upstream: fix all-zero check in kexc25519_shared_key

from markus@ ok djm@

OpenBSD-Commit-ID: 60b1d364e0d9d34d1d1ef1620cb92e36cf06712d

1 files changed, 7 insertions, 5 deletions

diff --git a/kexc25519.c b/kexc25519.c
index 712dd523d..acddcab37 100644
--- a/kexc25519.c
+++ b/kexc25519.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexc25519.c,v 1.11 2018/12/27 03:25:25 djm Exp $ */
+/* $OpenBSD: kexc25519.c,v 1.12 2019/01/21 09:49:37 djm Exp $ */
 /*
  * Copyright (c) 2001, 2013 Markus Friedl.  All rights reserved.
  * Copyright (c) 2010 Damien Miller.  All rights reserved.
@@ -64,14 +64,16 @@ kexc25519_shared_key(const u_char key[CURVE25519_SIZE],
     const u_char pub[CURVE25519_SIZE], struct sshbuf *out)
 {
         u_char shared_key[CURVE25519_SIZE];
+        u_char zero[CURVE25519_SIZE];
         int r;
 
-        /* Check for all-zero public key */
-        explicit_bzero(shared_key, CURVE25519_SIZE);
-        if (timingsafe_bcmp(pub, shared_key, CURVE25519_SIZE) == 0)
+        crypto_scalarmult_curve25519(shared_key, key, pub);
+
+        /* Check for all-zero shared secret */
+        explicit_bzero(zero, CURVE25519_SIZE);
+        if (timingsafe_bcmp(zero, shared_key, CURVE25519_SIZE) == 0)
                 return SSH_ERR_KEY_INVALID_EC_VALUE;
 
-        crypto_scalarmult_curve25519(shared_key, key, pub);
 #ifdef DEBUG_KEXECDH
         dump_digest("shared secret", shared_key, CURVE25519_SIZE);
 #endif