Update DTLS code to match CBC decoding in TLS.

This change updates the DTLS code to match the constant-time CBC behaviour in the TLS.
openssl · Jan 28, 2013 · 9f27de1 · 9f27de1
1 parent 6cb19b7
commit 9f27de1
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Showing 5 changed files with 67 additions and 57 deletions.
diff --git a/ssl/d1_enc.c b/ssl/d1_enc.c
@@ -126,6 +126,14 @@
 #include <openssl/des.h>
 #endif
 
+/* dtls1_enc encrypts/decrypts the record in |s->wrec| / |s->rrec|, respectively.
+ *
+ * Returns:
+ *   0: (in non-constant time) if the record is publically invalid (i.e. too
+ *       short etc).
+ *   1: if the record's padding is valid / the encryption was successful.
+ *   -1: if the record's padding/AEAD-authenticator is invalid or, if sending,
+ *       an internal error occured. */
 int dtls1_enc(SSL *s, int send)
 	{
 	SSL3_RECORD *rec;
@@ -165,8 +173,7 @@ int dtls1_enc(SSL *s, int send)
 		if (EVP_MD_CTX_md(s->read_hash))
 			{
 			mac_size=EVP_MD_CTX_size(s->read_hash);
-			if (mac_size < 0)
-				return -1;
+			OPENSSL_assert(mac_size >= 0);
 			}
 		ds=s->enc_read_ctx;
 		rec= &(s->s3->rrec);
@@ -231,7 +238,7 @@ int dtls1_enc(SSL *s, int send)
 		if (!send)
 			{
 			if (l == 0 || l%bs != 0)
-				return -1;
+				return 0;
 			}
 
 		EVP_Cipher(ds,rec->data,rec->input,l);

diff --git a/ssl/d1_pkt.c b/ssl/d1_pkt.c
@@ -376,15 +376,11 @@ static int
 dtls1_process_record(SSL *s)
 {
 	int i,al;
-	int clear=0;
 	int enc_err;
 	SSL_SESSION *sess;
 	SSL3_RECORD *rr;
 	unsigned int mac_size;
 	unsigned char md[EVP_MAX_MD_SIZE];
-	int decryption_failed_or_bad_record_mac = 0;
-	unsigned char *mac = NULL;
-
 
 	rr= &(s->s3->rrec);
 	sess = s->session;
@@ -417,12 +413,16 @@ dtls1_process_record(SSL *s)
 	rr->orig_len=rr->length;
 
 	enc_err = s->method->ssl3_enc->enc(s,0);
-	if (enc_err <= 0)
+	/* enc_err is:
+	 *    0: (in non-constant time) if the record is publically invalid.
+	 *    1: if the padding is valid
+	 *    -1: if the padding is invalid */
+	if (enc_err == 0)
 		{
-		/* To minimize information leaked via timing, we will always
-		 * perform all computations before discarding the message.
-		 */
-		decryption_failed_or_bad_record_mac = 1;
+		/* For DTLS we simply ignore bad packets. */
+		rr->length = 0;
+		s->packet_length = 0;
+		goto err;
 		}
 
 #ifdef TLS_DEBUG
@@ -432,45 +432,59 @@ printf("\n");
 #endif
 
 	/* r->length is now the compressed data plus mac */
-	if (	(sess == NULL) ||
-		(s->enc_read_ctx == NULL) ||
-		(s->read_hash == NULL))
-		clear=1;
-
-	if (!clear)
+	if ((sess != NULL) &&
+	    (s->enc_read_ctx != NULL) &&
+	    (EVP_MD_CTX_md(s->read_hash) != NULL))
 		{
-		/* !clear => s->read_hash != NULL => mac_size != -1 */
-		int t;
-		t=EVP_MD_CTX_size(s->read_hash);
-		OPENSSL_assert(t >= 0);
-		mac_size=t;
-
-		if (rr->length > SSL3_RT_MAX_COMPRESSED_LENGTH+mac_size)
+		/* s->read_hash != NULL => mac_size != -1 */
+		unsigned char *mac = NULL;
+		unsigned char mac_tmp[EVP_MAX_MD_SIZE];
+		mac_size=EVP_MD_CTX_size(s->read_hash);
+		OPENSSL_assert(mac_size <= EVP_MAX_MD_SIZE);
+
+		/* orig_len is the length of the record before any padding was
+		 * removed. This is public information, as is the MAC in use,
+		 * therefore we can safely process the record in a different
+		 * amount of time if it's too short to possibly contain a MAC.
+		 */
+		if (rr->orig_len < mac_size ||
+		    /* CBC records must have a padding length byte too. */
+		    (EVP_CIPHER_CTX_mode(s->enc_read_ctx) == EVP_CIPH_CBC_MODE &&
+		     rr->orig_len < mac_size+1))
 			{
-#if 0 /* OK only for stream ciphers (then rr->length is visible from ciphertext anyway) */
-			al=SSL_AD_RECORD_OVERFLOW;
-			SSLerr(SSL_F_DTLS1_PROCESS_RECORD,SSL_R_PRE_MAC_LENGTH_TOO_LONG);
+			al=SSL_AD_DECODE_ERROR;
+			SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_LENGTH_TOO_SHORT);
 			goto f_err;
-#else
-			decryption_failed_or_bad_record_mac = 1;
-#endif			
 			}
-		/* check the MAC for rr->input (it's in mac_size bytes at the tail) */
-		if (rr->length >= mac_size)
+
+		if (EVP_CIPHER_CTX_mode(s->enc_read_ctx) == EVP_CIPH_CBC_MODE)
 			{
+			/* We update the length so that the TLS header bytes
+			 * can be constructed correctly but we need to extract
+			 * the MAC in constant time from within the record,
+			 * without leaking the contents of the padding bytes.
+			 * */
+			mac = mac_tmp;
+			ssl3_cbc_copy_mac(mac_tmp, rr, mac_size);
 			rr->length -= mac_size;
-			mac = &rr->data[rr->length];
 			}
 		else
-			rr->length = 0;
-		i=s->method->ssl3_enc->mac(s,md,0);
-		if (i < 0 || mac == NULL || CRYPTO_memcmp(md,mac,mac_size) != 0)
 			{
-			decryption_failed_or_bad_record_mac = 1;
+			/* In this case there's no padding, so |rec->orig_len|
+			 * equals |rec->length| and we checked that there's
+			 * enough bytes for |mac_size| above. */
+			rr->length -= mac_size;
+			mac = &rr->data[rr->length];
 			}
+
+		i=s->method->ssl3_enc->mac(s,md,0 /* not send */);
+		if (i < 0 || mac == NULL || CRYPTO_memcmp(md, mac, (size_t)mac_size) != 0)
+			enc_err = -1;
+		if (rr->length > SSL3_RT_MAX_COMPRESSED_LENGTH+mac_size)
+			enc_err = -1;
 		}
 
-	if (decryption_failed_or_bad_record_mac)
+	if (enc_err < 0)
 		{
 		/* decryption failed, silently discard message */
 		rr->length = 0;

diff --git a/ssl/s3_enc.c b/ssl/s3_enc.c
@@ -531,11 +531,7 @@ int ssl3_enc(SSL *s, int send)
 		if (!send)
 			{
 			if (l == 0 || l%bs != 0)
-				{
-				SSLerr(SSL_F_SSL3_ENC,SSL_R_BLOCK_CIPHER_PAD_IS_WRONG);
-				ssl3_send_alert(s,SSL3_AL_FATAL,SSL_AD_DECRYPTION_FAILED);
 				return 0;
-				}
 			/* otherwise, rec->length >= bs */
 			}
 

diff --git a/ssl/s3_pkt.c b/ssl/s3_pkt.c
@@ -291,7 +291,6 @@ static int ssl3_get_record(SSL *s)
 	unsigned char md[EVP_MAX_MD_SIZE];
 	short version;
 	unsigned mac_size;
-	int clear=0;
 	size_t extra;
 
 	rr= &(s->s3->rrec);
@@ -408,8 +407,9 @@ fprintf(stderr, "Record type=%d, Length=%d\n", rr->type, rr->length);
 	 *    -1: if the padding is invalid */
 	if (enc_err == 0)
 		{
-		/* SSLerr() and ssl3_send_alert() have been called */
-		goto err;
+		al=SSL_AD_DECRYPTION_FAILED;
+		SSLerr(SSL_F_TLS1_ENC,SSL_R_BLOCK_CIPHER_PAD_IS_WRONG);
+		goto f_err;
 		}
 
 #ifdef TLS_DEBUG
@@ -419,14 +419,11 @@ printf("\n");
 #endif
 
 	/* r->length is now the compressed data plus mac */
-	if (	(sess == NULL) ||
-		(s->enc_read_ctx == NULL) ||
-		(EVP_MD_CTX_md(s->read_hash) == NULL))
-		clear=1;
-
-	if (!clear)
+	if ((sess != NULL) &&
+	    (s->enc_read_ctx != NULL) &&
+	    (EVP_MD_CTX_md(s->read_hash) != NULL))
 		{
-		/* !clear => s->read_hash != NULL => mac_size != -1 */
+		/* s->read_hash != NULL => mac_size != -1 */
 		unsigned char *mac = NULL;
 		unsigned char mac_tmp[EVP_MAX_MD_SIZE];
 		mac_size=EVP_MD_CTX_size(s->read_hash);

diff --git a/ssl/t1_enc.c b/ssl/t1_enc.c
@@ -825,11 +825,7 @@ int tls1_enc(SSL *s, int send)
 		if (!send)
 			{
 			if (l == 0 || l%bs != 0)
-				{
-				SSLerr(SSL_F_TLS1_ENC,SSL_R_BLOCK_CIPHER_PAD_IS_WRONG);
-				ssl3_send_alert(s,SSL3_AL_FATAL,SSL_AD_DECRYPTION_FAILED);
 				return 0;
-				}
 			}
 
 		i = EVP_Cipher(ds,rec->data,rec->input,l);