add crypto_cipher_rewind to reverse crypto_cipher_advance

svn:r957

src/common/crypto.c

@@ -807,6 +807,12 @@ int crypto_cipher_decrypt(crypto_cipher_env_t *env, unsigned char *from, unsigne
   }
 }
 
+int
+crypto_cipher_rewind(crypto_cipher_env_t *env, long delta)
+{
+  return crypto_cipher_advance(env, -delta);
+}
+
 int
 crypto_cipher_advance(crypto_cipher_env_t *env, long delta)
 {
@@ -841,8 +847,8 @@ crypto_new_digest_env(int type)
 
 void
 crypto_free_digest_env(crypto_digest_env_t *digest) {
-  assert(digest);
-  tor_free(digest);
+  if(digest)
+    free(digest);
 }
 
 void

src/common/crypto.h

@@ -92,6 +92,7 @@ int crypto_cipher_encrypt(crypto_cipher_env_t *env, unsigned char *from, unsigne
 int crypto_cipher_decrypt(crypto_cipher_env_t *env, unsigned char *from, unsigned int fromlen, unsigned char *to);
 
 /* only implemented for CRYPTO_CIPHER_AES_CTR */
+int crypto_cipher_rewind(crypto_cipher_env_t *env, long delta);
 int crypto_cipher_advance(crypto_cipher_env_t *env, long delta);
 
 /* convenience function: wraps crypto_create_crypto_env, set_key, set_iv, and init. */