--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_prime_rand.c	Tue May 26 17:36:47 2020 +0200
@@ -0,0 +1,141 @@
+#include "tommath_private.h"
+#ifdef BN_MP_PRIME_RAND_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* makes a truly random prime of a given size (bits),
+ *
+ * Flags are as follows:
+ *
+ *   MP_PRIME_BBS      - make prime congruent to 3 mod 4
+ *   MP_PRIME_SAFE     - make sure (p-1)/2 is prime as well (implies MP_PRIME_BBS)
+ *   MP_PRIME_2MSB_ON  - make the 2nd highest bit one
+ *
+ * You have to supply a callback which fills in a buffer with random bytes.  "dat" is a parameter you can
+ * have passed to the callback (e.g. a state or something).  This function doesn't use "dat" itself
+ * so it can be NULL
+ *
+ */
+
+/* This is possibly the mother of all prime generation functions, muahahahahaha! */
+mp_err s_mp_prime_random_ex(mp_int *a, int t, int size, int flags, private_mp_prime_callback cb, void *dat)
+{
+   unsigned char *tmp, maskAND, maskOR_msb, maskOR_lsb;
+   int bsize, maskOR_msb_offset;
+   mp_bool res;
+   mp_err err;
+
+   /* sanity check the input */
+   if ((size <= 1) || (t <= 0)) {
+      return MP_VAL;
+   }
+
+   /* MP_PRIME_SAFE implies MP_PRIME_BBS */
+   if ((flags & MP_PRIME_SAFE) != 0) {
+      flags |= MP_PRIME_BBS;
+   }
+
+   /* calc the byte size */
+   bsize = (size>>3) + ((size&7)?1:0);
+
+   /* we need a buffer of bsize bytes */
+   tmp = (unsigned char *) MP_MALLOC((size_t)bsize);
+   if (tmp == NULL) {
+      return MP_MEM;
+   }
+
+   /* calc the maskAND value for the MSbyte*/
+   maskAND = ((size&7) == 0) ? 0xFFu : (unsigned char)(0xFFu >> (8 - (size & 7)));
+
+   /* calc the maskOR_msb */
+   maskOR_msb        = 0;
+   maskOR_msb_offset = ((size & 7) == 1) ? 1 : 0;
+   if ((flags & MP_PRIME_2MSB_ON) != 0) {
+      maskOR_msb       |= (unsigned char)(0x80 >> ((9 - size) & 7));
+   }
+
+   /* get the maskOR_lsb */
+   maskOR_lsb         = 1u;
+   if ((flags & MP_PRIME_BBS) != 0) {
+      maskOR_lsb     |= 3u;
+   }
+
+   do {
+      /* read the bytes */
+      if (cb(tmp, bsize, dat) != bsize) {
+         err = MP_VAL;
+         goto error;
+      }
+
+      /* work over the MSbyte */
+      tmp[0]    &= maskAND;
+      tmp[0]    |= (unsigned char)(1 << ((size - 1) & 7));
+
+      /* mix in the maskORs */
+      tmp[maskOR_msb_offset]   |= maskOR_msb;
+      tmp[bsize-1]             |= maskOR_lsb;
+
+      /* read it in */
+      /* TODO: casting only for now until all lengths have been changed to the type "size_t"*/
+      if ((err = mp_from_ubin(a, tmp, (size_t)bsize)) != MP_OKAY) {
+         goto error;
+      }
+
+      /* is it prime? */
+      if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY) {
+         goto error;
+      }
+      if (res == MP_NO) {
+         continue;
+      }
+
+      if ((flags & MP_PRIME_SAFE) != 0) {
+         /* see if (a-1)/2 is prime */
+         if ((err = mp_sub_d(a, 1uL, a)) != MP_OKAY) {
+            goto error;
+         }
+         if ((err = mp_div_2(a, a)) != MP_OKAY) {
+            goto error;
+         }
+
+         /* is it prime? */
+         if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY) {
+            goto error;
+         }
+      }
+   } while (res == MP_NO);
+
+   if ((flags & MP_PRIME_SAFE) != 0) {
+      /* restore a to the original value */
+      if ((err = mp_mul_2(a, a)) != MP_OKAY) {
+         goto error;
+      }
+      if ((err = mp_add_d(a, 1uL, a)) != MP_OKAY) {
+         goto error;
+      }
+   }
+
+   err = MP_OKAY;
+error:
+   MP_FREE_BUFFER(tmp, (size_t)bsize);
+   return err;
+}
+
+static int s_mp_rand_cb(unsigned char *dst, int len, void *dat)
+{
+   (void)dat;
+   if (len <= 0) {
+      return len;
+   }
+   if (s_mp_rand_source(dst, (size_t)len) != MP_OKAY) {
+      return 0;
+   }
+   return len;
+}
+
+mp_err mp_prime_rand(mp_int *a, int t, int size, int flags)
+{
+   return s_mp_prime_random_ex(a, t, size, flags, s_mp_rand_cb, NULL);
+}
+
+#endif