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
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- This manual is last updated 4 March 2015 for version
3.5.4 of GnuTLS.
Copyright (C) 2001-2015 Free Software Foundation, Inc.\\
Copyright (C) 2001-2015 Nikos Mavrogiannopoulos
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". -->
<!-- Created by GNU Texinfo 6.1, http://www.gnu.org/software/texinfo/ -->
<head>
<title>GnuTLS 3.5.4: Selecting cryptographic key sizes</title>
<meta name="description" content="GnuTLS 3.5.4: Selecting cryptographic key sizes">
<meta name="keywords" content="GnuTLS 3.5.4: Selecting cryptographic key sizes">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="Function-and-Data-Index.html#Function-and-Data-Index" rel="index" title="Function and Data Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="How-to-use-GnuTLS-in-applications.html#How-to-use-GnuTLS-in-applications" rel="up" title="How to use GnuTLS in applications">
<link href="Advanced-topics.html#Advanced-topics" rel="next" title="Advanced topics">
<link href="Priority-Strings.html#Priority-Strings" rel="prev" title="Priority Strings">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
body {
margin: 2%;
padding: 0 5%;
background: #ffffff;
}
h1,h2,h3,h4,h5 {
font-weight: bold;
padding: 5px 5px 5px 5px;
background-color: #c2e0ff;
color: #336699;
}
h1 {
padding: 2em 2em 2em 5%;
color: white;
background: #336699;
text-align: center;
letter-spacing: 3px;
}
h2 { text-decoration: underline; }
pre {
margin: 0 5%;
padding: 0.5em;
}
pre.example,pre.verbatim {
padding-bottom: 1em;
border: solid #c2e0ff;
background: #f0faff;
border-width: 1px 1px 1px 5px;
margin: 1em auto;
width: 90%;
}
div.node {
margin: 0 -5% 0 -2%;
padding: 0.5em 0.5em;
margin-top: 0.5em;
margin-bottom: 0.5em;
font-weight: bold;
}
dd, li {
padding-top: 0.1em;
padding-bottom: 0.1em;
}
div.float {
margin-bottom: 0.5em;
text-align: center;
}
table {
text-align: left;
margin-left:auto;
margin-right:auto;
border-spacing: 7px;
width: 50%;
}
th {
padding: 0;
color: #336699;
background-color: #c2e0ff;
border: solid #000000;
border-width: 0px;
margin: 1em auto;
text-align: center;
margin-left:auto;
margin-right:auto;
}
td {
padding: 0;
border: solid #000000;
background-color: #f0faff;
border-width: 0px;
margin: 1em auto;
text-align: left;
margin-left:auto;
margin-right:auto;
padding-left: 1em;
}
dl {
text-align: left;
margin-left:auto;
margin-right:auto;
width: 50%;
padding-left: 1em;
border: solid #c2e0ff;
background: #f0faff;
border-width: 5px 1px 1px 1px;
margin: 1em auto;
}
-->
</style>
</head>
<body lang="en">
<a name="Selecting-cryptographic-key-sizes"></a>
<div class="header">
<p>
Next: <a href="Advanced-topics.html#Advanced-topics" accesskey="n" rel="next">Advanced topics</a>, Previous: <a href="Priority-Strings.html#Priority-Strings" accesskey="p" rel="prev">Priority Strings</a>, Up: <a href="How-to-use-GnuTLS-in-applications.html#How-to-use-GnuTLS-in-applications" accesskey="u" rel="up">How to use GnuTLS in applications</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Selecting-cryptographic-key-sizes-1"></a>
<h3 class="section">6.11 Selecting cryptographic key sizes</h3>
<a name="index-key-sizes"></a>
<p>Because many algorithms are involved in TLS, it is not easy to set
a consistent security level. For this reason in <a href="#tab_003akey_002dsizes">Table 6.7</a> we
present some correspondence between key sizes of symmetric algorithms
and public key algorithms based on [<em>ECRYPT</em>].
Those can be used to generate certificates with
appropriate key sizes as well as select parameters for Diffie-Hellman and SRP
authentication.
</p>
<div class="float"><a name="tab_003akey_002dsizes"></a>
<table>
<thead><tr><th width="10%">Security bits</th><th width="12%">RSA, DH and SRP parameter size</th><th width="10%">ECC key size</th><th width="20%">Security parameter (profile)</th><th width="32%">Description</th></tr></thead>
<tr><td width="10%"><64</td><td width="12%"><768</td><td width="10%"><128</td><td width="20%"><code>INSECURE</code></td><td width="32%">Considered to be insecure</td></tr>
<tr><td width="10%">64</td><td width="12%">768</td><td width="10%">128</td><td width="20%"><code>VERY WEAK</code></td><td width="32%">Short term protection against individuals</td></tr>
<tr><td width="10%">72</td><td width="12%">1008</td><td width="10%">160</td><td width="20%"><code>WEAK</code></td><td width="32%">Short term protection against small organizations</td></tr>
<tr><td width="10%">80</td><td width="12%">1024</td><td width="10%">160</td><td width="20%"><code>LOW</code></td><td width="32%">Very short term protection against agencies (corresponds to ENISA legacy level)</td></tr>
<tr><td width="10%">96</td><td width="12%">1776</td><td width="10%">192</td><td width="20%"><code>LEGACY</code></td><td width="32%">Legacy standard level</td></tr>
<tr><td width="10%">112</td><td width="12%">2048</td><td width="10%">224</td><td width="20%"><code>MEDIUM</code></td><td width="32%">Medium-term protection</td></tr>
<tr><td width="10%">128</td><td width="12%">3072</td><td width="10%">256</td><td width="20%"><code>HIGH</code></td><td width="32%">Long term protection (corresponds to ENISA future level)</td></tr>
<tr><td width="10%">192</td><td width="12%">8192</td><td width="10%">384</td><td width="20%"><code>ULTRA</code></td><td width="32%">Even longer term protection</td></tr>
<tr><td width="10%">256</td><td width="12%">15424</td><td width="10%">512</td><td width="20%"><code>FUTURE</code></td><td width="32%">Foreseeable future</td></tr>
</table>
<div class="float-caption"><p><strong>Table 6.7: </strong>Key sizes and security parameters.</p></div></div>
<p>The first column provides a security parameter in a number of bits. This
gives an indication of the number of combinations to be tried by an adversary
to brute force a key. For example to test all possible keys in a 112 bit security parameter
<em>2^{112}</em> combinations have to be tried. For today’s technology this is infeasible.
The next two columns correlate the security
parameter with actual bit sizes of parameters for DH, RSA, SRP and ECC algorithms.
A mapping to <code>gnutls_sec_param_t</code> value is given for each security parameter, on
the next column, and finally a brief description of the level.
</p>
<p>Note, however, that the values suggested here are nothing more than an
educated guess that is valid today. There are no guarantees 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.
The NIST publication SP 800-57 [<em>NISTSP80057</em>] contains a similar
table.
</p>
<p>When using <acronym>GnuTLS</acronym> and a decision on bit sizes for a public
key algorithm is required, use of the following functions is
recommended:
</p>
<dl>
<dt><a name="index-gnutls_005fsec_005fparam_005fto_005fpk_005fbits"></a>Function: <em>unsigned int</em> <strong>gnutls_sec_param_to_pk_bits</strong> <em>(gnutls_pk_algorithm_t <var>algo</var>, gnutls_sec_param_t <var>param</var>)</em></dt>
<dd><p><var>algo</var>: is a public key algorithm
</p>
<p><var>param</var>: is a security parameter
</p>
<p>When generating private and public key pairs a difficult question
is which size of "bits" the modulus will be in RSA and the group size
in DSA. The easy answer is 1024, which is also wrong. This function
will convert a human understandable security parameter to an
appropriate size for the specific algorithm.
</p>
<p><strong>Returns:</strong> The number of bits, or (0).
</p>
<p><strong>Since:</strong> 2.12.0
</p></dd></dl>
<dl>
<dt><a name="index-gnutls_005fpk_005fbits_005fto_005fsec_005fparam"></a>Function: <em>gnutls_sec_param_t</em> <strong>gnutls_pk_bits_to_sec_param</strong> <em>(gnutls_pk_algorithm_t <var>algo</var>, unsigned int <var>bits</var>)</em></dt>
<dd><p><var>algo</var>: is a public key algorithm
</p>
<p><var>bits</var>: is the number of bits
</p>
<p>This is the inverse of <code>gnutls_sec_param_to_pk_bits()</code> . Given an algorithm
and the number of bits, it will return the security parameter. This is
a rough indication.
</p>
<p><strong>Returns:</strong> The security parameter.
</p>
<p><strong>Since:</strong> 2.12.0
</p></dd></dl>
<p>Those functions will convert a human understandable security parameter
of <code>gnutls_sec_param_t</code> type, to a number of bits suitable for a public
key algorithm.
</p>
<dl compact="compact">
<dt><code><var>const char *</var> <a href="Core-TLS-API.html#gnutls_005fsec_005fparam_005fget_005fname">gnutls_sec_param_get_name</a> (gnutls_sec_param_t <var>param</var>)</code></dt>
</dl>
<p>The following functions will set the minimum acceptable group size for Diffie-Hellman
and SRP authentication.
</p><dl compact="compact">
<dt><code><var>void</var> <a href="Core-TLS-API.html#gnutls_005fdh_005fset_005fprime_005fbits">gnutls_dh_set_prime_bits</a> (gnutls_session_t <var>session</var>, unsigned int <var>bits</var>)</code></dt>
<dt><code><var>void</var> <a href="Core-TLS-API.html#gnutls_005fsrp_005fset_005fprime_005fbits">gnutls_srp_set_prime_bits</a> (gnutls_session_t <var>session</var>, unsigned int <var>bits</var>)</code></dt>
</dl>
<hr>
<div class="header">
<p>
Next: <a href="Advanced-topics.html#Advanced-topics" accesskey="n" rel="next">Advanced topics</a>, Previous: <a href="Priority-Strings.html#Priority-Strings" accesskey="p" rel="prev">Priority Strings</a>, Up: <a href="How-to-use-GnuTLS-in-applications.html#How-to-use-GnuTLS-in-applications" accesskey="u" rel="up">How to use GnuTLS in applications</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
</body>
</html>
|
