--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtomcrypt/src/pk/dh/dh_shared_secret.c	Fri Feb 09 21:44:05 2018 +0800
@@ -0,0 +1,80 @@
+/* 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.
+ */
+
+#include "tomcrypt.h"
+
+#ifdef LTC_MDH
+
+/**
+   Create a DH shared secret.
+   @param private_key     The private DH key in the pair
+   @param public_key      The public DH key in the pair
+   @param out             [out] The destination of the shared data
+   @param outlen          [in/out] The max size and resulting size of the shared data.
+   @return CRYPT_OK if successful
+*/
+int dh_shared_secret(dh_key *private_key, dh_key *public_key,
+                     unsigned char *out, unsigned long *outlen)
+{
+   void *tmp;
+   unsigned long x;
+   int err;
+
+   LTC_ARGCHK(private_key != NULL);
+   LTC_ARGCHK(public_key  != NULL);
+   LTC_ARGCHK(out         != NULL);
+   LTC_ARGCHK(outlen      != NULL);
+
+   /* types valid? */
+   if (private_key->type != PK_PRIVATE) {
+      return CRYPT_PK_NOT_PRIVATE;
+   }
+
+   /* same DH group? */
+   if (mp_cmp(private_key->prime, public_key->prime) != LTC_MP_EQ) { return CRYPT_PK_TYPE_MISMATCH; }
+   if (mp_cmp(private_key->base, public_key->base) != LTC_MP_EQ)   { return CRYPT_PK_TYPE_MISMATCH; }
+
+   /* init big numbers */
+   if ((err = mp_init(&tmp)) != CRYPT_OK) {
+      return err;
+   }
+
+   /* check public key */
+   if ((err = dh_check_pubkey(public_key)) != CRYPT_OK) {
+      goto error;
+   }
+
+   /* compute tmp = y^x mod p */
+   if ((err = mp_exptmod(public_key->y, private_key->x, private_key->prime, tmp)) != CRYPT_OK)  {
+      goto error;
+   }
+
+   /* enough space for output? */
+   x = (unsigned long)mp_unsigned_bin_size(tmp);
+   if (*outlen < x) {
+      *outlen = x;
+      err = CRYPT_BUFFER_OVERFLOW;
+      goto error;
+   }
+   if ((err = mp_to_unsigned_bin(tmp, out)) != CRYPT_OK) {
+      goto error;
+   }
+   *outlen = x;
+   err = CRYPT_OK;
+
+error:
+   mp_clear(tmp);
+   return err;
+}
+
+#endif /* LTC_MDH */
+
+/* ref:         $Format:%D$ */
+/* git commit:  $Format:%H$ */
+/* commit time: $Format:%ai$ */