Use constant time memcmp when comparing HMACs in openvpn_decrypt.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Signed-off-by: Gert Doering <gert@greenie.muc.de>

1 files changed, 19 insertions, 1 deletions

diff --git a/src/openvpn/crypto.c b/src/openvpn/crypto.c
index 405c0aa..d9adf5b 100644
--- a/src/openvpn/crypto.c
+++ b/src/openvpn/crypto.c
@@ -65,6 +65,24 @@
 #define CRYPT_ERROR(format) \
   do { msg (D_CRYPT_ERRORS, "%s: " format, error_prefix); goto error_exit; } while (false)
 
+/**
+ * As memcmp(), but constant-time.
+ * Returns 0 when data is equal, non-zero otherwise.
+ */
+static int
+memcmp_constant_time (const void *a, const void *b, size_t size) {
+  const uint8_t * a1 = a;
+  const uint8_t * b1 = b;
+  int ret = 0;
+  size_t i;
+
+  for (i = 0; i < size; i++) {
+      ret |= *a1++ ^ *b1++;
+  }
+
+  return ret;
+}
+
 void
 openvpn_encrypt (struct buffer *buf, struct buffer work,
 		 const struct crypto_options *opt,
@@ -244,7 +262,7 @@ openvpn_decrypt (struct buffer *buf, struct buffer work,
 	  hmac_ctx_final (ctx->hmac, local_hmac);
 
 	  /* Compare locally computed HMAC with packet HMAC */
-	  if (memcmp (local_hmac, BPTR (buf), hmac_len))
+	  if (memcmp_constant_time (local_hmac, BPTR (buf), hmac_len))
 	    CRYPT_ERROR ("packet HMAC authentication failed");
 
 	  ASSERT (buf_advance (buf, hmac_len));