Make CBC decoding constant time.

This patch makes the decoding of SSLv3 and TLS CBC records constant
time. Without this, a timing side-channel can be used to build a padding
oracle and mount Vaudenay's attack.

This patch also disables the stitched AESNI+SHA mode pending a similar
fix to that code.

In order to be easy to backport, this change is implemented in ssl/,
rather than as a generic AEAD mode. In the future this should be changed
around so that HMAC isn't in ssl/, but crypto/ as FIPS expects.

1 files changed, 7 insertions, 42 deletions

diff --git a/ssl/d1_enc.c b/ssl/d1_enc.c
index 07a5e97ce5..c13b495a08 100644
--- a/ssl/d1_enc.c
+++ b/ssl/d1_enc.c
@@ -131,15 +131,15 @@ int dtls1_enc(SSL *s, int send)
 	SSL3_RECORD *rec;
 	EVP_CIPHER_CTX *ds;
 	unsigned long l;
-	int bs,i,ii,j,k,n=0;
+	int bs,i,j,k,mac_size=0;
 	const EVP_CIPHER *enc;
 
 	if (send)
 		{
 		if (EVP_MD_CTX_md(s->write_hash))
 			{
-			n=EVP_MD_CTX_size(s->write_hash);
-			if (n < 0)
+			mac_size=EVP_MD_CTX_size(s->write_hash);
+			if (mac_size < 0)
 				return -1;
 			}
 		ds=s->enc_write_ctx;
@@ -164,8 +164,8 @@ int dtls1_enc(SSL *s, int send)
 		{
 		if (EVP_MD_CTX_md(s->read_hash))
 			{
-			n=EVP_MD_CTX_size(s->read_hash);
-			if (n < 0)
+			mac_size=EVP_MD_CTX_size(s->read_hash);
+			if (mac_size < 0)
 				return -1;
 			}
 		ds=s->enc_read_ctx;
@@ -245,44 +245,9 @@ int dtls1_enc(SSL *s, int send)
                 }
 #endif	/* KSSL_DEBUG */
 
+		rec->orig_len = rec->length;
 		if ((bs != 1) && !send)
-			{
-			ii=i=rec->data[l-1]; /* padding_length */
-			i++;
-			if (s->options&SSL_OP_TLS_BLOCK_PADDING_BUG)
-				{
-				/* First packet is even in size, so check */
-				if ((memcmp(s->s3->read_sequence,
-					"\0\0\0\0\0\0\0\0",8) == 0) && !(ii & 1))
-					s->s3->flags|=TLS1_FLAGS_TLS_PADDING_BUG;
-				if (s->s3->flags & TLS1_FLAGS_TLS_PADDING_BUG)
-					i--;
-				}
-			/* TLS 1.0 does not bound the number of padding bytes by the block size.
-			 * All of them must have value 'padding_length'. */
-			if (i + bs > (int)rec->length)
-				{
-				/* Incorrect padding. SSLerr() and ssl3_alert are done
-				 * by caller: we don't want to reveal whether this is
-				 * a decryption error or a MAC verification failure
-				 * (see http://www.openssl.org/~bodo/tls-cbc.txt) 
-				 */
-				return -1;
-				}
-			for (j=(int)(l-i); j<(int)l; j++)
-				{
-				if (rec->data[j] != ii)
-					{
-					/* Incorrect padding */
-					return -1;
-					}
-				}
-			rec->length-=i;
-
-			rec->data += bs;    /* skip the implicit IV */
-			rec->input += bs;
-			rec->length -= bs;
-			}
+			return tls1_cbc_remove_padding(s, rec, bs, mac_size);
 		}
 	return(1);
 	}