mp_prime_rand.c

   1 #include "tommath_private.h"
   2 #ifdef MP_PRIME_RAND_C
   3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
   4 /* SPDX-License-Identifier: Unlicense */
   5
   6 /* makes a truly random prime of a given size (bits),
   7  *
   8  * Flags are as follows:
   9  *
  10  *   MP_PRIME_BBS      - make prime congruent to 3 mod 4
  11  *   MP_PRIME_SAFE     - make sure (p-1)/2 is prime as well (implies MP_PRIME_BBS)
  12  *   MP_PRIME_2MSB_ON  - make the 2nd highest bit one
  13  *
  14  * You have to supply a callback which fills in a buffer with random bytes.  "dat" is a parameter you can
  15  * have passed to the callback (e.g. a state or something).  This function doesn't use "dat" itself
  16  * so it can be NULL
  17  *
  18  */
  19
  20 /* This is possibly the mother of all prime generation functions, muahahahahaha! */
  21 mp_err mp_prime_rand(mp_int *a, int t, int size, int flags)
  22 {
  23    uint8_t *tmp, maskAND, maskOR_msb, maskOR_lsb;
  24    int bsize, maskOR_msb_offset;
  25    bool res;
  26    mp_err err;
  27
  28    /* sanity check the input */
  29    if (size <= 1) {
  30       return MP_VAL;
  31    }
  32
  33    /* MP_PRIME_SAFE implies MP_PRIME_BBS */
  34    if ((flags & MP_PRIME_SAFE) != 0) {
  35       flags |= MP_PRIME_BBS;
  36    }
  37
  38    /* calc the byte size */
  39    bsize = (size>>3) + ((size&7)?1:0);
  40
  41    /* we need a buffer of bsize bytes */
  42    tmp = (uint8_t *) MP_MALLOC((size_t)bsize);
  43    if (tmp == NULL) {
  44       return MP_MEM;
  45    }
  46
  47    /* calc the maskAND value for the MSbyte*/
  48    maskAND = ((size&7) == 0) ? 0xFFu : (uint8_t)(0xFFu >> (8 - (size & 7)));
  49
  50    /* calc the maskOR_msb */
  51    maskOR_msb        = 0;
  52    maskOR_msb_offset = ((size & 7) == 1) ? 1 : 0;
  53    if ((flags & MP_PRIME_2MSB_ON) != 0) {
  54       maskOR_msb       |= (uint8_t)(0x80 >> ((9 - size) & 7));
  55    }
  56
  57    /* get the maskOR_lsb */
  58    maskOR_lsb         = 1u;
  59    if ((flags & MP_PRIME_BBS) != 0) {
  60       maskOR_lsb     |= 3u;
  61    }
  62
  63    do {
  64       /* read the bytes */
  65       if ((err = s_mp_rand_source(tmp, (size_t)bsize)) != MP_OKAY) {
  66          goto LBL_ERR;
  67       }
  68
  69       /* work over the MSbyte */
  70       tmp[0]    &= maskAND;
  71       tmp[0]    |= (uint8_t)(1 << ((size - 1) & 7));
  72
  73       /* mix in the maskORs */
  74       tmp[maskOR_msb_offset]   |= maskOR_msb;
  75       tmp[bsize-1]             |= maskOR_lsb;
  76
  77       /* read it in */
  78       /* TODO: casting only for now until all lengths have been changed to the type "size_t"*/
  79       if ((err = mp_from_ubin(a, tmp, (size_t)bsize)) != MP_OKAY) {
  80          goto LBL_ERR;
  81       }
  82
  83       /* is it prime? */
  84       if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY) {
  85          goto LBL_ERR;
  86       }
  87       if (!res) {
  88          continue;
  89       }
  90
  91       if ((flags & MP_PRIME_SAFE) != 0) {
  92          /* see if (a-1)/2 is prime */
  93          if ((err = mp_sub_d(a, 1uL, a)) != MP_OKAY) {
  94             goto LBL_ERR;
  95          }
  96          if ((err = mp_div_2(a, a)) != MP_OKAY) {
  97             goto LBL_ERR;
  98          }
  99
 100          /* is it prime? */
 101          if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY) {
 102             goto LBL_ERR;
 103          }
 104       }
 105    } while (!res);
 106
 107    if ((flags & MP_PRIME_SAFE) != 0) {
 108       /* restore a to the original value */
 109       if ((err = mp_mul_2(a, a)) != MP_OKAY) {
 110          goto LBL_ERR;
 111       }
 112       if ((err = mp_add_d(a, 1uL, a)) != MP_OKAY) {
 113          goto LBL_ERR;
 114       }
 115    }
 116
 117    err = MP_OKAY;
 118 LBL_ERR:
 119    MP_FREE_BUF(tmp, (size_t)bsize);
 120    return err;
 121 }
 122
 123 #endif