--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rsa_exptmod.c	Mon May 31 18:25:41 2004 +0000
@@ -0,0 +1,87 @@
+/* LibTomCrypt, modular cryptographic library -- Tom St Denis
+ *
+ * LibTomCrypt is a library that provides various cryptographic
+ * algorithms in a highly modular and flexible manner.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, [email protected], http://libtomcrypt.org
+ */
+
+/* RSA Code by Tom St Denis */
+#include "mycrypt.h"
+
+#ifdef MRSA
+
+int rsa_exptmod(const unsigned char *in,  unsigned long inlen,
+                      unsigned char *out, unsigned long *outlen, int which,
+                      rsa_key *key)
+{
+   mp_int tmp, tmpa, tmpb;
+   unsigned long x;
+   int err;
+
+   _ARGCHK(in     != NULL);
+   _ARGCHK(out    != NULL);
+   _ARGCHK(outlen != NULL);
+   _ARGCHK(key    != NULL);
+
+   if (which == PK_PRIVATE && (key->type != PK_PRIVATE && key->type != PK_PRIVATE_OPTIMIZED)) {
+      return CRYPT_PK_NOT_PRIVATE;
+   }
+
+   /* must be a private or public operation */
+   if (which != PK_PRIVATE && which != PK_PUBLIC) {
+      return CRYPT_PK_INVALID_TYPE;
+   }
+
+   /* init and copy into tmp */
+   if ((err = mp_init_multi(&tmp, &tmpa, &tmpb, NULL)) != MP_OKAY)                     { goto error; }
+   if ((err = mp_read_unsigned_bin(&tmp, (unsigned char *)in, (int)inlen)) != MP_OKAY) { goto error; }
+
+   /* sanity check on the input */
+   if (mp_cmp(&key->N, &tmp) == MP_LT) {
+      err = CRYPT_PK_INVALID_SIZE;
+      goto done;
+   }
+
+   /* are we using the private exponent and is the key optimized? */
+   if (which == PK_PRIVATE && key->type == PK_PRIVATE_OPTIMIZED) {
+      /* tmpa = tmp^dP mod p */
+      if ((err = mp_exptmod(&tmp, &key->dP, &key->p, &tmpa)) != MP_OKAY)    { goto error; }
+
+      /* tmpb = tmp^dQ mod q */
+      if ((err = mp_exptmod(&tmp, &key->dQ, &key->q, &tmpb)) != MP_OKAY)    { goto error; }
+
+      /* tmp = tmpa*qP + tmpb*pQ mod N */
+      if ((err = mp_mul(&tmpa, &key->qP, &tmpa)) != MP_OKAY)                { goto error; }
+      if ((err = mp_mul(&tmpb, &key->pQ, &tmpb)) != MP_OKAY)                { goto error; }
+      if ((err = mp_addmod(&tmpa, &tmpb, &key->N, &tmp)) != MP_OKAY)        { goto error; }
+   } else {
+      /* exptmod it */
+      if ((err = mp_exptmod(&tmp, which==PK_PRIVATE?&key->d:&key->e, &key->N, &tmp)) != MP_OKAY) { goto error; }
+   }
+
+   /* read it back */
+   x = (unsigned long)mp_unsigned_bin_size(&tmp);
+   if (x > *outlen) {
+      err = CRYPT_BUFFER_OVERFLOW;
+      goto done;
+   }
+   *outlen = x;
+
+   /* convert it */
+   if ((err = mp_to_unsigned_bin(&tmp, out)) != MP_OKAY)                    { goto error; }
+
+   /* clean up and return */
+   err = CRYPT_OK;
+   goto done;
+error:
+   err = mpi_to_ltc_error(err);
+done:
+   mp_clear_multi(&tmp, &tmpa, &tmpb, NULL);
+   return err;
+}
+
+#endif