### diff libtommath/bn_mp_prime_fermat.c @ 389:5ff8218bcee9

author Matt Johnston Thu, 11 Jan 2007 03:14:55 +0000 eed26cff980b 60fc6476e044
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```--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_prime_fermat.c	Thu Jan 11 03:14:55 2007 +0000
@@ -0,0 +1,62 @@
+#include <tommath.h>
+#ifdef BN_MP_PRIME_FERMAT_C
+/* 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.com
+ */
+
+/* performs one Fermat test.
+ *
+ * If "a" were prime then b**a == b (mod a) since the order of
+ * the multiplicative sub-group would be phi(a) = a-1.  That means
+ * it would be the same as b**(a mod (a-1)) == b**1 == b (mod a).
+ *
+ * Sets result to 1 if the congruence holds, or zero otherwise.
+ */
+int mp_prime_fermat (mp_int * a, mp_int * b, int *result)
+{
+  mp_int  t;
+  int     err;
+
+  /* default to composite  */
+  *result = MP_NO;
+
+  /* ensure b > 1 */
+  if (mp_cmp_d(b, 1) != MP_GT) {
+     return MP_VAL;
+  }
+
+  /* init t */
+  if ((err = mp_init (&t)) != MP_OKAY) {
+    return err;
+  }
+
+  /* compute t = b**a mod a */
+  if ((err = mp_exptmod (b, a, a, &t)) != MP_OKAY) {
+    goto LBL_T;
+  }
+
+  /* is it equal to b? */
+  if (mp_cmp (&t, b) == MP_EQ) {
+    *result = MP_YES;
+  }
+
+  err = MP_OKAY;
+LBL_T:mp_clear (&t);
+  return err;
+}
+#endif
+
+/* \$Source: /cvs/libtom/libtommath/bn_mp_prime_fermat.c,v \$ */
+/* \$Revision: 1.3 \$ */
+/* \$Date: 2006/03/31 14:18:44 \$ */```