--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/bn_mp_prime_next_prime.c	Mon May 31 18:23:46 2004 +0000
@@ -0,0 +1,164 @@
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+ *
+ * LibTomMath is a library that provides multiple-precision
+ * integer arithmetic as well as number theoretic functionality.
+ *
+ * The library was designed directly after the MPI library by
+ * Michael Fromberger but has been written from scratch with
+ * additional optimizations in place.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, [email protected], http://math.libtomcrypt.org
+ */
+#include <tommath.h>
+
+/* finds the next prime after the number "a" using "t" trials
+ * of Miller-Rabin.
+ *
+ * bbs_style = 1 means the prime must be congruent to 3 mod 4
+ */
+int mp_prime_next_prime(mp_int *a, int t, int bbs_style)
+{
+   int      err, res, x, y;
+   mp_digit res_tab[PRIME_SIZE], step, kstep;
+   mp_int   b;
+
+   /* ensure t is valid */
+   if (t <= 0 || t > PRIME_SIZE) {
+      return MP_VAL;
+   }
+
+   /* force positive */
+   a->sign = MP_ZPOS;
+
+   /* simple algo if a is less than the largest prime in the table */
+   if (mp_cmp_d(a, __prime_tab[PRIME_SIZE-1]) == MP_LT) {
+      /* find which prime it is bigger than */
+      for (x = PRIME_SIZE - 2; x >= 0; x--) {
+          if (mp_cmp_d(a, __prime_tab[x]) != MP_LT) {
+             if (bbs_style == 1) {
+                /* ok we found a prime smaller or
+                 * equal [so the next is larger]
+                 *
+                 * however, the prime must be
+                 * congruent to 3 mod 4
+                 */
+                if ((__prime_tab[x + 1] & 3) != 3) {
+                   /* scan upwards for a prime congruent to 3 mod 4 */
+                   for (y = x + 1; y < PRIME_SIZE; y++) {
+                       if ((__prime_tab[y] & 3) == 3) {
+                          mp_set(a, __prime_tab[y]);
+                          return MP_OKAY;
+                       }
+                   }
+                }
+             } else {
+                mp_set(a, __prime_tab[x + 1]);
+                return MP_OKAY;
+             }
+          }
+      }
+      /* at this point a maybe 1 */
+      if (mp_cmp_d(a, 1) == MP_EQ) {
+         mp_set(a, 2);
+         return MP_OKAY;
+      }
+      /* fall through to the sieve */
+   }
+
+   /* generate a prime congruent to 3 mod 4 or 1/3 mod 4? */
+   if (bbs_style == 1) {
+      kstep   = 4;
+   } else {
+      kstep   = 2;
+   }
+
+   /* at this point we will use a combination of a sieve and Miller-Rabin */
+
+   if (bbs_style == 1) {
+      /* if a mod 4 != 3 subtract the correct value to make it so */
+      if ((a->dp[0] & 3) != 3) {
+         if ((err = mp_sub_d(a, (a->dp[0] & 3) + 1, a)) != MP_OKAY) { return err; };
+      }
+   } else {
+      if (mp_iseven(a) == 1) {
+         /* force odd */
+         if ((err = mp_sub_d(a, 1, a)) != MP_OKAY) {
+            return err;
+         }
+      }
+   }
+
+   /* generate the restable */
+   for (x = 1; x < PRIME_SIZE; x++) {
+      if ((err = mp_mod_d(a, __prime_tab[x], res_tab + x)) != MP_OKAY) {
+         return err;
+      }
+   }
+
+   /* init temp used for Miller-Rabin Testing */
+   if ((err = mp_init(&b)) != MP_OKAY) {
+      return err;
+   }
+
+   for (;;) {
+      /* skip to the next non-trivially divisible candidate */
+      step = 0;
+      do {
+         /* y == 1 if any residue was zero [e.g. cannot be prime] */
+         y     =  0;
+
+         /* increase step to next candidate */
+         step += kstep;
+
+         /* compute the new residue without using division */
+         for (x = 1; x < PRIME_SIZE; x++) {
+             /* add the step to each residue */
+             res_tab[x] += kstep;
+
+             /* subtract the modulus [instead of using division] */
+             if (res_tab[x] >= __prime_tab[x]) {
+                res_tab[x]  -= __prime_tab[x];
+             }
+
+             /* set flag if zero */
+             if (res_tab[x] == 0) {
+                y = 1;
+             }
+         }
+      } while (y == 1 && step < ((((mp_digit)1)<<DIGIT_BIT) - kstep));
+
+      /* add the step */
+      if ((err = mp_add_d(a, step, a)) != MP_OKAY) {
+         goto __ERR;
+      }
+
+      /* if didn't pass sieve and step == MAX then skip test */
+      if (y == 1 && step >= ((((mp_digit)1)<<DIGIT_BIT) - kstep)) {
+         continue;
+      }
+
+      /* is this prime? */
+      for (x = 0; x < t; x++) {
+          mp_set(&b, __prime_tab[t]);
+          if ((err = mp_prime_miller_rabin(a, &b, &res)) != MP_OKAY) {
+             goto __ERR;
+          }
+          if (res == MP_NO) {
+             break;
+          }
+      }
+
+      if (res == MP_YES) {
+         break;
+      }
+   }
+
+   err = MP_OKAY;
+__ERR:
+   mp_clear(&b);
+   return err;
+}
+