item You have to send us a test case that makes it possible for us to
reproduce the bug.

@item You also have to explain what is wrong; if you get a crash, or
if the results printed are not good and in that case, in what way.
Make sure that the bug report includes all information you would need
to fix this kind of bug for someone else.

@end itemize

Please make an effort to produce a self-contained report, with
something definite that can be tested or debugged.  Vague queries or
piecemeal messages are difficult to act on and don't help the
development effort.

If your bug report is good, we will do our best to help you to get a
corrected version of the software; if the bug report is poor, we won't
do anything about it (apart from asking you to send better bug
reports).

If you think something in this manual is unclear, or downright
incorrect, or if the language needs to be improved, please also send a
note.

Send your bug report to:

@center @samp{bug-gnutls@@gnu.org}

@node Contributing
@section Contributing
@cindex Contributing
@cindex Hacking

If you want to submit a patch for inclusion -- from solve a typo you
discovered, up to adding support for a new feature -- you should
submit it as a bug report (@pxref{Bug Reports}).  There are some
things that you can do to increase the chances for it to be included
in the official package.

Unless your patch is very small (say, under 10 lines) we require that
you assign the copyright of your work to the Free Software Foundation.
This is to protect the freedom of the project.  If you have not
already signed papers, we will send you the necessary information when
you submit your contribution.

For contributions that doesn't consist of actual programming code, the
only guidelines are common sense.  Use it.

For code contributions, a number of style guides will help you:

@itemize @bullet

@item Coding Style.
Follow the GNU Standards document (@pxref{top, GNU Coding Standards,,
standards}).

If you normally code using another coding standard, there is no
problem, but you should use @samp{indent} to reformat the code
(@pxref{top, GNU Indent,, indent}) before submitting your work.

@item Use the unified diff format @samp{diff -u}.

@item Return errors.
No reason whatsoever should abort the execution of the library.  Even
memory allocation errors, e.g. when malloc return NULL, should work
although result in an error code.

@item Design with thread safety in mind.
Don't use global variables.  Don't even write to per-handle global
variables unless the documented behaviour of the function you write is
to write to the per-handle global variable.

@item Avoid using the C math library.
It causes problems for embedded implementations, and in most
situations it is very easy to avoid using it.

@item Document your functions.
Use comments before each function headers, that, if properly
formatted, are extracted into Texinfo manuals and GTK-DOC web pages.

@item Supply a ChangeLog and NEWS entries, where appropriate.

@end itemize
@node The Library
@chapter The Library

In brief @acronym{GnuTLS} can be described as a library which offers an API
to access secure communication protocols. These protocols provide
privacy over insecure lines, and were designed to prevent
eavesdropping, tampering, or message forgery.

Technically @acronym{GnuTLS} is a portable ANSI C based library which
implements the TLS 1.1 and SSL 3.0 protocols (@xref{Introduction to
TLS}, for a more detailed description of the protocols), accompanied
with the required framework for authentication and public key
infrastructure.  Important features of the @acronym{GnuTLS} library
include:

@itemize

@item Support for TLS 1.0, TLS 1.1, and SSL 3.0 protocols.

@item Support for both @acronym{X.509} and @acronym{OpenPGP} certificates.

@item Support for handling and verification of certificates.

@item Support for @acronym{SRP} for TLS authentication.

@item Support for @acronym{PSK} for TLS authentication.

@item Support for TLS Extension mechanism.

@item Support for TLS Compression Methods.

@end itemize

Additionally @acronym{GnuTLS} provides a limited emulation API for the
widely used OpenSSL@footnote{@url{http://www.openssl.org/}} library,
to ease integration with existing applications.

@acronym{GnuTLS} consists of three independent parts, namely the ``TLS
protocol part'', the ``Certificate part'', and the ``Cryptographic
backend'' part.  The `TLS protocol part' is the actual protocol
implementation, and is entirely implemented within the
@acronym{GnuTLS} library.  The `Certificate part' consists of the
certificate parsing, and verification functions which is partially
implemented in the @acronym{GnuTLS} library.  The
@acronym{Libtasn1}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/libtasn1/}},
a library which offers @acronym{ASN.1} parsing capabilities, is used
for the @acronym{X.509} certificate parsing functions.  A smaller
version of
@acronym{OpenCDK}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/opencdk/}}
is used for the @acronym{OpenPGP} key support in @acronym{GnuTLS}.
The ``Cryptographic backend'' is provided by the
@acronym{Libgcrypt}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/libgcrypt/}}
library@footnote{On current versions of GnuTLS it is possible to
override the default crypto backend. Check @pxref{Cryptographic
Backend} for details}.

In order to ease integration in embedded systems, parts of the
@acronym{GnuTLS} library can be disabled at compile time. That way a
small library, with the required features, can be generated.

@menu
* General Idea::
* Error handling::
* Memory handling::
* Callback functions::
@end menu

@node General Idea
@section General Idea

A brief description of how @acronym{GnuTLS} works internally is shown
at the figure below. This section may be easier to understand after
having seen the examples (@pxref{examples}).

@image{gnutls-internals,12cm,8cm}

As shown in the figure, there is a read-only global state that is
initialized once by the global initialization function.  This global
structure, among others, contains the memory allocation functions
used, and some structures needed for the @acronym{ASN.1} parser.  This
structure is never modified by any @acronym{GnuTLS} function, except
for the deinitialization function which frees all memory allocated in
the global structure and is called after the program has permanently
finished using @acronym{GnuTLS}.

The credentials structure is used by some authentication methods, such
as certificate authentication (@pxref{Certificate Authentication}).  A
credentials structure may contain certificates, private keys,
temporary parameters for Diffie-Hellman or RSA key exchange, and other
stuff that may be shared between several TLS sessions.

This structure should be initialized using the appropriate
initialization functions. For example an application which uses
certificate authentication would probably initialize the credentials,
using the appropriate functions, and put its trusted certificates in
this structure. The next step is to associate the credentials
structure with each @acronym{TLS} session.

A @acronym{GnuTLS} session contains all the required stuff for a
session to handle one secure connection. This session calls directly
to the transport layer functions, in order to communicate with the
peer.  Every session has a unique session ID shared with the peer.

Since TLS sessions can be resumed, servers would probably need a
database backend to hold the session's parameters.  Every
@acronym{GnuTLS} session after a successful handshake calls the
appropriate backend function (@xref{resume}, for information on
initialization) to store the newly negotiated session. The session
database is examined by the server just after having received the
client hello@footnote{The first message in a @acronym{TLS} handshake},
and if the session ID sent by the client, matches a stored session,
the stored session will be retrieved, and the new session will be a
resumed one, and will share the same session ID with the previous one.

@node Error handling
@section Error Handling

In @acronym{GnuTLS} most functions return an integer type as a result.
In almost all cases a zero or a positive number means success, and a
negative number indicates failure, or a situation that some action has
to be taken. Thus negative error codes may be fatal or not.

Fatal errors terminate the connection immediately and further sends
and receives will be disallowed. An example of a fatal error code is
@code{GNUTLS_E_DECRYPTION_FAILED}. Non-fatal errors may warn about
something, i.e., a warning alert was received, or indicate the some
action has to be taken. This is the case with the error code
@code{GNUTLS_E_REHANDSHAKE} returned by @ref{gnutls_record_recv}.
This error code indicates that the server requests a re-handshake. The
client may ignore this request, or may reply with an alert.  You can
test if an error code is a fatal one by using the
@ref{gnutls_error_is_fatal}.

If any non fatal errors, that require an action, are to be returned by
a function, these error codes will be documented in the function's
reference.  @xref{Error Codes}, for all the error codes.

@node Memory handling
@section Memory Handling

@acronym{GnuTLS} internally handles heap allocated objects
differently, depending on the sensitivity of the data they
contain. However for performance reasons, the default memory functions
do not overwrite sensitive data from memory, nor protect such objects
from being written to the swap.  In order to change the default
behavior the @ref{gnutls_global_set_mem_functions} function is
available which can be used to set other memory handlers than the
defaults.

The @acronym{Libgcrypt} library on which @acronym{GnuTLS} depends, has
such secure memory allocation functions available. These should be
used in cases where even the system's swap memory is not considered
secure. See the documentation of @acronym{Libgcrypt} for more
information.

@node Callback functions
@section Callback Functions
@cindex Callback functions

There are several cases where @acronym{GnuTLS} may need some out of
band input from your program. This is now implemented using some
callback functions, which your program is expected to register.

An example of this type of functions are the push and pull callbacks
which are used to specify the functions that will retrieve and send
data to the transport layer.

@itemize

@item @ref{gnutls_transport_set_push_function}

@item @ref{gnutls_transport_set_pull_function}

@end itemize

Other callback functions such as the one set by
@ref{gnutls_srp_set_server_credentials_function}, may require more
complicated input, including data to be allocated.  These callbacks
should allocate and free memory using the functions shown below.

@itemize

@item @ref{gnutls_malloc}

@item @ref{gnutls_free}

@end itemize

@node Introduction to TLS
@chapter Introduction to @acronym{TLS}

@acronym{TLS} stands for ``Transport Layer Security'' and is the
successor of SSL, the Secure Sockets Layer protocol @xcite{SSL3}
designed by Netscape.  @acronym{TLS} is an Internet protocol, defined
by @acronym{IETF}@footnote{IETF, or Internet Engineering Task Force,
is a large open international community of network designers,
operators, vendors, and researchers concerned with the evolution of
the Internet architecture and the smooth operation of the Internet.
It is open to any interested individual.}, described in @acronym{RFC}
4346 and also in @xcite{RESCORLA}.  The protocol provides
confidentiality, and authentication layers over any reliable transport
layer.  The description, below, refers to @acronym{TLS} 1.0 but also
applies to @acronym{TLS} 1.1 @xcite{RFC4346} and @acronym{SSL} 3.0,
since the differences of these protocols are minor.  Older protocols
such as @acronym{SSL} 2.0 are not discussed nor implemented in
@acronym{GnuTLS} since they are not considered secure today.  GnuTLS
also supports @acronym{X.509} and @acronym{OpenPGP} @xcite{RFC4880}.

@menu
* TLS layers::
* The transport layer::
* The TLS record protocol::
* The TLS Alert Protocol::
* The TLS Handshake Protocol::
* TLS Extensions::
* Selecting cryptographic key sizes::
* On SSL 2 and older protocols::
* On Record Padding::
* Safe Renegotiation::
@end menu

@node TLS layers
@section TLS Layers
@cindex TLS Layers

@acronym{TLS} is a layered protocol, and consists of the Record
Protocol, the Handshake Protocol and the Alert Protocol. The Record
Protocol is to serve all other protocols and is above the transport
layer.  The Record protocol offers symmetric encryption, data
authenticity, and optionally compression.

The Alert protocol offers some signaling to the other protocols. It
can help informing the peer for the cause of failures and other error
conditions.  @xref{The Alert Protocol}, for more information.  The
alert protocol is above the record protocol.

The Handshake protocol is responsible for the security parameters'
negotiation, the initial key exchange and authentication.  @xref{The
Handshake Protocol}, for more information about the handshake
protocol.  The protocol layering in TLS is shown in the figure below.

@image{gnutls-layers,12cm,8cm}

@node The transport layer
@section The Transport Layer
@cindex Transport protocol

@acronym{TLS} is not limited to one transport layer, it can be used
above any transport layer, as long as it is a reliable one.  A set of
functions is provided and their purpose is to load to @acronym{GnuTLS} the
required callbacks to access the transport layer.

@itemize
@item @ref{gnutls_transport_set_push_function}
@item @ref{gnutls_transport_set_pull_function}
@item @ref{gnutls_transport_set_ptr}
@item @ref{gnutls_transport_set_lowat}
@item @ref{gnutls_transport_set_errno}
@end itemize

These functions accept a callback function as a parameter.  The
callback functions should return the number of bytes written, or -1 on
error and should set @code{errno} appropriately.

In some environments, setting @code{errno} is unreliable, for example
Windows have several errno variables in different CRTs, or it may be
that errno is not a thread-local variable.  If this is a concern to
you, call @code{gnutls_transport_set_errno} with the intended errno
value instead of setting @code{errno} directly.

@acronym{GnuTLS} currently only interprets the EINTR and EAGAIN errno
values and returns the corresponding @acronym{GnuTLS} error codes
@code{GNUTLS_E_INTERRUPTED} and @code{GNUTLS_E_AGAIN}.  These values
are usually returned by interrupted system calls, or when non blocking
IO is used.  All @acronym{GnuTLS} functions can be resumed (called
again), if any of these error codes is returned.  The error codes
above refer to the system call, not the @acronym{GnuTLS} function,
since signals do not interrupt @acronym{GnuTLS}' functions.

For non blocking sockets or other custom made pull/push functions
the @ref{gnutls_transport_set_lowat} must be called, with a zero
low water mark value.

By default, if the transport functions are not set, @acronym{GnuTLS}
will use the Berkeley Sockets functions.  In this case
@acronym{GnuTLS} will use some hacks in order for @code{select} to
work, thus making it easy to add @acronym{TLS} support to existing
TCP/IP servers.

@node The TLS record protocol
@section The TLS Record Protocol
@cindex Record protocol

The Record protocol is the secure communications provider. Its purpose
is to encrypt, authenticate and ---optionally--- compress packets.
The following functions are available:

@table @asis

@item @ref{gnutls_record_send}:
To send a record packet (with application data).

@item @ref{gnutls_record_recv}:
To receive a record packet (with application data).

@item @ref{gnutls_record_get_direction}:
To get the direction of the last interrupted function call.
@end table

As you may have already noticed, the functions which access the Record
protocol, are quite limited, given the importance of this protocol in
@acronym{TLS}.  This is because the Record protocol's parameters are
all set by the Handshake protocol.

The Record protocol initially starts with NULL parameters, which means
no encryption, and no MAC is used. Encryption and authentication begin
just after the handshake protocol has finished.

@menu
* Encryption algorithms used in the record layer::
* Compression algorithms used in the record layer::
* Weaknesses and countermeasures::
@end menu

@node Encryption algorithms used in the record layer
@subsection Encryption Algorithms Used in the Record Layer
@cindex Symmetric encryption algorithms

Confidentiality in the record layer is achieved by using symmetric
block encryption algorithms like @code{3DES}, @code{AES}@footnote{AES,
or Advanced Encryption Standard, is actually the RIJNDAEL algorithm.
This is the algorithm that replaced DES.}, or stream algorithms like
@code{ARCFOUR_128}@footnote{@code{ARCFOUR_128} is a compatible
algorithm with RSA's RC4 algorithm, which is considered to be a trade
secret.}. Ciphers are encryption algorithms that use a single, secret,
key to encrypt and decrypt data. Block algorithms in TLS also provide
protection against statistical analysis of the data.  Thus, if you're
using the @acronym{TLS} protocol, a random number of blocks will be
appended to data, to prevent eavesdroppers from guessing the actual
data size.

Supported cipher algorithms:

@table @code
@item 3DES_CBC
@code{3DES_CBC} is the DES block cipher algorithm used with triple
encryption (EDE). Has 64 bits block size and is used in CBC mode.

@item ARCFOUR_128
ARCFOUR is a fast stream cipher.

@item ARCFOUR_40
This is the ARCFOUR cipher that is fed with a 40 bit key,
which is considered weak.

@item AES_CBC
AES or RIJNDAEL is the block cipher algorithm that replaces the old
DES algorithm.  Has 128 bits block size and is used in CBC mode. This
is not officially supported in TLS.
@end table

Supported MAC algorithms:

@table @code
@item MAC_MD5
MD5 is a cryptographic hash algorithm designed by Ron Rivest. Outputs
128 bits of data.

@item MAC_SHA
SHA is a cryptographic hash algorithm designed by NSA. Outputs 160
bits of data.

@end table

@node Compression algorithms used in the record layer
@subsection Compression Algorithms Used in the Record Layer
@cindex Compression algorithms

The TLS record layer also supports compression.  The algorithms
implemented in @acronym{GnuTLS} can be found in the table below.
All the algorithms except for DEFLATE which is
referenced in @xcite{RFC3749}, should be considered as
@acronym{GnuTLS}' extensions@footnote{You should use
@ref{gnutls_handshake_set_private_extensions} to enable private
extensions.}, and should be advertised only when the peer is known to
have a compliant client, to avoid interoperability problems.

The included algorithms perform really good when text, or other
compressible data are to be transfered, but offer nothing on already
compressed data, such as compressed images, zipped archives etc.
These compression algorithms, may be useful in high bandwidth TLS
tunnels, and in cases where network usage has to be minimized. As a
drawback, compression increases latency.

The record layer compression in @acronym{GnuTLS} is implemented based
on the proposal @xcite{RFC3749}.
The supported compression algorithms are:

@table @code
@item DEFLATE
Zlib compression, using the deflate algorithm.

@item LZO
LZO is a very fast compression algorithm.  This algorithm is only
available if the @acronym{GnuTLS-extra} library has been initialized
and the private extensions are enabled, and if GnuTLS was built with
LZO support.

@end table

@node Weaknesses and countermeasures
@subsection Weaknesses and Countermeasures

Some weaknesses that may affect the security of the Record layer have
been found in @acronym{TLS} 1.0 protocol. These weaknesses can be
exploited by active attackers, and exploit the facts that

@enumerate

@item
@acronym{TLS} has separate alerts for ``decryption_failed'' and
``bad_record_mac''

@item
The decryption failure reason can be detected by timing the response
time.

@item
The IV for CBC encrypted packets is the last block of the previous
encrypted packet.

@end enumerate

Those weaknesses were solved in @acronym{TLS} 1.1 @xcite{RFC4346}
which is implemented in @acronym{GnuTLS}. For a detailed discussion
see the archives of the TLS Working Group mailing list and the paper
@xcite{CBCATT}.

@node The TLS Alert Protocol
@section The TLS Alert Protocol
@anchor{The Alert Protocol}
@cindex Alert protocol

The Alert protocol is there to allow signals to be sent between peers.
These signals are mostly used to inform the peer about the cause of a
protocol failure. Some of these signals are used internally by the
protocol and the application protocol does not have to cope with them
(see @code{GNUTLS_A_CLOSE_NOTIFY}), and others refer to the
application protocol solely (see @code{GNUTLS_A_USER_CANCELLED}).  An
alert signal includes a level indication which may be either fatal or
warning. Fatal alerts always terminate the current connection, and
prevent future renegotiations using the current session ID.

The alert messages are protected by the record protocol, thus the
information that is included does not leak. You must take extreme care
for the alert information not to leak to a possible attacker, via
public log files etc.

@table @asis
@item @ref{gnutls_alert_send}:
To send an alert signal.

@item @ref{gnutls_error_to_alert}:
To map a gnutls error number to an alert signal.

@item @ref{gnutls_alert_get}:
Returns the last received alert.

@item @ref{gnutls_alert_get_name}:
Returns the name, in a character array, of the given alert.

@end table

@node The TLS Handshake Protocol
@section The TLS Handshake Protocol
@anchor{The Handshake Protocol}
@cindex Handshake protocol

The Handshake protocol is responsible for the ciphersuite negotiation,
the initial key exchange, and the authentication of the two peers.
This is fully controlled by the application layer, thus your program
has to set up the required parameters. Available functions to control
the handshake protocol include:

@table @asis
@item @ref{gnutls_priority_init}:
To initialize a priority set of ciphers.

@item @ref{gnutls_priority_deinit}:
To deinitialize a priority set of ciphers.

@item @ref{gnutls_priority_set}:
To associate a priority set with a @acronym{TLS} session.

@item @ref{gnutls_priority_set_direct}:
To directly associate a session with a given priority string.

@item @ref{gnutls_credentials_set}:
To set the appropriate credentials structures.

@item @ref{gnutls_certificate_server_set_request}:
To set whether client certificate is required or not.

@item @ref{gnutls_handshake}:
To initiate the handshake.
@end table

@subsection TLS Cipher Suites

The Handshake Protocol of @acronym{TLS} negotiates cipher suites of
the form @code{TLS_DHE_RSA_WITH_3DES_CBC_SHA}.  The usual cipher
suites contain these parameters:

@itemize

@item The key exchange algorithm.
@code{DHE_RSA} in the example.

@item The Symmetric encryption algorithm and mode
@code{3DES_CBC} in this example.

@item The MAC@footnote{MAC stands for Message Authentication Code. It can be described as a keyed hash algorithm. See RFC2104.} algorithm used for authentication.
@code{MAC_SHA} is used in the above example.

@end itemize

The cipher suite negotiated in the handshake protocol will affect the
Record Protocol, by enabling encryption and data authentication.  Note
that you should not over rely on @acronym{TLS} to negotiate the
strongest available cipher suite. Do not enable ciphers and algorithms
that you consider weak.

The priority functions, dicussed above, allow the application layer to
enable and set priorities on the individual ciphers. It may imply that
all combinations of ciphersuites are allowed, but this is not
true. For several reasons, not discussed here, some combinations were
not defined in the @acronym{TLS} protocol.  The supported ciphersuites
are shown in @ref{ciphersuites}.

@subsection Client Authentication
@cindex Client Certificate authentication

In the case of ciphersuites that use certificate authentication, the
authentication of the client is optional in @acronym{TLS}.  A server
may request a certificate from the client --- using the
@ref{gnutls_certificate_server_set_request} function. If a certificate
is to be requested from the client during the handshake, the server
will send a certificate request message that contains a list of
acceptable certificate signers. In @acronym{GnuTLS} the certificate
signers list is constructed using the trusted Certificate Authorities
by the server. That is the ones set using
@itemize
@item @ref{gnutls_certificate_set_x509_trust_file}
@item @ref{gnutls_certificate_set_x509_trust_mem}
@end itemize

Sending of the names of the CAs can be controlled using
@ref{gnutls_certificate_send_x509_rdn_sequence}. The client, then, may
send a certificate, signed by one of the server's acceptable signers.

@subsection Resuming Sessions
@anchor{resume}
@cindex Resuming sessions

The @ref{gnutls_handshake} function, is expensive since a lot of
calculations are performed. In order to support many fast connections
to the same server a client may use session resuming. @strong{Session
resuming} is a feature of the @acronym{TLS} protocol which allows a
client to connect to a server, after a successful handshake, without
the expensive calculations.  This is achieved by using the previously
established keys. @acronym{GnuTLS} supports this feature, and the
example (@pxref{ex:resume-client}) illustrates a typical use of it.

Keep in mind that sessions are expired after some time, for security
reasons, thus it may be normal for a server not to resume a session
even if you requested that.  Also note that you must enable, using the
priority functions, at least the algorithms used in the last session.

@subsection Resuming Internals

The resuming capability, mostly in the server side, is one of the
problems of a thread-safe TLS implementations. The problem is that all
threads must share information in order to be able to resume
sessions. The gnutls approach is, in case of a client, to leave all
the burden of resuming to the client. I.e., copy and keep the
necessary parameters. See the functions:

@itemize

@item @ref{gnutls_session_get_data}

@item @ref{gnutls_session_get_id}

@item @ref{gnutls_session_set_data}

@end itemize

The server side is different. A server has to specify some callback
functions which store, retrieve and delete session data. These can be
registered with:

@itemize

@item @ref{gnutls_db_set_remove_function}

@item @ref{gnutls_db_set_store_function}

@item @ref{gnutls_db_set_retrieve_function}

@item @ref{gnutls_db_set_ptr}

@end itemize

It might also be useful to be able to check for expired sessions in
order to remove them, and save space. The function
@ref{gnutls_db_check_entry} is provided for that reason.

@node TLS Extensions
@section TLS Extensions
@cindex TLS Extensions

A number of extensions to the @acronym{TLS} protocol have been
proposed mainly in @xcite{TLSEXT}. The extensions supported
in @acronym{GnuTLS} are:

@itemize
@item Maximum fragment length negotiation
@item Server name indication
@item Session tickets
@end itemize

and they will be discussed in the subsections that follow.

@subsection Maximum Fragment Length Negotiation
@cindex TLS Extensions
@cindex Maximum fragment length

This extension allows a @acronym{TLS} implementation to negotiate a
smaller value for record packet maximum length. This extension may be
useful to clients with constrained capabilities. See the
@ref{gnutls_record_set_max_size} and the
@ref{gnutls_record_get_max_size} functions.

@subsection Server Name Indication
@anchor{serverind}
@cindex TLS Extensions
@cindex Server name indication

A common problem in @acronym{HTTPS} servers is the fact that the
@acronym{TLS} protocol is not aware of the hostname that a client
connects to, when the handshake procedure begins. For that reason the
@acronym{TLS} server has no way to know which certificate to send.

This extension solves that problem within the @acronym{TLS} protocol,
and allows a client to send the HTTP hostname before the handshake
begins within the first handshake packet.  The functions
@ref{gnutls_server_name_set} and @ref{gnutls_server_name_get} can be
used to enable this extension, or to retrieve the name sent by a
client.

@subsection Session Tickets
@cindex TLS Extensions
@cindex Session Tickets
@cindex Ticket

To resume a TLS session the server normally store some state.  This
complicates deployment, and typical situations the client can cache
information and send it to the server instead.  The Session Ticket
extension implements this idea, and it is documented in
RFC 5077 @xcite{TLSTKT}.

Clients can enable support for TLS tickets with
@ref{gnutls_session_ticket_enable_client} and servers use
@ref{gnutls_session_ticket_key_generate} to generate a key and
@ref{gnutls_session_ticket_enable_server} to enable the extension.
Clients resume sessions using the ticket using the normal session
resume functions, @ref{resume}.

@node Selecting cryptographic key sizes
@section Selecting Cryptographic Key Sizes
@cindex key sizes

In TLS, since a lot of algorithms are involved, it is not easy to set
a consistent security level.  For this reason this section will
present some correspondance between key sizes of symmetric algorithms
and public key algorithms based on the most conservative values of
@xcite{SELKEY}.  Those can be used to generate certificates with
appropriate key sizes as well as parameters for Diffie-Hellman and SRP
authentication.

@multitable @columnfractions .15 .20 .20 .20

@item Year
@tab Symmetric key size
@tab RSA key size, DH and SRP prime size
@tab ECC key size

@item 1982
@tab 56
@tab 417
@tab 105

@item 1988
@tab 61
@tab 566
@tab 114

@item 2002
@tab 72
@tab 1028
@tab 139

@item 2015
@tab 82
@tab 1613
@tab 173

@item 2028
@tab 92
@tab 2362
@tab 210

@item 2040
@tab 101
@tab 3214
@tab 244

@item 2050
@tab 109
@tab 4047
@tab 272

@end multitable

The first column provides an estimation of the year until these
parameters are considered safe and the rest of the columns list the
parameters for the various algorithms.

Note however that the values suggested here are nothing more than an
educated guess that is valid today. There are no guarrantees that an
algorithm will remain unbreakable or that these values will remain
constant in time. There could be scientific breakthroughs that cannot
be predicted or total failure of the current public key systems by
quantum computers. On the other hand though the cryptosystems used in
TLS are selected in a conservative way and such catastrophic
breakthroughs or failures are believed to be unlikely.

NIST publication SP 800-57 @xcite{NISTSP80057} contains a similar
table that extends beyond the key sizes given above.

@multitable @columnfractions .15 .20 .20 .20

@item Bits of security
@tab Symmetric key algorithms
@tab RSA key size, DSA, DH and SRP prime size
@tab ECC key size

@item 80
@tab 2TDEA
@tab 1024
@tab 160-223

@item 112
@tab 3DES
@tab 2048
@tab 224-255

@item 128
@tab AES-128
@tab 3072
@tab 256-383

@item 192
@tab AES-192
@tab 7680
@tab 384-511

@item 256
@tab AES-256
@tab 15360
@tab 512+

@end multitable

The recommendations are fairly consistent.

@node On SSL 2 and older protocols
@section On SSL 2 and Older Protocols
@cindex SSL 2

One of the initial decisions in the @acronym{GnuTLS} development was
to implement the known security protocols for the transport layer.
Initially @acronym{TLS} 1.0 was implemented since it was the latest at
that time, and was considered to be the most advanced in security
properties.  Later the @acronym{SSL} 3.0 protocol was implemented
since it is still the only protocol supported by several servers and
there are no serious security vulnerabilities known.

One question that may arise is why we didn't implement @acronym{SSL}
2.0 in the library.  There are several reasons, most important being
that it has serious security flaws, unacceptable for a modern security
library.  Other than that, this protocol is barely used by anyone
these days since it has been deprecated since 1996.  The security
problems in @acronym{SSL} 2.0 include:

@itemize

@item Message integrity compromised.
The @acronym{SSLv2} message authentication uses the MD5 function, and
is insecure.

@item Man-in-the-middle attack.
There is no protection of the handshake in @acronym{SSLv2}, which
permits a man-in-the-middle attack.

@item Truncation attack.
@acronym{SSLv2} relies on TCP FIN to close the session, so the
attacker can forge a TCP FIN, and the peer cannot tell if it was a
legitimate end of data or not.

@item Weak message integrity for export ciphers.
The cryptographic keys in @acronym{SSLv2} are used for both message
authentication and encryption, so if weak encryption schemes are
negotiated (say 40-bit keys) the message authentication code use the
same weak key, which isn't necessary.

@end itemize

@cindex PCT
Other protocols such as Microsoft's @acronym{PCT} 1 and @acronym{PCT}
2 were not implemented because they were also abandoned and deprecated
by @acronym{SSL} 3.0 and later @acronym{TLS} 1.0.

@node On Record Padding
@section On Record Padding
@cindex Record padding
@cindex Bad record MAC

The TLS protocol allows for random padding of records, to make it more
difficult to perform analysis on the length of exchanged messages.
(In RFC 4346 this is specified in section 6.2.3.2.)  GnuTLS appears to
be one of few implementation that take advantage of this text, and pad
records by a random length.

The TLS implementation in the Symbian operating system, frequently
used by Nokia and Sony-Ericsson mobile phones, cannot handle
non-minimal record padding.  What happens when one of these clients
handshake with a GnuTLS server is that the client will fail to compute
the correct MAC for the record.  The client sends a TLS alert
(@code{bad_record_mac}) and disconnects.  Typically this will result
in error messages such as 'A TLS fatal alert has been received', 'Bad
record MAC', or both, on the GnuTLS server side.

GnuTLS implements a work around for this problem.  However, it has to
be enabled specifically.  It can be enabled by using
@ref{gnutls_record_disable_padding}, or @ref{gnutls_priority_set} with
the @code{%COMPAT} priority string.

If you implement an application that have a configuration file, we
recommend that you make it possible for users or administrators to
specify a GnuTLS protocol priority string, which is used by your
application via @ref{gnutls_priority_set}.  To allow the best
flexibility, make it possible to have a different priority string for
different incoming IP addresses.

To enable the workaround in the @code{gnutls-cli} client or the
@code{gnutls-serv} server, for testing of other implementations, use
the following parameter: @code{--priority "%COMPAT"}.

This problem has been discussed on mailing lists and in bug reports.
This section tries to collect all pieces of information that we know
about the problem.  If you wish to go back to the old discussions,
here are some links:

@url{http://bugs.debian.org/390712}

@url{http://bugs.debian.org/402861}

@url{http://bugs.debian.org/438137}

@url{http://thread.gmane.org/gmane.ietf.tls/3079}

@node Safe Renegotiation
@section Safe Renegotiation
@cindex renegotiation

Some application protocols and implementations uses the TLS
renegotiation feature in a manner that enables attackers to insert
content of his choice in the beginning of a TLS session.

One easy to understand vulnerability is HTTPS when servers request
client certificates optionally for certain parts of a web site.  The
attack works by having the attacker simulate a client and connect to a
server, with server-only authentication, and send some data intended
to cause harm.  When the proper client attempts to contact the server,
the attacker hijacks that connection and uses the TLS renegotiation
feature with the server and splices in the client connection to the
already established connection between the attacker and server.  The
attacker will not be able to read the data exchanged between the
client and the server.  However, the server will (incorrectly) assume
that the data sent by the attacker was sent by the now authenticated
client.  The result is a prefix plain-text injection attack.

The above is just one example.  Other vulnerabilities exists that do
not rely on the TLS renegotiation to change the client's authenticated
status (either TLS or application layer).

While fixing these application protocols and implementations would be
one natural reaction, an extension to TLS has been designed that
cryptographically binds together any renegotiated handshakes with the
initial negotiation.  When the extension is used, the attack is
detected and the session can be terminated.  The extension is
specified in @xcite{RFC5746}.

GnuTLS supports the safe renegotiation extension.  By default, GnuTLS
clients will attempt to negotiate the safe renegotiation extension
when talking to servers.  Also by default, GnuTLS servers will accept
the extension when presented by clients.  However, by default GnuTLS
client and servers will not refuse renegotiation attempts when the
extension has not been negotiated, as this would break backwards
compatibility and cause too much operational problems.  We will likely
reconsider these defaults in the future.

To modify the default behaviour, we have introduced four new priority
strings.  The priority strings can be used by applications
(@pxref{gnutls_priority_set}) and end users (e.g., @code{--priority}
parameter to @code{gnutls-cli} and @code{gnutls-serv}).

The @code{%UNSAFE_RENEGOTIATION} priority string requests what is
today the default behaviour, i.e., that handshakes without the safe
renegotiation extension is permitted.  To make more use of the
extension, you may provide the @code{%SAFE_RENEGOTIATION} priority
string.  In this mode, clients will require that the server supports
the extension for the initial handshake, and servers will require that
the client supports the extension for renegotiated handshakes.  If you
want to make a server refuse even initial handshakes without the safe
renegotiation extension, use the @code{%INITIAL_SAFE_RENEGOTIATION}
priority string.  It is possible to disable use of the extension
completely by using the @code{%DISABLE_SAFE_RENEGOTIATION} priority
string however this is recommended against except for debugging.

For applications we have introduced a new API related to safe
renegotiation.  The @ref{gnutls_safe_renegotiation_status} function is
used to check if the extension has been negotiated on a session, and
can be used both by clients and servers.

@node Authentication methods
@chapter Authentication Methods

The @acronym{TLS} protocol provides confidentiality and encryption,
but also offers authentication, which is a prerequisite for a secure
connection. The available authentication methods in @acronym{GnuTLS}
are:

@itemize

@item Certificate authentication

@item Anonymous authentication

@item @acronym{SRP} authentication

@item @acronym{PSK} authentication

@end itemize

@menu
* Certificate authentication::
* Anonymous authentication::
* Authentication using SRP::
* Authentication using PSK::
* Authentication and credentials::
* Parameters stored in credentials::
@end menu

@node Certificate authentication
@section Certificate Authentication

@subsection Authentication Using @acronym{X.509} Certificates
@cindex @acronym{X.509} certificates

@acronym{X.509} certificates contain the public parameters, of a
public key algorithm, and an authority's signature, which proves the
authenticity of the parameters.  @xref{The X.509 trust model}, for
more information on @acronym{X.509} protocols.

@subsection Authentication Using @acronym{OpenPGP} Keys
@cindex @acronym{OpenPGP} Keys

@acronym{OpenPGP} keys also contain public parameters of a public key
algorithm, and signatures from several other parties. Depending on
whether a signer is trusted the key is considered trusted or not.
@acronym{GnuTLS}'s @acronym{OpenPGP} authentication implementation is
based on the @xcite{TLSPGP} proposal.

@xref{The OpenPGP trust model}, for more information about the
@acronym{OpenPGP} trust model.  For a more detailed introduction to
@acronym{OpenPGP} and @acronym{GnuPG} see @xcite{GPGH}.

@subsection Using Certificate Authentication

In @acronym{GnuTLS} both the @acronym{OpenPGP} and @acronym{X.509}
certificates are part of the certificate authentication and thus are
handled using a common API.

When using certificates the server is required to have at least one
certificate and private key pair. A client may or may not have such a
pair. The certificate and key pair should be loaded, before any
@acronym{TLS} session is initialized, in a certificate credentials
structure. This should be done by using
@ref{gnutls_certificate_set_x509_key_file} or
@ref{gnutls_certificate_set_openpgp_key_file} depending on the
certificate type.  In the @acronym{X.509} case, the functions will
also accept and use a certificate list that leads to a trusted
authority. The certificate list must be ordered in such way that every
certificate certifies the one before it. The trusted authority's
certificate need not to be included, since the peer should possess it
already.

As an alternative, a callback may be used so the server or the client
specify the certificate and the key at the handshake time.  That
callback can be set using the functions:

@itemize

@item @ref{gnutls_certificate_server_set_retrieve_function}

@item @ref{gnutls_certificate_client_set_retrieve_function}

@end itemize

Clients and servers that will select certificates using callback
functions should select a certificate according the peer's signature
algorithm preferences. To get those preferences use
@ref{gnutls_sign_algorithm_get_requested}.

Certificate verification is possible by loading the trusted
authorities into the credentials structure by using
@ref{gnutls_certificate_set_x509_trust_file} or
@ref{gnutls_certificate_set_openpgp_keyring_file} for openpgp
keys. Note however that the peer's certificate is not automatically
verified, you should call @ref{gnutls_certificate_verify_peers2},
after a successful handshake, to verify the signatures of the
certificate.  An alternative way, which reports a more detailed
verification output, is to use @ref{gnutls_certificate_get_peers} to
obtain the raw certificate of the peer and verify it using the
functions discussed in @ref{The X.509 trust model}.

In a handshake, the negotiated cipher suite depends on the
certificate's parameters, so not all key exchange methods will be
available with some certificates. @acronym{GnuTLS} will disable
ciphersuites that are not compatible with the key, or the enabled
authentication methods.  For example keys marked as sign-only, will
not be able to access the plain RSA ciphersuites, but only the
@code{DHE_RSA} ones. It is recommended not to use RSA keys for both
signing and encryption. If possible use the same key for the
@code{DHE_RSA} and @code{RSA_EXPORT} ciphersuites, which use signing,
and a different key for the plain RSA ciphersuites, which use
encryption.  All the key exchange methods shown below are available in
certificate authentication.

Note that the DHE key exchange methods are generally
slower@footnote{It really depends on the group used.  Primes with
lesser bits are always faster, but also easier to break.  Values less
than 768 should not be used today} than plain RSA and require Diffie
Hellman parameters to be generated and associated with a credentials
structure, by the server.  The @code{RSA-EXPORT} method also requires
512 bit RSA parameters, that should also be generated and associated
with the credentials structure.  See the functions:

@itemize

@item @ref{gnutls_dh_params_generate2}

@item @ref{gnutls_certificate_set_dh_params}

@item @ref{gnutls_rsa_params_generate2}

@item @ref{gnutls_certificate_set_rsa_export_params}

@end itemize

Sometimes in order to avoid bottlenecks in programs it is usefull to
store and read parameters from formats that can be generated by
external programs such as @code{certtool}. This is possible with
@acronym{GnuTLS} by using the following functions:

@itemize

@item @ref{gnutls_dh_params_import_pkcs3}

@item @ref{gnutls_rsa_params_import_pkcs1}

@item @ref{gnutls_dh_params_export_pkcs3}

@item @ref{gnutls_rsa_params_export_pkcs1}

@end itemize

Key exchange algorithms for @acronym{OpenPGP} and @acronym{X.509}
certificates:

@table @code

@item RSA:
The RSA algorithm is used to encrypt a key and send it to the peer.
The certificate must allow the key to be used for encryption.

@item RSA_EXPORT:
The RSA algorithm is used to encrypt a key and send it to the peer.
In the EXPORT algorithm, the server signs temporary RSA parameters of
512 bits --- which are considered weak --- and sends them to the
client.

@item DHE_RSA:
The RSA algorithm is used to sign Ephemeral Diffie-Hellman parameters
which are sent to the peer. The key in the certificate must allow the
key to be used for signing. Note that key exchange algorithms which
use Ephemeral Diffie-Hellman parameters, offer perfect forward
secrecy. That means that even if the private key used for signing is
compromised, it cannot be used to reveal past session data.

@item DHE_DSS:
The DSS algorithm is used to sign Ephemeral Diffie-Hellman parameters
which are sent to the peer. The certificate must contain DSA
parameters to use this key exchange algorithm. DSS stands for Digital
Signature Standard.

@end table

@node Anonymous authentication
@section Anonymous Authentication
@cindex Anonymous authentication

The anonymous key exchange performs encryption but there is no
indication of the identity of the peer.  This kind of authentication
is vulnerable to a man in the middle attack, but this protocol can be
used even if there is no prior communication and trusted parties with
the peer, or when full anonymity is required.  Unless really required,
do not use anonymous authentication.  Available key exchange methods
are shown below.

Note that the key exchange methods for anonymous authentication
require Diffie-Hellman parameters to be generated by the server and
associated with an anonymous credentials structure.

Supported anonymous key exchange algorithms:

@table @code

@item ANON_DH:
This algorithm exchanges Diffie-Hellman parameters.

@end table

@node Authentication using SRP
@section Authentication using @acronym{SRP}
@cindex @acronym{SRP} authentication

Authentication via the Secure Remote Password protocol,
@acronym{SRP}@footnote{@acronym{SRP} is described in @xcite{RFC2945}},
is supported.  The @acronym{SRP} key exchange is an extension to the
@acronym{TLS} protocol, and it is a password based authentication
(unlike @acronym{X.509} or @acronym{OpenPGP} that use certificates).
The two peers can be identified using a single password, or there can
be combinations where the client is authenticated using @acronym{SRP}
and the server using a certificate.

The advantage of @acronym{SRP} authentication, over other proposed
secure password authentication schemes, is that @acronym{SRP} does not
require the server to hold the user's password.  This kind of
protection is similar to the one used traditionally in the @emph{UNIX}
@file{/etc/passwd} file, where the contents of this file did not cause
harm to the system security if they were revealed.  The @acronym{SRP}
needs instead of the plain password something called a verifier, which
is calculated using the user's password, and if stolen cannot be used
to impersonate the user. Check @xcite{TOMSRP} for a detailed
description of the @acronym{SRP} protocol and the Stanford
@acronym{SRP} libraries, which includes a PAM module that synchronizes
the system's users passwords with the @acronym{SRP} password
files. That way @acronym{SRP} authentication could be used for all the
system's users.

The implementation in @acronym{GnuTLS} is based on paper
@xcite{TLSSRP}.  The supported @acronym{SRP} key exchange methods are:

@table @code

@item SRP:
Authentication using the @acronym{SRP} protocol.

@item SRP_DSS:
Client authentication using the @acronym{SRP} protocol. Server is
authenticated using a certificate with DSA parameters.

@item SRP_RSA:
Client authentication using the @acronym{SRP} protocol. Server is
authenticated using a certificate with RSA parameters.

@end table

If clients supporting @acronym{SRP} know the username and password
before the connection, should initialize the client credentials and
call the function @ref{gnutls_srp_set_client_credentials}.
Alternatively they could specify a callback function by using the
function @ref{gnutls_srp_set_client_credentials_function}.  This has
the advantage that allows probing the server for @acronym{SRP}
support.  In that case the callback function will be called twice per
handshake.  The first time is before the ciphersuite is negotiated,
and if the callback returns a negative error code, the callback will
be called again if @acronym{SRP} has been negotiated.  This uses a
special @acronym{TLS}-@acronym{SRP} handshake idiom in order to avoid,
in interactive applications, to ask the user for @acronym{SRP}
password and username if the server does not negotiate an
@acronym{SRP} ciphersuite.

In server side the default behaviour of @acronym{GnuTLS} is to read
the usernames and @acronym{SRP} verifiers from password files. These
password files are the ones used by the @emph{Stanford srp libraries}
and can be specified using the
@ref{gnutls_srp_set_server_credentials_file}.  If a different
password file format is to be used, then the function
@ref{gnutls_srp_set_server_credentials_function}, should be called,
in order to set an appropriate callback.

Some helper functions such as

@itemize

@item @ref{gnutls_srp_verifier}

@item @ref{gnutls_srp_base64_encode}

@item @ref{gnutls_srp_base64_decode}

@end itemize

are included in @acronym{GnuTLS}, and can be used to generate and
maintain @acronym{SRP} verifiers and password files.  A program to
manipulate the required parameters for @acronym{SRP} authentication is
also included.  @xref{srptool}, for more information.


@node Authentication using PSK
@section Authentication using @acronym{PSK}
@cindex @acronym{PSK} authentication

Authentication using Pre-shared keys is a method to authenticate using
usernames and binary keys. This protocol avoids making use of public
key infrastructure and expensive calculations, thus it is suitable for
constraint clients.

The implementation in @acronym{GnuTLS} is based on paper
@xcite{TLSPSK}.  The supported @acronym{PSK} key exchange methods are:

@table @code

@item PSK:
Authentication using the @acronym{PSK} protocol.

@item DHE-PSK:
Authentication using the @acronym{PSK} protocol and Diffie-Hellman key
exchange.  This method offers perfect forward secrecy.

@end table

Clients supporting @acronym{PSK} should supply the username and key
before the connection to the client credentials by calling the
function @ref{gnutls_psk_set_client_credentials}.  Alternatively they
could specify a callback function by using the function
@ref{gnutls_psk_set_client_credentials_function}.  This has the
advantage that the callback will be called only if @acronym{PSK} has
been negotiated.

In server side the default behaviour of @acronym{GnuTLS} is to read
the usernames and @acronym{PSK} keys from a password file. The
password file should contain usernames and keys in hexadecimal
format. The name of the password file can be stored to the credentials
structure by calling @ref{gnutls_psk_set_server_credentials_file}.  If
a different password file format is to be used, then the function
@ref{gnutls_psk_set_server_credentials_function}, should be used
instead.

The server can help the client chose a suitable username and password,
by sending a hint.  In the server, specify the hint by calling
@ref{gnutls_psk_set_server_credentials_hint}.  The client can retrieve
the hint, for example in the callback function, using
@ref{gnutls_psk_client_get_hint}.

There is no standard mechanism to derive a PSK key from a password
specified by the TLS PSK document.  However, GnuTLS provides
@ref{gnutls_psk_netconf_derive_key} which follows the algorithm
specified in @file{draft-ietf-netconf-tls-02.txt}.

Some helper functions such as:

@itemize

@item @ref{gnutls_hex_encode}

@item @ref{gnutls_hex_decode}

@end itemize

are included in @acronym{GnuTLS}, and may be used to generate and
maintain @acronym{PSK} keys.


@node Authentication and credentials
@section Authentication and Credentials

In @acronym{GnuTLS} every key exchange method is associated with a
credentials type. So in order to enable to enable a specific method,
the corresponding credentials type should be initialized and set using
@ref{gnutls_credentials_set}.  A mapping is shown below.

Key exchange algorithms and the corresponding credential types:

@multitable @columnfractions .3 .3 .3

@headitem Key exchange @tab Client credentials @tab Server credentials

@item @code{KX_RSA}
@item @code{KX_DHE_RSA}
@item @code{KX_DHE_DSS}
@item @code{KX_RSA_EXPORT}
@tab @code{CRD_CERTIFICATE}
@tab @code{CRD_CERTIFICATE}

@item @code{KX_SRP_RSA}
@tab @code{CRD_SRP}
@tab @code{CRD_SRP}
@item @code{KX_SRP_DSS}
@tab
@tab @code{CRD_CERTIFICATE}

@item @code{KX_SRP}
@tab @code{CRD_SRP}
@tab @code{CRD_SRP}

@item @code{KX_ANON_DH}
@tab @code{CRD_ANON}
@tab @code{CRD_ANON}

@item @code{KX_PSK}
@tab @code{CRD_PSK}
@tab @code{CRD_PSK}

@end multitable

@node Parameters stored in credentials
@section Parameters Stored in Credentials

Several parameters such as the ones used for Diffie-Hellman
authentication are stored within the credentials structures, so all
sessions can access them. Those parameters are stored in structures
such as @code{gnutls_dh_params_t} and @code{gnutls_rsa_params_t}, and
functions like @ref{gnutls_certificate_set_dh_params} and
@ref{gnutls_certificate_set_rsa_export_params} can be used to
associate those parameters with the given credentials structure.

Since those parameters need to be renewed from time to time and a
global structure such as the credentials, may not be easy to modify
since it is accessible by all sessions, an alternative interface is
available using a callback function.  This can be set using the
@ref{gnutls_certificate_set_params_function}.  An example is shown
below.

@example
#include <gnutls.h>

gnutls_rsa_params_t rsa_params;
gnutls_dh_params_t dh_params;

/* This function will be called once a session requests DH
 * or RSA parameters. The parameters returned (if any) will
 * be used for the first handshake only.
 */
static int get_params( gnutls_session_t session,
        gnutls_params_type_t type,
        gnutls_params_st *st)
@{
   if (type == GNUTLS_PARAMS_RSA_EXPORT)
      st->params.rsa_export = rsa_params;
   else if (type == GNUTLS_PARAMS_DH)
      st->params.dh = dh_params;
   else return -1;

   st->type = type;
   /* do not deinitialize those parameters.
    */
   st->deinit = 0;

   return 0;
@}

int main()
@{
   gnutls_certificate_credentials_t cert_cred;

   initialize_params();

   /* ...
    */

   gnutls_certificate_set_params_function( cert_cred, get_params);
@}
@end example

@node More on certificate authentication
@chapter More on Certificate Authentication
@anchor{Certificate Authentication}
@cindex Certificate authentication

@menu
* The X.509 trust model::
* The OpenPGP trust model::
* Digital signatures::
@end menu

@node The X.509 trust model
@section The @acronym{X.509} Trust Model
@cindex @acronym{X.509} certificates

The @acronym{X.509} protocols rely on a hierarchical trust model. In
this trust model Certification Authorities (CAs) are used to certify
entities.  Usually more than one certification authorities exist, and
certification authorities may certify other authorities to issue
certificates as well, following a hierarchical model.

@image{gnutls-x509,7cm,9.5cm}

One needs to trust one or more CAs for his secure communications. In
that case only the certificates issued by the trusted authorities are
acceptable.  See the figure above for a typical example.  The API for
handling @acronym{X.509} certificates is described at section
@ref{sec:x509api}.  Some examples are listed below.

@menu
* X.509 certificates::
* Verifying X.509 certificate paths::
* PKCS #10 certificate requests::
* PKCS #12 structures::
@end menu

@node X.509 certificates
@subsection @acronym{X.509} Certificates

An @acronym{X.509} certificate usually contains information about the
certificate holder, the signer, a unique serial number, expiration
dates and some other fields @xcite{PKIX} as shown in the table below.

@table @code

@item version:
The field that indicates the version of the certificate.

@item serialNumber:
This field holds a unique serial number per certificate.

@item issuer:
Holds the issuer's distinguished name.

@item validity:
The activation and expiration dates.

@item subject:
The subject's distinguished name of the certificate.

@item extensions:
The extensions are fields only present in version 3 certificates.

@end table

The certificate's @emph{subject or issuer name} is not just a single
string.  It is a Distinguished name and in the @acronym{ASN.1}
notation is a sequence of several object IDs with their corresponding
values. Some of available OIDs to be used in an @acronym{X.509}
distinguished name are defined in @file{gnutls/x509.h}.

The @emph{Version} field in a certificate has values either 1 or 3 for
version 3 certificates.  Version 1 certificates do not support the
extensions field so it is not possible to distinguish a CA from a
person, thus their usage should be avoided.

The @emph{validity} dates are there to indicate the date that the
specific certificate was activated and the date the certificate's key
would be considered invalid.

Certificate @emph{extensions} are there to include information about
the certificate's subject that did not fit in the typical certificate
fields. Those may be e-mail addresses, flags that indicate whether the
belongs to a CA etc.  All the supported @acronym{X.509} version 3
extensions are shown in the table below.

@table @code

@item subject key id (2.5.29.14):
An identifier of the key of the subject.

@item authority key id (2.5.29.35):
An identifier of the authority's key used to sign the certificate.

@item subject alternative name (2.5.29.17):
Alternative names to subject's distinguished name.

@item key usage (2.5.29.15):
Constraints the key's usage of the certificate.

@item extended key usage (2.5.29.37):
Constraints the purpose of the certificate.

@item basic constraints (2.5.29.19):
Indicates whether this is a CA certificate or not, and specify the
maximum path lengths of certificate chains.

@item CRL distribution points (2.5.29.31):
This extension is set by the CA, in order to inform about the issued
CRLs.

@item Proxy Certification Information (1.3.6.1.5.5.7.1.14):
Proxy Certificates includes this extension that contains the OID of
the proxy policy language used, and can specify limits on the maximum
lengths of proxy chains.  Proxy Certificates are specified in
@xcite{RFC3820}.

@end table

In @acronym{GnuTLS} the @acronym{X.509} certificate structures are
handled using the @code{gnutls_x509_crt_t} type and the corresponding
private keys with the @code{gnutls_x509_privkey_t} type.  All the
available functions for @acronym{X.509} certificate handling have
their prototypes in @file{gnutls/x509.h}. An example program to
demonstrate the @acronym{X.509} parsing capabilities can be found at
section @ref{ex:x509-info}.

@node Verifying X.509 certificate paths
@subsection Verifying @acronym{X.509} Certificate Paths
@cindex Verifying certificate paths

Verifying certificate paths is important in @acronym{X.509}
authentication. For this purpose the function
@ref{gnutls_x509_crt_verify} is provided. The output of this function
is the bitwise OR of the elements of the
@code{gnutls_certificate_status_t} enumeration.  A detailed
description of these elements can be found in figure below.  The
function @ref{gnutls_certificate_verify_peers2} is equivalent to the
previous one, and will verify the peer's certificate in a TLS session.

@table @code

@item GNUTLS_CERT_INVALID:
The certificate is not signed by one of the known authorities, or
the signature is invalid.

@item GNUTLS_CERT_REVOKED:
The certificate has been revoked by its CA.

@item GNUTLS_CERT_SIGNER_NOT_FOUND:
The certificate's issuer is not known. This is the case when the
issuer is not in the trusted certificates list.

@item GNUTLS_CERT_SIGNER_NOT_CA:
The certificate's signer was not a CA. This may happen if
this was a version 1 certificate, which is common with some CAs, or
a version 3 certificate without the basic constrains extension.

@anchor{GNUTLS_CERT_INSECURE_ALGORITHM}
@item GNUTLS_CERT_INSECURE_ALGORITHM:
The certificate was signed using an insecure algorithm such as MD2 or
MD5.  These algorithms have been broken and should not be trusted.

@end table

There is also to possibility to pass some input to the verification
functions in the form of flags. For @ref{gnutls_x509_crt_verify} the
flags are passed straightforward, but
@ref{gnutls_certificate_verify_peers2} depends on the flags set by
calling @ref{gnutls_certificate_set_verify_flags}.  All the available
flags are part of the enumeration
@ref{gnutls_certificate_verify_flags} and are explained in the table
below.

@anchor{gnutls_certificate_verify_flags}
@tindex gnutls_certificate_verify_flags
@table @code
@item GNUTLS_VERIFY_DISABLE_CA_SIGN:
If set a signer does not have to be a certificate authority. This
flag should normaly be disabled, unless you know what this means.

@item GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT:
Allow only trusted CA certificates that have version 1.  This is
safer than GNUTLS_VERIFY_ALLOW_ANY_X509_V1_CA_CRT, and should be
used instead. That way only signers in your trusted list will be
allowed to have certificates of version 1.

@item GNUTLS_VERIFY_ALLOW_ANY_X509_V1_CA_CRT:
Allow CA certificates that have version 1 (both root and
intermediate). This is dangerous since those haven't the
basicConstraints extension. Must be used in combination with
GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT.

@item GNUTLS_VERIFY_DO_NOT_ALLOW_SAME:
If a certificate is not signed by anyone trusted but exists in
the trusted CA list do not treat it as trusted.

@item GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD2:
Allow certificates to be signed using the old MD2 algorithm.

@item GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5:
Allow certificates to be signed using the broken MD5 algorithm.
@end table

Although the verification of a certificate path indicates that the
certificate is signed by trusted authority, does not reveal anything
about the peer's identity. It is required to verify if the
certificate's owner is the one you expect. For more information
consult @xcite{RFC2818} and section @ref{ex:verify} for an example.

@node PKCS #10 certificate requests
@subsection @acronym{PKCS} #10 Certificate Requests
@cindex Certificate requests
@cindex @acronym{PKCS} #10

A certificate request is a structure, which contain information about
an applicant of a certificate service.  It usually contains a private
key, a distinguished name and secondary data such as a challenge
password. @acronym{GnuTLS} supports the requests defined in
@acronym{PKCS} #10 @xcite{RFC2986}. Other certificate request's format
such as PKIX's @xcite{RFC4211} are not currently supported.

In @acronym{GnuTLS} the @acronym{PKCS} #10 structures are handled
using the @code{gnutls_x509_crq_t} type.  An example of a certificate
request generation can be found at section @ref{ex:crq}.

@node PKCS #12 structures
@subsection @acronym{PKCS} #12 Structures
@cindex @acronym{PKCS} #12

A @acronym{PKCS} #12 structure @xcite{PKCS12} usually contains a user's
private keys and certificates. It is commonly used in browsers to
export and import the user's identities.

In @acronym{GnuTLS} the @acronym{PKCS} #12 structures are handled
using the @code{gnutls_pkcs12_t} type. This is an abstract type that
may hold several @code{gnutls_pkcs12_bag_t} types.  The Bag types are
the holders of the actual data, which may be certificates, private
keys or encrypted data.  An Bag of type encrypted should be decrypted
in order for its data to be accessed.

An example of a @acronym{PKCS} #12 structure generation can be found
at section @ref{ex:pkcs12}.

@node The OpenPGP trust model
@section The @acronym{OpenPGP} Trust Model
@cindex @acronym{OpenPGP} Keys

The @acronym{OpenPGP} key authentication relies on a distributed trust
model, called the ``web of trust''. The ``web of trust'' uses a
decentralized system of trusted introducers, which are the same as a
CA. @acronym{OpenPGP} allows anyone to sign anyone's else public
key. When Alice signs Bob's key, she is introducing Bob's key to
anyone who trusts Alice. If someone trusts Alice to introduce keys,
then Alice is a trusted introducer in the mind of that observer.

@image{gnutls-pgp,11cm,9cm}

For example: If David trusts Alice to be an introducer, and Alice
signed Bob's key, Dave also trusts Bob's key to be the real one.

There are some key points that are important in that model. In the
example Alice has to sign Bob's key, only if she is sure that the key
belongs to Bob. Otherwise she may also make Dave falsely believe that
this is Bob's key. Dave has also the responsibility to know who to
trust.  This model is similar to real life relations.

Just see how Charlie behaves in the previous example. Although he has
signed Bob's key - because he knows, somehow, that it belongs to Bob -
he does not trust Bob to be an introducer. Charlie decided to trust
only Kevin, for some reason. A reason could be that Bob is lazy
enough, and signs other people's keys without being sure that they
belong to the actual owner.

@subsection @acronym{OpenPGP} Keys

In @acronym{GnuTLS} the @acronym{OpenPGP} key structures
@xcite{RFC2440} are handled using the @code{gnutls_openpgp_crt_t} type
and the corresponding private keys with the
@code{gnutls_openpgp_privkey_t} type. All the prototypes for the key
handling functions can be found at @file{gnutls/openpgp.h}.

@subsection Verifying an @acronym{OpenPGP} Key

The verification functions of @acronym{OpenPGP} keys, included in
@acronym{GnuTLS}, are simple ones, and do not use the features of the
``web of trust''.  For that reason, if the verification needs are
complex, the assistance of external tools like @acronym{GnuPG} and
GPGME (@url{http://www.gnupg.org/related_software/gpgme/}) is
recommended.

There is one verification function in @acronym{GnuTLS}, the
@ref{gnutls_openpgp_crt_verify_ring}.  This checks an
@acronym{OpenPGP} key against a given set of public keys (keyring) and
returns the key status. The key verification status is the same as in
@acronym{X.509} certificates, although the meaning and interpretation
are different. For example an @acronym{OpenPGP} key may be valid, if
the self signature is ok, even if no signers were found.  The meaning
of verification status is shown in the figure below.

@table @code

@item CERT_INVALID:
A signature on the key is invalid. That means that the key was
modified by somebody, or corrupted during transport.

@item CERT_REVOKED:
The key has been revoked by its owner.

@item CERT_SIGNER_NOT_FOUND:
The key was not signed by a known signer.

@item GNUTLS_CERT_INSECURE_ALGORITHM:
The certificate was signed using an insecure algorithm such as MD2 or
MD5.  These algorithms have been broken and should not be trusted.

@end table

@node Digital signatures
@section Digital Signatures
@cindex Digital signatures

In this section we will provide some information about digital
signatures, how they work, and give the rationale for disabling some
of the algorithms used.

Digital signatures work by using somebody's secret key to sign some
arbitrary data.  Then anybody else could use the public key of that
person to verify the signature.  Since the data may be arbitrary it is
not suitable input to a cryptographic digital signature algorithm. For
this reason and also for performance cryptographic hash algorithms are
used to preprocess the input to the signature algorithm. This works as
long as it is difficult enough to generate two different messages with
the same hash algorithm output. In that case the same signature could
be used as a proof for both messages. Nobody wants to sign an innocent
message of donating 1 @euro{} to Greenpeace and find out that he
donated 1.000.000 @euro{} to Bad Inc.

For a hash algorithm to be called cryptographic the following three
requirements must hold:

@enumerate
@item Preimage resistance.
That means the algorithm must be one way and given the output of the
hash function @math{H(x)}, it is impossible to calculate @math{x}.

@item 2nd preimage resistance.
That means that given a pair @math{x,y} with @math{y=H(x)} it is
impossible to calculate an @math{x'} such that @math{y=H(x')}.

@item Collision resistance.
That means that it is impossible to calculate random @math{x} and
@math{x'} such @math{H(x')=H(x)}.
@end enumerate

The last two requirements in the list are the most important in
digital signatures. These protect against somebody who would like to
generate two messages with the same hash output. When an algorithm is
considered broken usually it means that the Collision resistance of
the algorithm is less than brute force. Using the birthday paradox the
brute force attack takes
@iftex
@math{2^{(\rm{hash\ size}) / 2}}
@end iftex
@ifnottex
@math{2^{((hash size) / 2)}}
@end ifnottex
operations. Today colliding certificates using the MD5 hash algorithm
have been generated as shown in @xcite{WEGER}.

There has been cryptographic results for the SHA-1 hash algorithms as
well, although they are not yet critical.  Before 2004, MD5 had a
presumed collision strength of @math{2^{64}}, but it has been showed
to have a collision strength well under @math{2^{50}}.  As of November
2005, it is believed that SHA-1's collision strength is around
@math{2^{63}}.  We consider this sufficiently hard so that we still
support SHA-1.  We anticipate that SHA-256/386/512 will be used in
publicly-distributed certificates in the future.  When @math{2^{63}}
can be considered too weak compared to the computer power available
sometime in the future, SHA-1 will be disabled as well.  The collision
attacks on SHA-1 may also get better, given the new interest in tools
for creating them.

@subsection Trading Security for Interoperability

If you connect to a server and use GnuTLS' functions to verify the
certificate chain, and get a @ref{GNUTLS_CERT_INSECURE_ALGORITHM}
validation error (@pxref{Verifying X.509 certificate paths}), it means
that somewhere in the certificate chain there is a certificate signed
using @code{RSA-MD2} or @code{RSA-MD5}.  These two digital signature
algorithms are considered broken, so GnuTLS fail when attempting to
verify the certificate.  In some situations, it may be useful to be
able to verify the certificate chain anyway, assuming an attacker did
not utilize the fact that these signatures algorithms are broken.
This section will give help on how to achieve that.

First, it is important to know that you do not have to enable any of
the flags discussed here to be able to use trusted root CA
certificates signed using @code{RSA-MD2} or @code{RSA-MD5}.  The only
attack today is that it is possible to generate certificates with
colliding signatures (collision resistance); you cannot generate a
certificate that has the same signature as an already existing
signature (2nd preimage resistance).

If you are using @ref{gnutls_certificate_verify_peers2} to verify the
certificate chain, you can call
@ref{gnutls_certificate_set_verify_flags} with the
@code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD2} or
@code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5} flag, as in:

@example
  gnutls_certificate_set_verify_flags (x509cred,
                                       GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5);
@end example

This will tell the verifier algorithm to enable @code{RSA-MD5} when
verifying the certificates.

If you are using @ref{gnutls_x509_crt_verify} or
@ref{gnutls_x509_crt_list_verify}, you can pass the
@code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5} parameter directly in the
@code{flags} parameter.

If you are using these flags, it may also be a good idea to warn the
user when verification failure occur for this reason.  The simplest is
to not use the flags by default, and only fall back to using them
after warning the user.  If you wish to inspect the certificate chain
yourself, you can use @ref{gnutls_certificate_get_peers} to extract
the raw server's certificate chain, then use
@ref{gnutls_x509_crt_import} to parse each of the certificates, and
then use @ref{gnutls_x509_crt_get_signature_algorithm} to find out the
signing algorithm used for each certificate.  If any of the
intermediary certificates are using @code{GNUTLS_SIGN_RSA_MD2} or
@code{GNUTLS_SIGN_RSA_MD5}, you could present a warning.



@node How to use TLS in application protocols
@chapter How To Use @acronym{TLS} in Application Protocols

This chapter is intended to provide some hints on how to use the
@acronym{TLS} over simple custom made application protocols.  The
discussion below mainly refers to the @emph{TCP/IP} transport layer
but may be extended to other ones too.

@menu
* Separate ports::
* Upward negotiation::
@end menu

@node Separate ports
@section Separate Ports

Traditionally @acronym{SSL} was used in application protocols by
assigning a new port number for the secure services. That way two
separate ports were assigned, one for the non secure sessions, and one
for the secured ones. This has the benefit that if a user requests a
secure session then the client will try to connect to the secure port
and fail otherwise. The only possible attack with this method is a
denial of service one. The most famous example of this method is the
famous ``HTTP over TLS'' or @acronym{HTTPS} protocol @xcite{RFC2818}.

Despite its wide use, this method is not as good as it seems.  This
approach starts the @acronym{TLS} Handshake procedure just after the
client connects on the ---so called--- secure port.  That way the
@acronym{TLS} protocol does not know anything about the client, and
popular methods like the host advertising in HTTP do not
work@footnote{See also the Server Name Indication extension on
@ref{serverind}.}.  There is no way for the client to say ``I
connected to YYY server'' before the Handshake starts, so the server
cannot possibly know which certificate to use.

Other than that it requires two separate ports to run a single
service, which is unnecessary complication. Due to the fact that there
is a limitation on the available privileged ports, this approach was
soon obsoleted.

@node Upward negotiation
@section Upward Negotiation

Other application protocols@footnote{See LDAP, IMAP etc.}  use a
different approach to enable the secure layer.  They use something
called the ``TLS upgrade'' method. This method is quite tricky but it
is more flexible. The idea is to extend the application protocol to
have a ``STARTTLS'' request, whose purpose it to start the TLS
protocols just after the client requests it.  This is a really neat
idea and does not require an extra port.

This method is used by almost all modern protocols and there is even
the @xcite{RFC2817} paper which proposes extensions to HTTP to support
it.

The tricky part, in this method, is that the ``STARTTLS'' request is
sent in the clear, thus is vulnerable to modifications.  A typical
attack is to modify the messages in a way that the client is fooled
and thinks that the server does not have the ``STARTTLS'' capability.
See a typical conversation of a hypothetical protocol:

@quotation
(client connects to the server)

CLIENT: HELLO I'M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

SERVER: OK

*** TLS STARTS

CLIENT: HERE ARE SOME CONFIDENTIAL DATA
@end quotation

And see an example of a conversation where someone is acting
in between:

@quotation
(client connects to the server)

CLIENT: HELLO I'M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

(here someone inserts this message)

SERVER: SORRY I DON'T HAVE THIS CAPABILITY

CLIENT: HERE ARE SOME CONFIDENTIAL DATA
@end quotation

As you can see above the client was fooled, and was dummy enough to
send the confidential data in the clear.

How to avoid the above attack? As you may have already thought this
one is easy to avoid. The client has to ask the user before it
connects whether the user requests @acronym{TLS} or not. If the user
answered that he certainly wants the secure layer the last
conversation should be:

@quotation
(client connects to the server)

CLIENT: HELLO I'M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

(here someone inserts this message)

SERVER: SORRY I DON'T HAVE THIS CAPABILITY

CLIENT: BYE

(the client notifies the user that the secure connection was not possible)
@end quotation

This method, if implemented properly, is far better than the
traditional method, and the security properties remain the same, since
only denial of service is possible. The benefit is that the server may
request additional data before the @acronym{TLS} Handshake protocol
starts, in order to send the correct certificate, use the correct
password file@footnote{in @acronym{SRP} authentication}, or anything
else!

@node How to use GnuTLS in applications
@chapter How To Use @acronym{GnuTLS} in Applications
@anchor{examples}
@cindex Example programs

@menu
* Preparation::
* Multi-threaded applications::
* Client examples::
* Server examples::
* Miscellaneous examples::
* Compatibility with the OpenSSL library::
* Opaque PRF Input TLS Extension::
* Keying Material Exporters::
@end menu

@node Preparation
@section Preparation

To use @acronym{GnuTLS}, you have to perform some changes to your
sources and your build system. The necessary changes are explained in
the following subsections.

@menu
* Headers::
* Initialization::
* Version check::
* Debugging::
* Building the source::
@end menu

@node Headers
@subsection Headers

All the data types and functions of the @acronym{GnuTLS} library are
defined in the header file @file{gnutls/gnutls.h}.  This must be
included in all programs that make use of the @acronym{GnuTLS}
library.

The extra functionality of the @acronym{GnuTLS-extra} library is
available by including the header file @file{gnutls/extra.h} in your
programs.

@node Initialization
@subsection Initialization

GnuTLS must be initialized before it can be used.  The library is
initialized by calling @ref{gnutls_global_init}.  The resources
allocated by the initialization process can be released if the
application no longer has a need to call GnuTLS functions, this is
done by calling @ref{gnutls_global_deinit}.

The extra functionality of the @acronym{GnuTLS-extra} library is
available after calling @ref{gnutls_global_init_extra}.

In order to take advantage of the internationalisation features in
GnuTLS, such as translated error messages, the application must set
the current locale using @code{setlocale} before initializing GnuTLS.

@node Version check
@subsection Version Check

It is often desirable to check that the version of `gnutls' used is
indeed one which fits all requirements.  Even with binary
compatibility new features may have been introduced but due to problem
with the dynamic linker an old version is actually used.  So you may
want to check that the version is okay right after program startup.
See the function @ref{gnutls_check_version}.

@node Debugging
@subsection Debugging

In many cases things may not go as expected and further information,
to assist debugging, from @acronym{GnuTLS} is desired. Those are the
case where the @ref{gnutls_global_set_log_level} and
@ref{gnutls_global_set_log_function} are to be used. Those will print
verbose information on the @acronym{GnuTLS} functions internal flow.

@node Building the source
@subsection Building the Source

If you want to compile a source file including the
@file{gnutls/gnutls.h} header file, you must make sure that the
compiler can find it in the directory hierarchy.  This is accomplished
by adding the path to the directory in which the header file is
located to the compilers include file search path (via the @option{-I}
option).

However, the path to the include file is determined at the time the
source is configured.  To solve this problem, the library uses the
external package @command{pkg-config} that knows the path to the
include file and other configuration options.  The options that need
to be added to the compiler invocation at compile time are output by
the @option{--cflags} option to @command{pkg-config gnutls}.  The
following example shows how it can be used at the command line:

@example
gcc -c foo.c `pkg-config gnutls --cflags`
@end example

Adding the output of @samp{pkg-config gnutls --cflags} to the
compilers command line will ensure that the compiler can find the
@file{gnutls/gnutls.h} header file.

A similar problem occurs when linking the program with the library.
Again, the compiler has to find the library files.  For this to work,
the path to the library files has to be added to the library search
path (via the @option{-L} option).  For this, the option
@option{--libs} to @command{pkg-config gnutls} can be used.  For
convenience, this option also outputs all other options that are
required to link the program with the libarary (for instance, the
@samp{-ltasn1} option).  The example shows how to link @file{foo.o}
with the library to a program @command{foo}.

@example
gcc -o foo foo.o `pkg-config gnutls --libs`
@end example

Of course you can also combine both examples to a single command by
specifying both options to @command{pkg-config}:

@example
gcc -o foo foo.c `pkg-config gnutls --cflags --libs`
@end example

@node Multi-threaded applications
@section Multi-Threaded Applications

Although the @acronym{GnuTLS} library is thread safe by design, some
parts of Libgcrypt, such as the random generator, are not.
Applications have to register callback functions to ensure proper
locking in the sensitive parts of @emph{libgcrypt}.

There are helper macros to help you properly initialize the libraries.
Examples are shown below.

@itemize

@item POSIX threads
@example
#include <gnutls.h>
#include <gcrypt.h>
#include <errno.h>
#include <pthread.h>
GCRY_THREAD_OPTION_PTHREAD_IMPL;

int main() 
@{
   /* The order matters.
    */
   gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread);
   gnutls_global_init();
@}
@end example

@item GNU PTH threads
@example
#include <gnutls.h>
#include <gcrypt.h>
#include <errno.h>
#include <pth.h>
GCRY_THREAD_OPTION_PTH_IMPL;

int main() 
@{
   gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pth);
   gnutls_global_init();
@}
@end example

@item Other thread packages
@example
/* The gcry_thread_cbs structure must have been
 * initialized.
 */
static struct gcry_thread_cbs gcry_threads_other = @{ ... @};

int main()
@{
   gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_other);
@}
@end example
@end itemize

@node Client examples
@section Client Examples

This section contains examples of @acronym{TLS} and @acronym{SSL}
clients, using @acronym{GnuTLS}.  Note that these examples contain
little or no error checking.  Some of the examples require functions
implemented by another example.

@menu
* Simple client example with anonymous authentication::
* Simple client example with X.509 certificate support::
* Obtaining session information::
* Verifying peer's certificate::
* Using a callback to select the certificate to use::
* Client with Resume capability example::
* Simple client example with SRP authentication::
* Simple client example with TLS/IA support::
* Simple client example in C++::
* Helper function for TCP connections::
@end menu

@node Simple client example with anonymous authentication
@subsection Simple Client Example with Anonymous Authentication

The simplest client using TLS is the one that doesn't do any
authentication.  This means no external certificates or passwords are
needed to set up the connection.  As could be expected, the connection
is vulnerable to man-in-the-middle (active or redirection) attacks.
However, the data is integrity and privacy protected.

@verbatiminclude examples/ex-client1.c

@node Simple client example with X.509 certificate support
@subsection Simple Client Example with @acronym{X.509} Certificate Support

Let's assume now that we want to create a TCP client which
communicates with servers that use @acronym{X.509} or
@acronym{OpenPGP} certificate authentication. The following client is
a very simple @acronym{TLS} client, it does not support session
resuming, not even certificate verification. The TCP functions defined
in this example are used in most of the other examples below, without
redefining them.

@verbatiminclude examples/ex-client2.c

@node Obtaining session information
@subsection Obtaining Session Information

Most of the times it is desirable to know the security properties of
the current established session.  This includes the underlying ciphers
and the protocols involved.  That is the purpose of the following
function.  Note that this function will print meaningful values only
if called after a successful @ref{gnutls_handshake}.

@verbatiminclude examples/ex-session-info.c

@node Verifying peer's certificate
@subsection Verifying Peer's Certificate
@anchor{ex:verify}

A @acronym{TLS} session is not secure just after the handshake
procedure has finished.  It must be considered secure, only after the
peer's certificate and identity have been verified. That is, you have
to verify the signature in peer's certificate, the hostname in the
certificate, and expiration dates.  Just after this step you should
treat the connection as being a secure one.

@verbatiminclude examples/ex-rfc2818.c

An other example is listed below which provides a more detailed
verification output.

@verbatiminclude examples/ex-verify.c

@node Using a callback to select the certificate to use
@subsection Using a Callback to Select the Certificate to Use

There are cases where a client holds several certificate and key
pairs, and may not want to load all of them in the credentials
structure.  The following example demonstrates the use of the
certificate selection callback.

@verbatiminclude examples/ex-cert-select.c

@node Client with Resume capability example
@subsection Client with Resume Capability Example
@anchor{ex:resume-client}

This is a modification of the simple client example. Here we
demonstrate the use of session resumption. The client tries to connect
once using @acronym{TLS}, close the connection and then try to
establish a new connection using the previously negotiated data.

@verbatiminclude examples/ex-client-resume.c

@node Simple client example with SRP authentication
@subsection Simple Client Example with @acronym{SRP} Authentication

The following client is a very simple @acronym{SRP} @acronym{TLS}
client which connects to a server and authenticates using a
@emph{username} and a @emph{password}. The server may authenticate
itself using a certificate, and in that case it has to be verified.

@verbatiminclude examples/ex-client-srp.c

@node Simple client example with TLS/IA support
@subsection Simple Client Example with @acronym{TLS/IA} Support

The following client is a simple client which uses the
@acronym{TLS/IA} extension to authenticate with the server.

@verbatiminclude examples/ex-client-tlsia.c

@node Simple client example in C++
@subsection Simple Client Example using the C++ API

The following client is a simple example of a client client utilizing
the GnuTLS C++ API.

@verbatiminclude examples/ex-cxx.cpp

@node Helper function for TCP connections
@subsection Helper Function for TCP Connections

This helper function abstracts away TCP connection handling from the
other examples.  It is required to build some examples.

@verbatiminclude examples/tcp.c

@node Server examples
@section Server Examples

This section contains examples of @acronym{TLS} and @acronym{SSL}
servers, using @acronym{GnuTLS}.

@menu
* Echo Server with X.509 authentication::
* Echo Server with X.509 authentication II::
* Echo Server with OpenPGP authentication::
* Echo Server with SRP authentication::
* Echo Server with anonymous authentication::
@end menu

@node Echo Server with X.509 authentication
@subsection Echo Server with @acronym{X.509} Authentication

This example is a very simple echo server which supports
@acronym{X.509} authentication, using the RSA ciphersuites.

@verbatiminclude examples/ex-serv1.c

@node Echo Server with X.509 authentication II
@subsection Echo Server with @acronym{X.509} Authentication II

The following example is a server which supports @acronym{X.509}
authentication.  This server supports the export-grade cipher suites,
the DHE ciphersuites and session resuming.

@verbatiminclude examples/ex-serv-export.c

@node Echo Server with OpenPGP authentication
@subsection Echo Server with @acronym{OpenPGP} Authentication
@cindex @acronym{OpenPGP} Server

The following example is an echo server which supports
@acronym{@acronym{OpenPGP}} key authentication. You can easily combine
this functionality ---that is have a server that supports both
@acronym{X.509} and @acronym{OpenPGP} certificates--- but we separated
them to keep these examples as simple as possible.

@verbatiminclude examples/ex-serv-pgp.c

@node Echo Server with SRP authentication
@subsection Echo Server with @acronym{SRP} Authentication

This is a server which supports @acronym{SRP} authentication. It is
also possible to combine this functionality with a certificate
server. Here it is separate for simplicity.

@verbatiminclude examples/ex-serv-srp.c

@node Echo Server with anonymous authentication
@subsection Echo Server with Anonymous Authentication

This example server support anonymous authentication, and could be
used to serve the example client for anonymous authentication.

@verbatiminclude examples/ex-serv-anon.c

@node Miscellaneous examples
@section Miscellaneous Examples

@menu
* Checking for an alert::
* X.509 certificate parsing example::
* Certificate request generation::
* PKCS #12 structure generation::
@end menu

@node Checking for an alert
@subsection Checking for an Alert

This is a function that checks if an alert has been received in the
current session.

@verbatiminclude examples/ex-alert.c

@node X.509 certificate parsing example
@subsection @acronym{X.509} Certificate Parsing Example
@anchor{ex:x509-info}

To demonstrate the @acronym{X.509} parsing capabilities an example program is
listed below.  That program reads the peer's certificate, and prints
information about it.

@verbatiminclude examples/ex-x509-info.c

@node Certificate request generation
@subsection Certificate Request Generation
@anchor{ex:crq}

The following example is about generating a certificate request, and a
private key. A certificate request can be later be processed by a CA,
which should return a signed certificate.

@verbatiminclude examples/ex-crq.c

@node PKCS #12 structure generation
@subsection @acronym{PKCS} #12 Structure Generation
@anchor{ex:pkcs12}

The following example is about generating a @acronym{PKCS} #12
structure.

@verbatiminclude examples/ex-pkcs12.c

@node Compatibility with the OpenSSL library
@section Compatibility with the OpenSSL Library
@cindex OpenSSL

To ease @acronym{GnuTLS}' integration with existing applications, a
compatibility layer with the widely used OpenSSL library is included
in the @code{gnutls-openssl} library. This compatibility layer is not
complete and it is not intended to completely reimplement the OpenSSL
API with @acronym{GnuTLS}.  It only provides source-level
compatibility. There is currently no attempt to make it
binary-compatible with OpenSSL.

The prototypes for the compatibility functions are in the
@file{gnutls/openssl.h} header file.

Current limitations imposed by the compatibility layer include:

@itemize

@item Error handling is not thread safe.

@end itemize

@node Opaque PRF Input TLS Extension
@section Opaque PRF Input TLS Extension
@cindex Opaque PRF Input

GnuTLS supports the Opaque PRF Input TLS extension
(@code{draft-rescorla-tls-opaque-prf-input-00.txt}).  The API consists
of one API for use in the client, @ref{gnutls_oprfi_enable_client},
and one API for use in the server, @ref{gnutls_oprfi_enable_server}.
You must invoke both functions before calling @ref{gnutls_handshake}.
The server utilizes a callback function into the application.  The
callback can look at the random string provided by the client, and
also set the server string.  The string lengths must be equal
according to the protocol.

@node Keying Material Exporters
@section Keying Material Exporters
@cindex Keying Material Exporters
@cindex Exporting Keying Material

The TLS PRF can be used by other protocols to derive data.  The API to
use is @ref{gnutls_prf}.  The function needs to be provided with the
label in the parameter @code{label}, and the extra data to mix in the
@code{extra} parameter.  Depending on whether you want to mix in the
client or server random data first, you can set the
@code{server_random_first} parameter.

For example, after establishing a TLS session using
@ref{gnutls_handshake}, you can invoke the TLS PRF with this call:

@smallexample
#define MYLABEL "EXPORTER-FOO"
#define MYCONTEXT "some context data"
char out[32];
rc = gnutls_prf (session, strlen (MYLABEL), MYLABEL, 0,
                 strlen (MYCONTEXT), MYCONTEXT, 32, out);
@end smallexample

If you don't want to mix in the client/server random, there is a more
low-level TLS PRF interface called @ref{gnutls_prf_raw}.

@node Included programs
@chapter Included Programs

Included with @acronym{GnuTLS} are also a few command line tools that
let you use the library for common tasks without writing an
application.  The applications are discussed in this chapter.

@menu
* Invoking certtool::
* Invoking gnutls-cli::
* Invoking gnutls-cli-debug::
* Invoking gnutls-serv::
* Invoking psktool::
* Invoking srptool::
@end menu

@node Invoking certtool
@section Invoking certtool
@cindex certtool

This is a program to generate @acronym{X.509} certificates, certificate
requests, CRLs and private keys.

@verbatim
Certtool help
Usage: certtool [options]
     -s, --generate-self-signed
                              Generate a self-signed certificate.
     -c, --generate-certificate
                              Generate a signed certificate.
     --generate-proxy         Generate a proxy certificate.
     --generate-crl           Generate a CRL.
     -u, --update-certificate
                              Update a signed certificate.
     -p, --generate-privkey   Generate a private key.
     -q, --generate-request   Generate a PKCS #10 certificate
                              request.
     -e, --verify-chain       Verify a PEM encoded certificate chain.
                              The last certificate in the chain must
                              be a self signed one.
     --verify-crl             Verify a CRL.
     --generate-dh-params     Generate PKCS #3 encoded Diffie-Hellman
                              parameters.
     --get-dh-params          Get the included PKCS #3 encoded Diffie
                              Hellman parameters.
     --load-privkey FILE      Private key file to use.
     --load-request FILE      Certificate request file to use.
     --load-certificate FILE
                              Certificate file to use.
     --load-ca-privkey FILE   Certificate authority's private key
                              file to use.
     --load-ca-certificate FILE
                              Certificate authority's certificate
                              file to use.
     --password PASSWORD      Password to use.
     -i, --certificate-info   Print information on a certificate.
     -l, --crl-info           Print information on a CRL.
     --p12-info               Print information on a PKCS #12
                              structure.
     --p7-info                Print information on a PKCS #7
                              structure.
     --smime-to-p7            Convert S/MIME to PKCS #7 structure.
     -k, --key-info           Print information on a private key.
     --fix-key                Regenerate the parameters in a private
                              key.
     --to-p12                 Generate a PKCS #12 structure.
     -8, --pkcs8              Use PKCS #8 format for private keys.
     --dsa                    Use DSA keys.
     --hash STR               Hash algorithm to use for signing
                              (MD5,SHA1,RMD160).
     --export-ciphers         Use weak encryption algorithms.
     --inder                  Use DER format for input certificates
                              and private keys.
     --outder                 Use DER format for output certificates
                              and private keys.
     --bits BITS              specify the number of bits for key
                              generation.
     --outfile FILE           Output file.
     --infile FILE            Input file.
     --template FILE          Template file to use for non
                              interactive operation.
     -d, --debug LEVEL        specify the debug level. Default is 1.
     -h, --help               shows this help text
     -v, --version            shows the program's version
@end verbatim

The program can be used interactively or non interactively by
specifying the @code{--template} command line option. See below for an
example of a template file.

How to use certtool interactively:

@itemize
@item
To generate parameters for Diffie-Hellman key exchange, use the command:
@example
$ certtool --generate-dh-params --outfile dh.pem
@end example

@item
To generate parameters for the RSA-EXPORT key exchange, use the command:
@example
$ certtool --generate-privkey --bits 512 --outfile rsa.pem
@end example

@end itemize

@itemize

@item
To create a self signed certificate, use the command:
@example
$ certtool --generate-privkey --outfile ca-key.pem
$ certtool --generate-self-signed --load-privkey ca-key.pem \
   --outfile ca-cert.pem
@end example

Note that a self-signed certificate usually belongs to a certificate
authority, that signs other certificates.

@item
To create a private key (RSA by default), run:

@example
$ certtool --generate-privkey --outfile key.pem
@end example

To create a DSA private key, run:

@example
$ certtool --dsa --generate-privkey --outfile key-dsa.pem
@end example

@item
To generate a certificate using the private key, use the command:

@example
$ certtool --generate-certificate --load-privkey key.pem \
   --outfile cert.pem --load-ca-certificate ca-cert.pem \
   --load-ca-privkey ca-key.pem
@end example

@item
To create a certificate request (needed when the certificate is issued by
another party), run:

@example
$ certtool --generate-request --load-privkey key.pem \
  --outfile request.pem
@end example

@item
To generate a certificate using the previous request, use the command:

@example
$ certtool --generate-certificate --load-request request.pem \
   --outfile cert.pem \
   --load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem
@end example

@item
To view the certificate information, use:

@example
$ certtool --certificate-info --infile cert.pem
@end example

@item
To generate a @acronym{PKCS} #12 structure using the previous key and
certificate, use the command:

@example
$ certtool --load-certificate cert.pem --load-privkey key.pem \
  --to-p12 --outder --outfile key.p12
@end example

Some tools (reportedly web browsers) have problems with that file
because it does not contain the CA certificate for the certificate.
To work around that problem in the tool, you can use the
@samp{--load-ca-certificate} parameter as follows:

@example
$ certtool --load-ca-certificate ca.pem \
  --load-certificate cert.pem --load-privkey key.pem \
  --to-p12 --outder --outfile key.p12
@end example

@item
Proxy certificate can be used to delegate your credential to a
temporary, typically short-lived, certificate.  To create one from the
previously created certificate, first create a temporary key and then
generate a proxy certificate for it, using the commands:

@example
$ certtool --generate-privkey > proxy-key.pem
$ certtool --generate-proxy --load-ca-privkey key.pem \
  --load-privkey proxy-key.pem --load-certificate cert.pem \
  --outfile proxy-cert.pem
@end example

@item
To create an empty Certificate Revocation List (CRL) do:

@example
$ certtool --generate-crl --load-ca-privkey x509-ca-key.pem --load-ca-certificate x509-ca.pem
@end example

To create a CRL that contains some revoked certificates, place the
certificates in a file and use @code{--load-certificate} as follows:

@example
$ certtool --generate-crl --load-ca-privkey x509-ca-key.pem --load-ca-certificate x509-ca.pem --load-certificate revoked-certs.pem
@end example

@item
To verify a Certificate Revocation List (CRL) do:

@example
$ certtool --verify-crl --load-ca-certificate x509-ca.pem < crl.pem
@end example

@end itemize

Certtool's template file format:

@itemize

@item
Firstly create a file named 'cert.cfg' that contains the information
about the certificate. An example file is listed below.

@item
Then execute:

@example
$ certtool --generate-certificate cert.pem --load-privkey key.pem  \
   --template cert.cfg \
   --load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem
@end example

@end itemize

An example certtool template file:

@example
# X.509 Certificate options
#
# DN options

# The organization of the subject.
organization = "Koko inc."

# The organizational unit of the subject.
unit = "sleeping dept."

# The locality of the subject.
# locality =

# The state of the certificate owner.
state = "Attiki"

# The country of the subject. Two letter code.
country = GR

# The common name of the certificate owner.
cn = "Cindy Lauper"

# A user id of the certificate owner.
#uid = "clauper"

# If the supported DN OIDs are not adequate you can set
# any OID here.
# For example set the X.520 Title and the X.520 Pseudonym
# by using OID and string pairs.
#dn_oid = "2.5.4.12" "Dr." "2.5.4.65" "jackal"

# This is deprecated and should not be used in new
# certificates.
# pkcs9_email = "none@@none.org"

# The serial number of the certificate
serial = 007

# In how many days, counting from today, this certificate will expire.
expiration_days = 700

# X.509 v3 extensions

# A dnsname in case of a WWW server.
#dns_name = "www.none.org"
#dns_name = "www.morethanone.org"

# An IP address in case of a server.
#ip_address = "192.168.1.1"

# An email in case of a person
email = "none@@none.org"

# An URL that has CRLs (certificate revocation lists)
# available. Needed in CA certificates.
#crl_dist_points = "http://www.getcrl.crl/getcrl/"

# Whether this is a CA certificate or not
#ca

# Whether this certificate will be used for a TLS client
#tls_www_client

# Whether this certificate will be used for a TLS server
#tls_www_server

# Whether this certificate will be used to sign data (needed
# in TLS DHE ciphersuites).
signing_key

# Whether this certificate will be used to encrypt data (needed
# in TLS RSA ciphersuites). Note that it is preferred to use different
# keys for encryption and signing.
#encryption_key

# Whether this key will be used to sign other certificates.
#cert_signing_key

# Whether this key will be used to sign CRLs.
#crl_signing_key

# Whether this key will be used to sign code.
#code_signing_key

# Whether this key will be used to sign OCSP data.
#ocsp_signing_key

# Whether this key will be used for time stamping.
#time_stamping_key
@end example

@node Invoking gnutls-cli
@section Invoking gnutls-cli
@cindex gnutls-cli

Simple client program to set up a TLS connection to some other
computer.  It sets up a TLS connection and forwards data from the
standard input to the secured socket and vice versa.

@verbatim
GnuTLS test client
Usage:  gnutls-cli [options] hostname

     -d, --debug integer      Enable debugging
     -r, --resume             Connect, establish a session. Connect
                              again and resume this session.
     -s, --starttls           Connect, establish a plain session and
                              start TLS when EOF or a SIGALRM is
                              received.
     --crlf                   Send CR LF instead of LF.
     --x509fmtder             Use DER format for certificates to read
                              from.
     -f, --fingerprint        Send the openpgp fingerprint, instead
                              of the key.
     --disable-extensions     Disable all the TLS extensions.
     --print-cert             Print the certificate in PEM format.
     --recordsize integer     The maximum record size to advertize.
     -V, --verbose            More verbose output.
     --ciphers cipher1 cipher2...
                              Ciphers to enable.
     --protocols protocol1 protocol2...
                              Protocols to enable.
     --comp comp1 comp2...    Compression methods to enable.
     --macs mac1 mac2...      MACs to enable.
     --kx kx1 kx2...          Key exchange methods to enable.
     --ctypes certType1 certType2...
                              Certificate types to enable.
     --priority PRIORITY STRING
                              Priorities string.
     --x509cafile FILE        Certificate file to use.
     --x509crlfile FILE       CRL file to use.
     --pgpkeyfile FILE        PGP Key file to use.
     --pgpkeyring FILE        PGP Key ring file to use.
     --pgpcertfile FILE       PGP Public Key (certificate) file to
                              use.
     --pgpsubkey HEX|auto     PGP subkey to use.
     --x509keyfile FILE       X.509 key file to use.
     --x509certfile FILE      X.509 Certificate file to use.
     --srpusername NAME       SRP username to use.
     --srppasswd PASSWD       SRP password to use.
     --pskusername NAME       PSK username to use.
     --pskkey KEY             PSK key (in hex) to use.
     --opaque-prf-input DATA
                              Use Opaque PRF Input DATA.
     -p, --port PORT          The port to connect to.
     --insecure               Don't abort program if server
                              certificate can't be validated.
     -l, --list               Print a list of the supported
                              algorithms and modes.
     -h, --help               prints this help
     -v, --version            prints the program's version number
@end verbatim

To connect to a server using PSK authentication, you may use something
like:

@smallexample
$ gnutls-cli -p 5556 test.gnutls.org --pskusername jas --pskkey 9e32cf7786321a828ef7668f09fb35db --priority NORMAL:+PSK:-RSA:-DHE-RSA -d 4711
@end smallexample

@menu
* Example client PSK connection::
@end menu

@node Example client PSK connection
@subsection Example client PSK connection
@cindex PSK client

If your server only supports the PSK ciphersuite, connecting to it
should be as simple as connecting to the server:

@smallexample
$ ./gnutls-cli -p 5556 localhost
Resolving 'localhost'...
Connecting to '127.0.0.1:5556'...
- PSK client callback. PSK hint 'psk_identity_hint'
Enter PSK identity: psk_identity
Enter password: 
- PSK authentication. PSK hint 'psk_identity_hint'
- Version: TLS1.1
- Key Exchange: PSK
- Cipher: AES-128-CBC
- MAC: SHA1
- Compression: NULL
- Handshake was completed

- Simple Client Mode:
@end smallexample

If the server supports several cipher suites, you may need to force it
to chose PSK by using a cipher priority parameter such as
@code{--priority NORMAL:+PSK:-RSA:-DHE-RSA:-DHE-PSK}.

@cindex Netconf
Instead of using the Netconf-way to derive the PSK key from a
password, you can also give the PSK username and key directly on the
command line:

@smallexample
$ ./gnutls-cli -p 5556 localhost --pskusername psk_identity --pskkey 88f3824b3e5659f52d00e959bacab954b6540344 
Resolving 'localhost'...
Connecting to '127.0.0.1:5556'...
- PSK authentication. PSK hint 'psk_identity_hint'
- Version: TLS1.1
- Key Exchange: PSK
- Cipher: AES-128-CBC
- MAC: SHA1
- Compression: NULL
- Handshake was completed

- Simple Client Mode:
@end smallexample

By keeping the @code{--pskusername} parameter and removing the
@code{--pskkey} parameter, it will query only for the password during
the handshake.

@node Invoking gnutls-cli-debug
@section Invoking gnutls-cli-debug
@cindex gnutls-cli-debug

This program was created to assist in debugging @acronym{GnuTLS}, but
it might be useful to extract a @acronym{TLS} server's capabilities.
It's purpose is to connect onto a @acronym{TLS} server, perform some
tests and print the server's capabilities. If called with the `-v'
parameter a more checks will be performed. An example output is:

@smallexample
crystal:/cvs/gnutls/src$ ./gnutls-cli-debug localhost -p 5556
Resolving 'localhost'...
Connecting to '127.0.0.1:5556'...
Checking for TLS 1.1 support... yes
Checking fallback from TLS 1.1 to... N/A
Checking for TLS 1.0 support... yes
Checking for SSL 3.0 support... yes
Checking for version rollback bug in RSA PMS... no
Checking for version rollback bug in Client Hello... no
Checking whether we need to disable TLS 1.0... N/A
Checking whether the server ignores the RSA PMS version... no
Checking whether the server can accept Hello Extensions... yes
Checking whether the server can accept cipher suites not in SSL 3.0 spec... yes
Checking whether the server can accept a bogus TLS record version in the client hello... yes
Checking for certificate information... N/A
Checking for trusted CAs... N/A
Checking whether the server understands TLS closure alerts... yes
Checking whether the server supports session resumption... yes
Checking for export-grade ciphersuite support... no
Checking RSA-export ciphersuite info... N/A
Checking for anonymous authentication support... no
Checking anonymous Diffie-Hellman group info... N/A
Checking for ephemeral Diffie-Hellman support... no
Checking ephemeral Diffie-Hellman group info... N/A
Checking for AES cipher support (TLS extension)... yes
Checking for 3DES cipher support... yes
Checking for ARCFOUR 128 cipher support... yes
Checking for ARCFOUR 40 cipher support... no
Checking for MD5 MAC support... yes
Checking for SHA1 MAC support... yes
Checking for ZLIB compression support (TLS extension)... yes
Checking for LZO compression support (GnuTLS extension)... yes
Checking for max record size (TLS extension)... yes
Checking for SRP authentication support (TLS extension)... yes
Checking for OpenPGP authentication support (TLS extension)... no
@end smallexample

@node Invoking gnutls-serv
@section Invoking gnutls-serv
@cindex gnutls-serv

Simple server program that listens to incoming TLS connections.

@verbatim
GnuTLS test server
Usage: gnutls-serv [options]

     -d, --debug integer      Enable debugging
     -g, --generate           Generate Diffie-Hellman Parameters.
     -p, --port integer       The port to connect to.
     -q, --quiet              Suppress some messages.
     --nodb                   Does not use the resume database.
     --http                   Act as an HTTP Server.
     --echo                   Act as an Echo Server.
     --dhparams FILE          DH params file to use.
     --x509fmtder             Use DER format for certificates
     --x509cafile FILE        Certificate file to use.
     --x509crlfile FILE       CRL file to use.
     --pgpkeyring FILE        PGP Key ring file to use.
     --pgpkeyfile FILE        PGP Key file to use.
     --pgpcertfile FILE       PGP Public Key (certificate) file to
                              use.
     --pgpsubkey HEX|auto     PGP subkey to use.
     --x509keyfile FILE       X.509 key file to use.
     --x509certfile FILE      X.509 Certificate file to use.
     --x509dsakeyfile FILE    Alternative X.509 key file to use.
     --x509dsacertfile FILE   Alternative X.509 certificate file to
                              use.
     -r, --require-cert       Require a valid certificate.
     -a, --disable-client-cert
                              Disable request for a client
                              certificate.
     --pskpasswd FILE         PSK password file to use.
     --pskhint HINT           PSK identity hint to use.
     --srppasswd FILE         SRP password file to use.
     --srppasswdconf FILE     SRP password conf file to use.
     --opaque-prf-input DATA
                              Use Opaque PRF Input DATA.
     --ciphers cipher1 cipher2...
                              Ciphers to enable.
     --protocols protocol1 protocol2...
                              Protocols to enable.
     --comp comp1 comp2...    Compression methods to enable.
     --macs mac1 mac2...      MACs to enable.
     --kx kx1 kx2...          Key exchange methods to enable.
     --ctypes certType1 certType2...
                              Certificate types to enable.
     --priority PRIORITY STRING
                              Priorities string.
     -l, --list               Print a list of the supported
                              algorithms  and modes.
     -h, --help               prints this help
     -v, --version            prints the program's version number
@end verbatim

@subsection Setting Up a Test HTTPS Server
@cindex HTTPS server
@cindex debug server

Running your own TLS server based on GnuTLS can be useful when
debugging clients and/or GnuTLS itself.  This section describes how to
use @code{gnutls-serv} as a simple HTTPS server.

The most basic server can be started as:

@example
gnutls-serv --http
@end example

It will only support anonymous ciphersuites, which many TLS clients
refuse to use.

The next step is to add support for X.509.  First we generate a CA:

@example
certtool --generate-privkey > x509-ca-key.pem
echo 'cn = GnuTLS test CA' > ca.tmpl
echo 'ca' >> ca.tmpl
echo 'cert_signing_key' >> ca.tmpl
certtool --generate-self-signed --load-privkey x509-ca-key.pem \
  --template ca.tmpl --outfile x509-ca.pem
...
@end example

Then generate a server certificate.  Remember to change the dns_name
value to the name of your server host, or skip that command to avoid
the field.

@example
certtool --generate-privkey > x509-server-key.pem
echo 'organization = GnuTLS test server' > server.tmpl
echo 'cn = test.gnutls.org' >> server.tmpl
echo 'tls_www_server' >> server.tmpl
echo 'encryption_key' >> server.tmpl
echo 'signing_key' >> server.tmpl
echo 'dns_name = test.gnutls.org' >> server.tmpl
certtool --generate-certificate --load-privkey x509-server-key.pem \
  --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
  --template server.tmpl --outfile x509-server.pem
...
@end example

For use in the client, you may want to generate a client certificate
as well.

@example
certtool --generate-privkey > x509-client-key.pem
echo 'cn = GnuTLS test client' > client.tmpl
echo 'tls_www_client' >> client.tmpl
echo 'encryption_key' >> client.tmpl
echo 'signing_key' >> client.tmpl
certtool --generate-certificate --load-privkey x509-client-key.pem \
  --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
  --template client.tmpl --outfile x509-client.pem
...
@end example

To be able to import the client key/certificate into some
applications, you will need to convert them into a PKCS#12 structure.
This also encrypts the security sensitive key with a password.

@example
certtool --to-p12 --load-ca-certificate x509-ca.pem --load-privkey x509-client-key.pem --load-certificate x509-client.pem --outder --outfile x509-client.p12
@end example

For icing, we'll create a proxy certificate for the client too.

@example
certtool --generate-privkey > x509-proxy-key.pem
echo 'cn = GnuTLS test client proxy' > proxy.tmpl
certtool --generate-proxy --load-privkey x509-proxy-key.pem \
  --load-ca-certificate x509-client.pem --load-ca-privkey x509-client-key.pem \
  --load-certificate x509-client.pem --template proxy.tmpl \
  --outfile x509-proxy.pem
...
@end example

Then start the server again:

@example
gnutls-serv --http \
            --x509cafile x509-ca.pem \
            --x509keyfile x509-server-key.pem \
            --x509certfile x509-server.pem
@end example

Try connecting to the server using your web browser.  Note that the
server listens to port 5556 by default.

While you are at it, to allow connections using DSA, you can also
create a DSA key and certificate for the server.  These credentials
will be used in the final example below.

@example
certtool --generate-privkey --dsa > x509-server-key-dsa.pem
certtool --generate-certificate --load-privkey x509-server-key-dsa.pem \
  --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
  --template server.tmpl --outfile x509-server-dsa.pem
...
@end example

The next step is to create OpenPGP credentials for the server.

@example
gpg --gen-key
...enter whatever details you want, use 'test.gnutls.org' as name...
@end example

Make a note of the OpenPGP key identifier of the newly generated key,
here it was @code{5D1D14D8}.  You will need to export the key for
GnuTLS to be able to use it.

@example
gpg -a --export 5D1D14D8 > openpgp-server.txt
gpg --export 5D1D14D8 > openpgp-server.bin
gpg --export-secret-keys 5D1D14D8 > openpgp-server-key.bin
gpg -a --export-secret-keys 5D1D14D8 > openpgp-server-key.txt
@end example

Let's start the server with support for OpenPGP credentials:

@example
gnutls-serv --http \
            --pgpkeyfile openpgp-server-key.txt \
            --pgpcertfile openpgp-server.txt
@end example

The next step is to add support for SRP authentication.

@example
srptool --create-conf srp-tpasswd.conf
srptool --passwd-conf srp-tpasswd.conf --username jas --passwd srp-passwd.txt
Enter password: [TYPE "foo"]
@end example

Start the server with SRP support:

@example
gnutls-serv --http \
            --srppasswdconf srp-tpasswd.conf \
            --srppasswd srp-passwd.txt
@end example

Let's also add support for PSK.

@example
$ psktool --passwd psk-passwd.txt
@end example

Start the server with PSK support:

@example
gnutls-serv --http \
            --pskpasswd psk-passwd.txt
@end example

Finally, we start the server with all the earlier parameters and you
get this command:

@example
gnutls-serv --http \
            --x509cafile x509-ca.pem \
            --x509keyfile x509-server-key.pem \
            --x509certfile x509-server.pem \
            --x509dsakeyfile x509-server-key-dsa.pem \
            --x509dsacertfile x509-server-dsa.pem \
            --pgpkeyfile openpgp-server-key.txt \
            --pgpcertfile openpgp-server.txt \
            --srppasswdconf srp-tpasswd.conf \
            --srppasswd srp-passwd.txt \
            --pskpasswd psk-passwd.txt
@end example

@menu
* Example server PSK connection::
@end menu

@node Example server PSK connection
@subsection Example server PSK connection
@cindex PSK server

To set up a PSK server with @code{gnutls-serv} you need to create PSK
password file (@pxref{Invoking psktool}).  In the example below, I
type @code{password} at the prompt.

@smallexample
$ ./psktool -u psk_identity -p psks.txt -n psk_identity_hint
Enter password:
Key stored to psks.txt
$ cat psks.txt
psk_identity:88f3824b3e5659f52d00e959bacab954b6540344
$
@end smallexample

After this, start the server pointing to the password file.  We
disable DHE-PSK.

@smallexample
$ ./gnutls-serv --pskpasswd psks.txt  --pskhint psk_identity_hint --priority NORMAL:-DHE-PSK
Set static Diffie-Hellman parameters, consider --dhparams.
Echo Server ready. Listening to port '5556'.
@end smallexample

You can now connect to the server using a PSK client (@pxref{Example
client PSK connection}).

@node Invoking psktool
@section Invoking psktool
@cindex psktool

This is a program to manage @acronym{PSK} username and keys.

@verbatim
PSKtool help
Usage : psktool [options]
     -u, --username username
                              specify username.
     -p, --passwd FILE        specify a password file.
     -n, --netconf-hint HINT
                              derive key from Netconf password, using 
                              HINT as the psk_identity_hint.
     -s, --keysize SIZE       specify the key size in bytes.
     -v, --version            prints the program's version number
     -h, --help               shows this help text
@end verbatim

Normally the file will generate random keys for the indicate username.
You may also derive PSK keys from passwords, using the algorithm
specified in @file{draft-ietf-netconf-tls-02.txt}.  The algorithm
needs a PSK identity hint, which you specify using
@code{--netconf-hint}.  To derive a PSK key from a password with an
empty PSK identity hint, using @code{--netconf-hint ""}.

@node Invoking srptool
@section Invoking srptool
@anchor{srptool}
@cindex srptool

The @file{srptool} is a very simple program that emulates the programs
in the @emph{Stanford SRP libraries}, see
@url{http://srp.stanford.edu/}.  It is intended for use in places
where you don't expect @acronym{SRP} authentication to be the used for
system users.

Traditionally @emph{libsrp} used two files. One called @code{tpasswd}
which holds usernames and verifiers, and @code{tpasswd.conf} which
holds generators and primes.

How to use srptool:

@itemize

@item
To create tpasswd.conf which holds the g and n values for
@acronym{SRP} protocol (generator and a large prime), run:

@example
$ srptool --create-conf /etc/tpasswd.conf
@end example

@item
This command will create /etc/tpasswd and will add user 'test' (you
will also be prompted for a password).  Verifiers are stored by
default in the way libsrp expects.

@example
$ srptool --passwd /etc/tpasswd \
    --passwd-conf /etc/tpasswd.conf -u test
@end example

@item
This command will check against a password.  If the password matches
the one in /etc/tpasswd you will get an ok.

@example
$ srptool --passwd /etc/tpasswd \
    --passwd-conf /etc/tpasswd.conf --verify -u test
@end example

@end itemize

@node Function reference
@chapter Function Reference
@cindex Function reference

@menu
* Core functions::
* X.509 certificate functions::
* GnuTLS-extra functions::
* OpenPGP functions::
* TLS Inner Application (TLS/IA) functions::
* Error codes and descriptions::
@end menu

@node Core functions
@section Core Functions

The prototypes for the following functions lie in
@file{gnutls/gnutls.h}.

@include gnutls-api.texi

@node X.509 certificate functions
@section @acronym{X.509} Certificate Functions
@anchor{sec:x509api}
@cindex @acronym{X.509} Functions

The following functions are to be used for @acronym{X.509} certificate handling.
Their prototypes lie in @file{gnutls/x509.h}.

@include x509-api.texi

@node GnuTLS-extra functions
@section @acronym{GnuTLS-extra} Functions
@cindex @acronym{GnuTLS-extra} functions

These functions are only available in the GPLv3+ version of the
library called @code{gnutls-extra}. The prototypes for this library
lie in @file{gnutls/extra.h}.

@include extra-api.texi

@node OpenPGP functions
@section @acronym{OpenPGP} Functions
@cindex @acronym{OpenPGP} functions
@anchor{sec:openpgpapi}

The following functions are to be used for @acronym{OpenPGP}
certificate handling.  Their prototypes lie in
@file{gnutls/openpgp.h}.

@include pgp-api.texi

@node TLS Inner Application (TLS/IA) functions
@section @acronym{TLS} Inner Application (@acronym{TLS/IA}) Functions
@cindex @acronym{TLS} Inner Application (@acronym{TLS/IA}) functions
@cindex Inner Application (@acronym{TLS/IA}) functions

The following functions are used for @acronym{TLS} Inner Application
(@acronym{TLS/IA}).  Their prototypes lie in @file{gnutls/extra.h}.
You need to link with @file{libgnutls-extra} to be able to use these
functions (@pxref{GnuTLS-extra functions}).

The typical control flow in an TLS/IA client (that would not require
an Application Phase for resumed sessions) would be similar to the
following:

@example
int client_avp (gnuls_session_t *session, void *ptr,
                const char *last, size_t lastlen,
		char **new, size_t *newlen)
@{
...
@}
...
int main ()
@{
  gnutls_ia_client_credentials_t iacred;
...
  gnutls_init (&session, GNUTLS_CLIENT);
...
  /* Enable TLS/IA. */
  gnutls_ia_allocate_client_credentials(&iacred);
  gnutls_ia_set_client_avp_function(iacred, client_avp);
  gnutls_credentials_set (session, GNUTLS_CRD_IA, iacred);
...
  ret = gnutls_handshake (session);
  // Error handling...
...
  if (gnutls_ia_handshake_p (session))
    @{
      ret = gnutls_ia_handshake (session);
      // Error handling...
...
@end example

See below for detailed descriptions of all the functions used above.

The function @code{client_avp} would have to be implemented by your
application.  The function is responsible for handling the AVP data.
See @code{gnutls_ia_set_client_avp_function} below for more
information on how that function should be implemented.

The control flow in a typical server is similar to the above, use
@code{gnutls_ia_server_credentials_t} instead of
@code{gnutls_ia_client_credentials_t}, and replace the call to the
client functions with the corresponding server functions.

@include ia-api.texi

@node Error codes and descriptions
@section Error Codes and Descriptions
@anchor{Error Codes}
@cindex Error codes

The error codes used throughout the library are described below.  The
return code @code{GNUTLS_E_SUCCESS} indicate successful operation, and
is guaranteed to have the value 0, so you can use it in logical
expressions.

@include error_codes.texi

@node All the supported ciphersuites in GnuTLS
@chapter All the Supported Ciphersuites in @acronym{GnuTLS}
@anchor{ciphersuites}
@cindex Ciphersuites

@include algorithms.texi

Some additional information regarding some of the algorithms:

@table @code
@item RSA
RSA is public key cryptosystem designed by Ronald Rivest, Adi Shamir
and Leonard Adleman.  It can be used with any hash functions.

@item DSA
DSA is the USA's Digital Signature Standard.  It uses only the SHA-1
hash algorithm.

@item MD2
MD2 is a cryptographic hash algorithm designed by Ron Rivest.  It is
optimized for 8-bit processors. Outputs 128 bits of data.  There are
no known weaknesses of this algorithm but since this algorithm is
rarely used and not really studied it should not be used today.

@item MD5
MD5 is a cryptographic hash algorithm designed by Ron Rivest. Outputs
128 bits of data.  It is considered to be broken.

@item SHA-1
SHA is a cryptographic hash algorithm designed by NSA. Outputs 160
bits of data.  It is also considered to be broken, though no practical
attacks have been found.

@item RMD160
RIPEMD is a cryptographic hash algorithm developed in the framework of
the EU project RIPE.  Outputs 160 bits of data.

@end table

@c
@c Guile Bindings
@c

@include guile.texi


@node Internal architecture of GnuTLS
@chapter Internal Architecture of GnuTLS
@cindex Internal architecture

This chapter is to give a brief description of the
way @acronym{GnuTLS} works. The focus is to give an idea
to potential developers and those who want to know what
happens inside the black box.

@menu
* The TLS Protocol::
* TLS Handshake Protocol::
* TLS Authentication Methods::
* TLS Extension Handling::
* Cryptographic Backend::
@end menu

@node The TLS Protocol
@section The TLS Protocol
The main needs for the TLS protocol to be used are
shown in the image below.

@image{gnutls-client-server-use-case,9cm}

This is being accomplished by the following object diagram.
Note that since @acronym{GnuTLS} is being developed in C
object are just structures with attributes. The operations listed
are functions that require the first parameter to be that object.
@image{gnutls-objects,15cm}

@node TLS Handshake Protocol
@section TLS Handshake Protocol
The @acronym{GnuTLS} handshake protocol is implemented as a state
machine that waits for input or returns immediately when the non-blocking
transport layer functions are used. The main idea is shown in the following
figure.

@image{gnutls-handshake-state,9cm}

Also the way the input is processed varies per ciphersuite. Several 
implementations of the internal handlers are available and 
@ref{gnutls_handshake} only multiplexes the input to the appropriate 
handler. For example a @acronym{PSK} ciphersuite has a different 
implementation of the @code{process_client_key_exchange} than a
certificate ciphersuite.

@image{gnutls-handshake-sequence,12cm}

@node TLS Authentication Methods
@section TLS Authentication Methods
In @acronym{GnuTLS} authentication methods can be implemented quite
easily.  Since the required changes to add a new authentication method
affect only the handshake protocol, a simple interface is used. An
authentication method needs only to implement the functions as seen in
the figure below.

@image{gnutls-mod_auth_st,12cm}

The functions that need to be implemented are the ones responsible for
interpreting the handshake protocol messages. It is common for such
functions to read data from one or more @code{credentials_t}
structures@footnote{such as the
@code{gnutls_certificate_credentials_t} structures} and write data,
such as certificates, usernames etc. to @code{auth_info_t} structures.

Simple examples of existing authentication methods can be seen in
@code{auth_psk.c} for PSK ciphersuites and @code{auth_srp.c} for SRP
ciphersuites. After implementing these functions the structure holding
its pointers has to be registered in @code{gnutls_algorithms.c} in the
@code{_gnutls_kx_algorithms} structure.

@node TLS Extension Handling
@section TLS Extension Handling
As with authentication methods, the TLS extensions handlers can be
implemented using the following interface.

@image{gnutls-extensions_st,12cm}

Here there are two functions, one for receiving the extension data
and one for sending. These functions have to check internally whether
they operate in client or server side. 

A simple example of an extension handler can be seen in
@code{ext_srp.c} After implementing these functions, together with the
extension number they handle, they have to be registered in
@code{gnutls_extensions.c} in the @code{_gnutls_extensions} structure.

@subsection Adding a New TLS Extension

Adding support for a new TLS extension is done from time to time, and
the process to do so is not difficult.  Here are the steps you need to
follow if you wish to do this yourself.  For sake of discussion, let's
consider adding support for the hypothetical TLS extension
@code{foobar}.

@enumerate

@item Add @code{configure} option like @code{--enable-foobar} or @code{--disable-foobar}.

Which to chose depends on whether you intend to make the extension be
enabled by default.  Look at existing checks (i.e., SRP, authz) for
how to model the code.  For example:

@example
AC_MSG_CHECKING([whether to disable foobar support])
AC_ARG_ENABLE(foobar,
	AS_HELP_STRING([--disable-foobar],
		[disable foobar support]),
	ac_enable_foobar=no)
if test x$ac_enable_foobar != xno; then
 AC_MSG_RESULT(no)
 AC_DEFINE(ENABLE_FOOBAR, 1, [enable foobar])
else
 ac_full=0
 AC_MSG_RESULT(yes)
fi
AM_CONDITIONAL(ENABLE_FOOBAR, test "$ac_enable_foobar" != "no")
@end example

These lines should go in @code{lib/m4/hooks.m4}.

@item Add IANA extension value to @code{extensions_t} in @code{gnutls_int.h}.

A good name for the value would be GNUTLS_EXTENSION_FOOBAR.  Check
with @url{http://www.iana.org/assignments/tls-extensiontype-values}
for allocated values.  For experiments, you could pick a number but
remember that some consider it a bad idea to deploy such modified
version since it will lead to interoperability problems in the future
when the IANA allocates that number to someone else, or when the
foobar protocol is allocated another number.

@item Add an entry to @code{_gnutls_extensions} in @code{gnutls_extensions.c}.

A typical entry would be:

@example
  int ret;

  /* ...
   */

#if ENABLE_FOOBAR
  ret = gnutls_ext_register (GNUTLS_EXTENSION_FOOBAR,
                             "FOOBAR",
                             GNUTLS_EXT_TLS,
                             _gnutls_foobar_recv_params,
                             _gnutls_foobar_send_params);
  if (ret != GNUTLS_E_SUCCESS)
    return ret;
#endif
@end example

The GNUTLS_EXTENSION_FOOBAR is the integer value you added to
@code{gnutls_int.h} earlier.  The two functions are new functions that
you will need to implement, most likely you'll need to add an
@code{#include "ext_foobar.h"} as well.

@item Add new files @code{ext_foobar.c} and @code{ext_foobar.h} that implements the extension.

The functions you are responsible to add are those mentioned in the
previous step.  As a starter, you could add this:

@example
int
_gnutls_foobar_recv_params (gnutls_session_t session,
                            const opaque * data,
                            size_t data_size)
@{
  return 0;
@}

int
_gnutls_foobar_send_params (gnutls_session_t session,
                            opaque * data,
                            size_t _data_size)
@{
  return 0;
@}
@end example

The @code{_gnutls_foobar_recv_params} function is responsible for
parsing incoming extension data (both in the client and server).

The @code{_gnutls_foobar_send_params} function is responsible for
sending extension data (both in the client and server).

If you receive length fields that doesn't match, return
@code{GNUTLS_E_UNEXPECTED_PACKET_LENGTH}.  If you receive invalid
data, return @code{GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER}.  You can use
other error codes too.  Return 0 on success.

The function typically store some information in the @code{session}
variable for later usage.  If you need to add new fields there, check
@code{tls_ext_st} in @code{gnutls_int.h} and compare with existing TLS
extension specific variables.

Recall that both the client and server both send and receives
parameters, and your code most likely will need to do different things
depending on which mode it is in.  It may be useful to make this
distinction explicit in the code.  Thus, for example, a better
template than above would be:

@example
int
_gnutls_foobar_recv_params (gnutls_session_t session,
                            const opaque * data,
                            size_t data_size)
@{
  if (session->security_parameters.entity == GNUTLS_CLIENT)
    return foobar_recv_client (session, data, data_size);
  else
    return foobar_recv_server (session, data, data_size);
@}

int
_gnutls_foobar_send_params (gnutls_session_t session,
                            opaque * data,
                            size_t data_size)
@{
  if (session->security_parameters.entity == GNUTLS_CLIENT)
    return foobar_send_client (session, data, data_size);
  else
    return foobar_send_server (session, data, data_size);
@}
@end example

The functions used would be declared as @code{static} functions, of
the appropriate prototype, in the same file.

When adding the files, you'll need to add them to @code{Makefile.am}
as well, for example:

@example
if ENABLE_FOOBAR
COBJECTS += ext_foobar.c
HFILES += ext_foobar.h
endif
@end example

@item Add API functions to enable/disable the extension.

Normally the client will have one API to request use of the extension,
and setting some extension specific data.  The server will have one
API to let the library know that it is willing to accept the
extension, often this is implemented through a callback but it doesn't
have to.

The APIs need to be added to @code{includes/gnutls/gnutls.h} or
@code{includes/gnutls/extra.h} as appropriate.  It is recommended that
if you don't have a requirement to use the LGPLv2.1+ license for your
extension, that you place your work under the GPLv3+ license and thus
in the libgnutls-extra library.

You can implement the API function in the @code{ext_foobar.c} file, or
if that file ends up becoming rather larger, add a
@code{gnutls_foobar.c} file.

To make the API available in the shared library you need to add the
symbol in @code{lib/libgnutls.map} or
@code{libextra/libgnutls-extra.map} as appropriate, so that the symbol
is exported properly.

When writing GTK-DOC style documentation for your new APIs, don't
forget to add @code{Since:} tags to indicate the GnuTLS version the
API was introduced in.

@end enumerate

@section Certificate Handling
What is provided by the certificate handling functions
is summarized in the following diagram.

@image{gnutls-certificate-user-use-case,12cm}

@node Cryptographic Backend
@section Cryptographic Backend
Several new systems provide hardware assisted cryptographic algorithm
implementations that offer implementations some orders of magnitude
faster than the software. For this reason GnuTLS supports by default
the /dev/crypto device usually found in FreeBSD and OpenBSD system, to
take advantage of installed hardware. 

In addition it is possible to override parts of the crypto backend or the
whole. It is possible to override them both at runtime and compile
time, however here we will discuss the runtime possibility. The API
available for this functionality is in @code{gnutls/crypto.h} header
file.

@subsection Override specific algorithms
When an optimized implementation of a single algorithm is available,
say a hardware assisted version of @acronym{AES-CBC} then the
following functions can be used to register those algorithms.

@itemize

@item @ref{gnutls_crypto_single_cipher_register2}
To register a cipher algorithm.

@ref{gnutls_crypto_single_digest_register2}
To register a hash (digest) or MAC algorithm.

@end itemize

Those registration functions will only replace the specified algorithm
and leave the rest of subsystem intact.

@subsection Override parts of the backend
In some systems, such as embedded ones, it might be desirable to
override big parts of the cryptographic backend, or even all of
them. For this reason the following functions are provided.

@itemize

@item @ref{gnutls_crypto_cipher_register2}
To override the cryptographic algorithms backend.

@item @ref{gnutls_crypto_digest_register2}
To override the digest algorithms backend.

@item @ref{gnutls_crypto_rnd_register2}
To override the random number generator backend.

@item @ref{gnutls_crypto_bigint_register2}
To override the big number number operations backend.

@item @ref{gnutls_crypto_pk_register2}
To override the public key encryption backend. This is tight to the
big number operations so either both of them should be updated or care
must be taken to use the same format.

@end itemize

If all of them are used then GnuTLS will no longer use libgcrypt.

@node Copying Information
@appendix Copying Information

@menu
* GNU Free Documentation License::   License for copying this manual.
* GNU LGPL::                     License for copying the core GnuTLS library.
* GNU GPL::                      License for copying GNUTLS extra and tools.
@end menu

@node GNU Free Documentation License
@appendixsec GNU Free Documentation License

@cindex FDL, GNU Free Documentation License

@include fdl-1.3.texi

@node GNU LGPL
@appendixsec GNU Lesser General Public License
@cindex LGPL, GNU Lesser General Public License
@cindex License, GNU LGPL

@include lgpl-2.1.texi

@node GNU GPL
@appendixsec GNU General Public License
@cindex GPL, GNU General Public License
@cindex License, GNU GPL

@include gpl-3.0.texi

@node Bibliography
@unnumbered Bibliography

@table @asis

@item @anchor{CBCATT}[CBCATT]
Bodo Moeller, "Security of CBC Ciphersuites in SSL/TLS: Problems and
Countermeasures", 2002, available from
@url{http://www.openssl.org/~bodo/tls-cbc.txt}.

@item @anchor{GPGH}[GPGH]
Mike Ashley, "The GNU Privacy Handbook", 2002, available from
@url{http://www.gnupg.org/gph/en/manual.pdf}.

@item @anchor{GUTPKI}[GUTPKI]
Peter Gutmann, "Everything you never wanted to know about PKI but were
forced to find out", Available from
@url{http://www.cs.auckland.ac.nz/~pgut001/}.

@item @anchor{NISTSP80057}[NISTSP80057]
NIST Special Publication 800-57, "Recommendation for Key Management -
Part 1: General (Revised)", March 2007, available from
@url{http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57-Part1-revised2_Mar08-2007.pdf}.

@item @anchor{RFC2246}[RFC2246]
Tim Dierks and Christopher Allen, "The TLS Protocol Version 1.0",
January 1999, Available from
@url{http://www.ietf.org/rfc/rfc2246.txt}.

@item @anchor{RFC4346}[RFC4346]
Tim Dierks and Eric Rescorla, "The TLS Protocol Version 1.1", Match
2006, Available from @url{http://www.ietf.org/rfc/rfc4346.txt}.

@item @anchor{RFC2440}[RFC2440]
Jon Callas, Lutz Donnerhacke, Hal Finney and Rodney Thayer, "OpenPGP
Message Format", November 1998, Available from
@url{http://www.ietf.org/rfc/rfc2440.txt}.

@item @anchor{RFC4880}[RFC4880]
Jon Callas, Lutz Donnerhacke, Hal Finney, David Shaw and Rodney
Thayer, "OpenPGP Message Format", November 2007, Available from
@url{http://www.ietf.org/rfc/rfc4880.txt}.

@item @anchor{RFC4211}[RFC4211]
J. Schaad, "Internet X.509 Public Key Infrastructure Certificate
Request Message Format (CRMF)", September 2005, Available from
@url{http://www.ietf.org/rfc/rfc4211.txt}.

@item @anchor{RFC2817}[RFC2817]
Rohit Khare and Scott Lawrence, "Upgrading to TLS Within HTTP/1.1",
May 2000, Available from @url{http://www.ietf.org/rfc/rfc2817.txt}

@item @anchor{RFC2818}[RFC2818]
Eric Rescorla, "HTTP Over TLS", May 2000, Available from
@url{http://www.ietf/rfc/rfc2818.txt}.

@item @anchor{RFC2945}[RFC2945]
Tom Wu, "The SRP Authentication and Key Exchange System", September
2000, Available from @url{http://www.ietf.org/rfc/rfc2945.txt}.

@item @anchor{RFC2986}[RFC2986]
Magnus Nystrom and Burt Kaliski, "PKCS 10 v1.7: Certification Request
Syntax Specification", November 2000, Available from
@url{http://www.ietf.org/rfc/rfc2986.txt}.

@item @anchor{PKIX}[PKIX]
D. Cooper, S. Santesson, S. Farrel, S. Boeyen, R. Housley, W. Polk,
"Internet X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", May 2008, available from
@url{http://www.ietf.org/rfc/rfc5280.txt}.

@item @anchor{RFC3749}[RFC3749]
Scott Hollenbeck, "Transport Layer Security Protocol Compression
Methods", May 2004, available from
@url{http://www.ietf.org/rfc/rfc3749.txt}.

@item @anchor{RFC3820}[RFC3820]
Steven Tuecke, Von Welch, Doug Engert, Laura Pearlman, and Mary
Thompson, "Internet X.509 Public Key Infrastructure (PKI) Proxy
Certificate Profile", June 2004, available from
@url{http://www.ietf.org/rfc/rfc3820}.

@item @anchor{RFC5746}[RFC5746]
E. Rescorla, M. Ray, S. Dispensa, and N. Oskov, "Transport Layer
Security (TLS) Renegotiation Indication Extension", February 2010,
available from @url{http://www.ietf.org/rfc/rfc5746}.

@item @anchor{TLSTKT}[TLSTKT]
Joseph Salowey, Hao Zhou, Pasi Eronen, Hannes Tschofenig, "Transport
Layer Security (TLS) Session Resumption without Server-Side State",
January 2008, available from @url{http://www.ietf.org/rfc/rfc5077}.

@item @anchor{PKCS12}[PKCS12]
RSA Laboratories, "PKCS 12 v1.0: Personal Information Exchange
Syntax", June 1999, Available from @url{http://www.rsa.com}.

@item @anchor{RESCORLA}[RESCORLA]
Eric Rescorla, "SSL and TLS: Designing and Building Secure Systems",
2001

@item @anchor{SELKEY}[SELKEY]
Arjen Lenstra and Eric Verheul, "Selecting Cryptographic Key Sizes",
2003, available from @url{http://www.win.tue.nl/~klenstra/key.pdf}.

@item @anchor{SSL3}[SSL3]
Alan Freier, Philip Karlton and Paul Kocher, "The SSL Protocol Version
3.0", November 1996, Available from
@url{http://wp.netscape.com/eng/ssl3/draft302.txt}.

@item @anchor{STEVENS}[STEVENS]
Richard Stevens, "UNIX Network Programming, Volume 1", Prentice Hall
PTR, January 1998

@item @anchor{TLSEXT}[TLSEXT]
Simon Blake-Wilson, Magnus Nystrom, David Hopwood, Jan Mikkelsen and
Tim Wright, "Transport Layer Security (TLS) Extensions", June 2003,
Available from @url{http://www.ietf.org/rfc/rfc3546.txt}.

@item @anchor{TLSPGP}[TLSPGP]
Nikos Mavrogiannopoulos, "Using OpenPGP keys for TLS authentication",
April 2004, November 2007. Available from
@url{http://www.ietf.org/rfc/rfc5081.txt}.

@item @anchor{TLSSRP}[TLSSRP]
David Taylor, Trevor Perrin, Tom Wu and Nikos Mavrogiannopoulos,
"Using SRP for TLS Authentication", November 2007. Available from
@url{http://www.ietf.org/rfc/rfc5054.txt}.

@item @anchor{TLSPSK}[TLSPSK]
Pasi Eronen and Hannes Tschofenig, "Pre-shared key Ciphersuites for
TLS", December 2005, Available from
@url{http://www.ietf.org/rfc/rfc4279.txt}.

@item @anchor{TOMSRP}[TOMSRP]
Tom Wu, "The Stanford SRP Authentication Project", Available at
@url{http://srp.stanford.edu/}.

@item @anchor{WEGER}[WEGER]
Arjen Lenstra and Xiaoyun Wang and Benne de Weger, "Colliding X.509
Certificates", Cryptology ePrint Archive, Report 2005/067, Available
at @url{http://eprint.iacr.org/}.

@end table

@node Function and Data Index
@unnumbered Function and Data Index

@printindex fn

@node Concept Index
@unnumbered Concept Index

@printindex cp

@bye