path: root/bignum-random-prime.c

/* bignum-random-prime.c
 *
 * Generation of random provable primes.
 */

/* nettle, low-level cryptographics library
 *
 * Copyright (C) 2010 Niels Möller
 *  
 * The nettle library is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or (at your
 * option) any later version.
 * 
 * The nettle library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 * License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with the nettle library; see the file COPYING.LIB.  If not, write to
 * the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

#if HAVE_CONFIG_H
# include "config.h"
#endif

#ifndef RANDOM_PRIME_VERBOSE
#define RANDOM_PRIME_VERBOSE 0
#endif

#include <assert.h>
#include <stdlib.h>

#if RANDOM_PRIME_VERBOSE
#include <stdio.h>
#define VERBOSE(x) (fputs((x), stderr))
#else
#define VERBOSE(x)
#endif

#include "bignum.h"

#include "macros.h"

/* Use a table of p_2 = 3 to p_{172} = 1021, used for sieving numbers
   of up to 20 bits. */

#define NPRIMES 171
#define TRIAL_DIV_BITS 20
#define TRIAL_DIV_MASK ((1 << TRIAL_DIV_BITS) - 1)

/* A 20-bit number x is divisible by p iff

     ((x * inverse) & TRIAL_DIV_MASK) <= limit
*/
struct trial_div_info {
  uint32_t inverse; /* p^{-1} (mod 2^20) */
  uint32_t limit;   /* floor( (2^20 - 1) / p) */
};

static const uint16_t
primes[NPRIMES] = {
  3,5,7,11,13,17,19,23,
  29,31,37,41,43,47,53,59,
  61,67,71,73,79,83,89,97,
  101,103,107,109,113,127,131,137,
  139,149,151,157,163,167,173,179,
  181,191,193,197,199,211,223,227,
  229,233,239,241,251,257,263,269,
  271,277,281,283,293,307,311,313,
  317,331,337,347,349,353,359,367,
  373,379,383,389,397,401,409,419,
  421,431,433,439,443,449,457,461,
  463,467,479,487,491,499,503,509,
  521,523,541,547,557,563,569,571,
  577,587,593,599,601,607,613,617,
  619,631,641,643,647,653,659,661,
  673,677,683,691,701,709,719,727,
  733,739,743,751,757,761,769,773,
  787,797,809,811,821,823,827,829,
  839,853,857,859,863,877,881,883,
  887,907,911,919,929,937,941,947,
  953,967,971,977,983,991,997,1009,
  1013,1019,1021,
};

static const uint32_t
prime_square[NPRIMES+1] = {
  9,25,49,121,169,289,361,529,
  841,961,1369,1681,1849,2209,2809,3481,
  3721,4489,5041,5329,6241,6889,7921,9409,
  10201,10609,11449,11881,12769,16129,17161,18769,
  19321,22201,22801,24649,26569,27889,29929,32041,
  32761,36481,37249,38809,39601,44521,49729,51529,
  52441,54289,57121,58081,63001,66049,69169,72361,
  73441,76729,78961,80089,85849,94249,96721,97969,
  100489,109561,113569,120409,121801,124609,128881,134689,
  139129,143641,146689,151321,157609,160801,167281,175561,
  177241,185761,187489,192721,196249,201601,208849,212521,
  214369,218089,229441,237169,241081,249001,253009,259081,
  271441,273529,292681,299209,310249,316969,323761,326041,
  332929,344569,351649,358801,361201,368449,375769,380689,
  383161,398161,410881,413449,418609,426409,434281,436921,
  452929,458329,466489,477481,491401,502681,516961,528529,
  537289,546121,552049,564001,573049,579121,591361,597529,
  619369,635209,654481,657721,674041,677329,683929,687241,
  703921,727609,734449,737881,744769,769129,776161,779689,
  786769,822649,829921,844561,863041,877969,885481,896809,
  908209,935089,942841,954529,966289,982081,994009,1018081,
  1026169,1038361,1042441,1L<<20
};

static const struct trial_div_info
trial_div_table[NPRIMES] = {
  {699051,349525},{838861,209715},{748983,149796},{953251,95325},
  {806597,80659},{61681,61680},{772635,55188},{866215,45590},
  {180789,36157},{1014751,33825},{793517,28339},{1023001,25575},
  {48771,24385},{870095,22310},{217629,19784},{710899,17772},
  {825109,17189},{281707,15650},{502135,14768},{258553,14364},
  {464559,13273},{934875,12633},{1001449,11781},{172961,10810},
  {176493,10381},{203607,10180},{568387,9799},{788837,9619},
  {770193,9279},{1032063,8256},{544299,8004},{619961,7653},
  {550691,7543},{182973,7037},{229159,6944},{427445,6678},
  {701195,6432},{370455,6278},{90917,6061},{175739,5857},
  {585117,5793},{225087,5489},{298817,5433},{228877,5322},
  {442615,5269},{546651,4969},{244511,4702},{83147,4619},
  {769261,4578},{841561,4500},{732687,4387},{978961,4350},
  {133683,4177},{65281,4080},{629943,3986},{374213,3898},
  {708079,3869},{280125,3785},{641833,3731},{618771,3705},
  {930477,3578},{778747,3415},{623751,3371},{40201,3350},
  {122389,3307},{950371,3167},{1042353,3111},{18131,3021},
  {285429,3004},{549537,2970},{166487,2920},{294287,2857},
  {919261,2811},{636339,2766},{900735,2737},{118605,2695},
  {10565,2641},{188273,2614},{115369,2563},{735755,2502},
  {458285,2490},{914767,2432},{370513,2421},{1027079,2388},
  {629619,2366},{462401,2335},{649337,2294},{316165,2274},
  {484655,2264},{65115,2245},{326175,2189},{1016279,2153},
  {990915,2135},{556859,2101},{462791,2084},{844629,2060},
  {404537,2012},{457123,2004},{577589,1938},{638347,1916},
  {892325,1882},{182523,1862},{1002505,1842},{624371,1836},
  {69057,1817},{210787,1786},{558769,1768},{395623,1750},
  {992745,1744},{317855,1727},{384877,1710},{372185,1699},
  {105027,1693},{423751,1661},{408961,1635},{908331,1630},
  {74551,1620},{36933,1605},{617371,1591},{506045,1586},
  {24929,1558},{529709,1548},{1042435,1535},{31867,1517},
  {166037,1495},{928781,1478},{508975,1458},{4327,1442},
  {779637,1430},{742091,1418},{258263,1411},{879631,1396},
  {72029,1385},{728905,1377},{589057,1363},{348621,1356},
  {671515,1332},{710453,1315},{84249,1296},{959363,1292},
  {685853,1277},{467591,1274},{646643,1267},{683029,1264},
  {439927,1249},{254461,1229},{660713,1223},{554195,1220},
  {202911,1215},{753253,1195},{941457,1190},{776635,1187},
  {509511,1182},{986147,1156},{768879,1151},{699431,1140},
  {696417,1128},{86169,1119},{808997,1114},{25467,1107},
  {201353,1100},{708087,1084},{1018339,1079},{341297,1073},
  {434151,1066},{96287,1058},{950765,1051},{298257,1039},
  {675933,1035},{167731,1029},{815445,1027},
};

/* Element j gives the index of the first prime of size 3+j bits */
static uint8_t
prime_by_size[9] = {
  1,3,5,10,17,30,53,96,171
};

/* Combined Miller-Rabin test to the base a, and checking the
   conditions from Pocklington's theorem. */
static int
miller_rabin_pocklington(mpz_t n, mpz_t nm1, mpz_t nm1dq, mpz_t a)
{
  mpz_t r;
  mpz_t y;
  int is_prime = 0;

  /* Avoid the mp_bitcnt_t type for compatibility with older GMP
     versions. */
  unsigned k;
  unsigned j;

  VERBOSE(".");

  if (mpz_even_p(n) || mpz_cmp_ui(n, 3) < 0)
    return 0;

  mpz_init(r);
  mpz_init(y);

  k = mpz_scan1(nm1, 0);
  assert(k > 0);

  mpz_fdiv_q_2exp (r, nm1, k);

  mpz_powm(y, a, r, n);

  if (mpz_cmp_ui(y, 1) == 0 || mpz_cmp(y, nm1) == 0)
    goto passed_miller_rabin;
    
  for (j = 1; j < k; j++)
    {
      mpz_powm_ui (y, y, 2, n);

      if (mpz_cmp_ui (y, 1) == 0)
	break;

      if (mpz_cmp (y, nm1) == 0)
	{
	passed_miller_rabin:
	  /* We know that a^{n-1} = 1 (mod n)

	     Remains to check that gcd(a^{(n-1)/q} - 1, n) == 1 */      
	  VERBOSE("x");

	  mpz_powm(y, a, nm1dq, n);
	  mpz_sub_ui(y, y, 1);
	  mpz_gcd(y, y, n);
	  is_prime = mpz_cmp_ui (y, 1) == 0;
	  VERBOSE(is_prime ? "\n" : "");
	  break;
	}

    }

  mpz_clear(r);
  mpz_clear(y);

  return is_prime;
}

/* The algorithm is based on the following special case of
   Pocklington's theorem:

   Assume that n = 1 + f q, where q is a prime, q > sqrt(n) - 1. If we
   can find an a such that

     a^{n-1} = 1 (mod n)
     gcd(a^f - 1, n) = 1

   then n is prime.

   Proof: Assume that n is composite, with smallest prime factor p <=
   sqrt(n). Since q is prime, and q > sqrt(n) - 1 >= p - 1, q and p-1
   are coprime, so that we can define u = q^{-1} (mod (p-1)). The
   assumption a^{n-1} = 1 (mod n) implies that also a^{n-1} = 1 (mod
   p). Since p is prime, we have a^{(p-1)} = 1 (mod p). Now, r =
   (n-1)/q = (n-1) u (mod (p-1)), and it follows that a^r = a^{(n-1)
   u} = 1 (mod p). Then p is a common factor of a^r - 1 and n. This
   contradicts gcd(a^r - 1, n) = 1, and concludes the proof.

   If n is specified as k bits, we need q of size ceil(k/2) + 1 bits
   (or more) to make the theorem apply.
*/

/* Generate a prime number p of size bits with 2 p0q dividing (p-1).
   p0 must be of size >= ceil(bits/2) + 1. The extra factor q can be
   omitted. */
void
_nettle_generate_pocklington_prime (mpz_t p, unsigned bits,
				    void *ctx, nettle_random_func random, 
				    const mpz_t p0,
				    const mpz_t q,
				    const mpz_t p0q)
{
  mpz_t i, r, pm1,a;
  
  assert (2*mpz_sizeinbase (p0, 2) > bits + 1);

  mpz_init (i);
  mpz_init (r);
  mpz_init (pm1);
  mpz_init (a);

  /* i = floor (2^{bits-2} / p0q) */
  mpz_init_set_ui (i, 1);
  mpz_mul_2exp (i, i, bits-2);
  mpz_fdiv_q (i, i, p0q);

  for (;;)
    {
      uint8_t buf[1];

      /* Generate r in the range i + 1 <= r <= 2*i */
      nettle_mpz_random (r, ctx, random, i);
      mpz_add (r, r, i);
      mpz_add_ui (r, r, 1);

      /* Set p = 2*r*p0q + 1 */
      mpz_mul_2exp(r, r, 1);
      mpz_mul (pm1, r, p0q);
      mpz_add_ui (p, pm1, 1);

      assert(mpz_sizeinbase(p, 2) == bits);

      /* Should use GMP trial division interface when that
	 materializes, we don't need any testing beyond trial
	 division. */
      if (!mpz_probab_prime_p (p, 1))
	continue;

      random(ctx, sizeof(buf), buf);
	  
      mpz_set_ui (a, buf[0] + 2);

      if (q)
	mpz_mul (r, r, q);
      
      if (miller_rabin_pocklington(p, pm1, r, a))
	break;
    }
  mpz_clear (i);
  mpz_clear (r);
  mpz_clear (pm1);
  mpz_clear (a);
}

/* Generate random prime of a given size. Maurer's algorithm (Alg.
   6.42 Handbook of applied cryptography), but with ratio = 1/2 (like
   the variant in fips186-3). FIXME: Force primes to start with two
   one bits? */
void
nettle_random_prime(mpz_t p, unsigned bits,
		    void *ctx, nettle_random_func random)
{
  assert (bits >= 3);
  if (bits <= 10)
    {
      unsigned first;
      unsigned choices;
      uint8_t buf;

      random (ctx, sizeof(buf), &buf);

      first = prime_by_size[bits-3];
      choices = prime_by_size[bits-2] - first;
      
      mpz_set_ui (p, primes[first + buf % choices]);
    }
  else if (bits <= 20)
    {
      unsigned long highbit;
      uint8_t buf[3];
      unsigned long x;
      unsigned j;
      
      highbit = 1L << (bits - 1);

    again:
      random (ctx, sizeof(buf), buf);
      x = READ_UINT24(buf);
      x &= (highbit - 1);
      x |= highbit | 1;

      for (j = 0; prime_square[j] <= x; j++)
	{
	  unsigned q = x * trial_div_table[j].inverse & TRIAL_DIV_MASK;
	  if (q <= trial_div_table[j].limit)
	    goto again;
	}
      mpz_set_ui (p, x);
    }
  else
    {
      mpz_t q;

      mpz_init (q);

     /* Bit size ceil(k/2) + 1, slightly larger than used in Alg. 4.62
	in Handbook of Applied Cryptography (which seems to be
	incorrect for odd k). */
      nettle_random_prime (q, (bits+3)/2, ctx, random);

      _nettle_generate_pocklington_prime (p, bits, ctx, random,
					  q, NULL, q);
      
      mpz_clear (q);
    }
}