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      1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
      2  *
      3  * LibTomCrypt is a library that provides various cryptographic
      4  * algorithms in a highly modular and flexible manner.
      5  *
      6  * The library is free for all purposes without any express
      7  * guarantee it works.
      8  *
      9  * Tom St Denis, tomstdenis (at) gmail.com, http://libtomcrypt.com
     10  */
     11 #include "tomcrypt.h"
     12 
     13 /**
     14   @file rand_prime.c
     15   Generate a random prime, Tom St Denis
     16 */
     17 
     18 #define USE_BBS 1
     19 
     20 int rand_prime(void *N, long len, prng_state *prng, int wprng)
     21 {
     22    int            err, res, type;
     23    unsigned char *buf;
     24 
     25    LTC_ARGCHK(N != NULL);
     26 
     27    /* get type */
     28    if (len < 0) {
     29       type = USE_BBS;
     30       len = -len;
     31    } else {
     32       type = 0;
     33    }
     34 
     35    /* allow sizes between 2 and 512 bytes for a prime size */
     36    if (len < 2 || len > 512) {
     37       return CRYPT_INVALID_PRIME_SIZE;
     38    }
     39 
     40    /* valid PRNG? Better be! */
     41    if ((err = prng_is_valid(wprng)) != CRYPT_OK) {
     42       return err;
     43    }
     44 
     45    /* allocate buffer to work with */
     46    buf = XCALLOC(1, len);
     47    if (buf == NULL) {
     48        return CRYPT_MEM;
     49    }
     50 
     51    do {
     52       /* generate value */
     53       if (prng_descriptor[wprng].read(buf, len, prng) != (unsigned long)len) {
     54          XFREE(buf);
     55          return CRYPT_ERROR_READPRNG;
     56       }
     57 
     58       /* munge bits */
     59       buf[0]     |= 0x80 | 0x40;
     60       buf[len-1] |= 0x01 | ((type & USE_BBS) ? 0x02 : 0x00);
     61 
     62       /* load value */
     63       if ((err = mp_read_unsigned_bin(N, buf, len)) != CRYPT_OK) {
     64          XFREE(buf);
     65          return err;
     66       }
     67 
     68       /* test */
     69       if ((err = mp_prime_is_prime(N, 8, &res)) != CRYPT_OK) {
     70          XFREE(buf);
     71          return err;
     72       }
     73    } while (res == LTC_MP_NO);
     74 
     75 #ifdef LTC_CLEAN_STACK
     76    zeromem(buf, len);
     77 #endif
     78 
     79    XFREE(buf);
     80    return CRYPT_OK;
     81 }
     82 
     83 
     84 
     85 /* $Source: /cvs/libtom/libtomcrypt/src/math/rand_prime.c,v $ */
     86 /* $Revision: 1.6 $ */
     87 /* $Date: 2006/03/31 14:15:35 $ */
     88