Implement FALLBACK_SCSV client-side
diff --git a/include/polarssl/config.h b/include/polarssl/config.h
index 50b4e33..ff0ccec 100644
--- a/include/polarssl/config.h
+++ b/include/polarssl/config.h
@@ -812,6 +812,23 @@
//#define POLARSSL_SSL_DEBUG_ALL
/**
+ * \def POLARSSL_SSL_FALLBACK_SCSV
+ *
+ * Enable support for FALLBACK_SCSV (draft-ietf-tls-downgrade-scsv-00).
+ *
+ * For servers, it is recommended to always enable this, unless you support
+ * only one version of TLS, or know for sure that none of your clients
+ * implements a fallback strategy.
+ *
+ * For clients, you only need this if you're using a fallback strategy, which
+ * is not recommended in the first place, unless you absolutely need it to
+ * interoperate with buggy (version-intolerant) servers.
+ *
+ * Comment this macro to disable support for FALLBACK_SCSV
+ */
+#define POLARSSL_SSL_FALLBACK_SCSV
+
+/**
* \def POLARSSL_SSL_HW_RECORD_ACCEL
*
* Enable hooking functions in SSL module for hardware acceleration of
diff --git a/include/polarssl/ssl.h b/include/polarssl/ssl.h
index 194e944..a36e742 100644
--- a/include/polarssl/ssl.h
+++ b/include/polarssl/ssl.h
@@ -206,6 +206,9 @@
#define SSL_IS_CLIENT 0
#define SSL_IS_SERVER 1
+#define SSL_IS_NOT_FALLBACK 0
+#define SSL_IS_FALLBACK 1
+
#define SSL_COMPRESS_NULL 0
#define SSL_COMPRESS_DEFLATE 1
@@ -308,6 +311,7 @@
* Signaling ciphersuite values (SCSV)
*/
#define SSL_EMPTY_RENEGOTIATION_INFO 0xFF /**< renegotiation info ext */
+#define SSL_FALLBACK_SCSV 0x5600 /**< draft-ietf-tls-downgrade-scsv-00 */
/*
* Supported Signature and Hash algorithms (For TLS 1.2)
@@ -697,6 +701,10 @@
int min_major_ver; /*!< min. major version used */
int min_minor_ver; /*!< min. minor version used */
+#if defined(POLARSSL_SSL_FALLBACK_SCSV) && defined(POLARSSL_SSL_CLI_C)
+ char fallback; /*!< flag for fallback connections */
+#endif
+
/*
* Callbacks (RNG, debug, I/O, verification)
*/
@@ -1367,7 +1375,6 @@
*/
void ssl_set_max_version( ssl_context *ssl, int major, int minor );
-
/**
* \brief Set the minimum accepted SSL/TLS protocol version
* (Default: SSL_MIN_MAJOR_VERSION, SSL_MIN_MINOR_VERSION)
@@ -1383,6 +1390,29 @@
*/
void ssl_set_min_version( ssl_context *ssl, int major, int minor );
+#if defined(POLARSSL_SSL_FALLBACK_SCSV) && defined(POLARSSL_SSL_CLI_C)
+/**
+ * \brief Set the fallback flag (client-side only).
+ * (Default: SSL_IS_NOT_FALLBACK).
+ *
+ * \note Set to SSL_IS_FALLBACK when preparing a fallback
+ * connection, that is a connection with max_version set to a
+ * lower value than the value you're willing to use. Such
+ * fallback connections are not recommended but are sometimes
+ * necessary to interoperate with buggy (version-intolerant)
+ * servers.
+ *
+ * \warning You should NOT set this to SSL_IS_FALLBACK for
+ * non-fallback connections! This would appear to work for a
+ * while, then cause failures when the server is upgraded to
+ * support a newer TLS version.
+ *
+ * \param ssl SSL context
+ * \param fallback SSL_IS_NOT_FALLBACK or SSL_IS_FALLBACK
+ */
+void ssl_set_fallback( ssl_context *ssl, char fallback );
+#endif /* POLARSSL_SSL_FALLBACK_SCSV && POLARSSL_SSL_CLI_C */
+
#if defined(POLARSSL_SSL_MAX_FRAGMENT_LENGTH)
/**
* \brief Set the maximum fragment length to emit and/or negotiate
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 27abb3e..50ab05f 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -596,6 +596,17 @@
*p++ = (unsigned char)( ciphersuites[i] );
}
+ /* Some versions of OpenSSL don't handle it correctly if not at end */
+#if defined(POLARSSL_SSL_FALLBACK_SCSV)
+ if( ssl->fallback == SSL_IS_FALLBACK )
+ {
+ SSL_DEBUG_MSG( 3, ( "adding FALLBACK_SCSV" ) );
+ *p++ = (unsigned char)( SSL_FALLBACK_SCSV >> 8 );
+ *p++ = (unsigned char)( SSL_FALLBACK_SCSV );
+ n++;
+ }
+#endif
+
*q++ = (unsigned char)( n >> 7 );
*q++ = (unsigned char)( n << 1 );
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 5f080de..bd4494f 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3977,6 +3977,13 @@
}
}
+#if defined(POLARSSL_SSL_FALLBACK_SCSV) && defined(POLARSSL_SSL_CLI_C)
+void ssl_set_fallback( ssl_context *ssl, char fallback )
+{
+ ssl->fallback = fallback;
+}
+#endif
+
#if defined(POLARSSL_SSL_MAX_FRAGMENT_LENGTH)
int ssl_set_max_frag_len( ssl_context *ssl, unsigned char mfl_code )
{
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index 5b7a488..8ca83eb 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -95,6 +95,7 @@
#define DFL_RECO_DELAY 0
#define DFL_TICKETS SSL_SESSION_TICKETS_ENABLED
#define DFL_ALPN_STRING NULL
+#define DFL_FALLBACK -1
#define GET_REQUEST "GET %s HTTP/1.0\r\nExtra-header: "
#define GET_REQUEST_END "\r\n\r\n"
@@ -132,6 +133,7 @@
int reco_delay; /* delay in seconds before resuming session */
int tickets; /* enable / disable session tickets */
const char *alpn_string; /* ALPN supported protocols */
+ int fallback; /* is this a fallback connection? */
} opt;
static void my_debug( void *ctx, int level, const char *str )
@@ -284,6 +286,13 @@
#define USAGE_ALPN ""
#endif /* POLARSSL_SSL_ALPN */
+#if defined(POLARSSL_SSL_FALLBACK_SCSV)
+#define USAGE_FALLBACK \
+ " fallback=0/1 default: (library default: off)\n"
+#else
+#define USAGE_FALLBACK ""
+#endif
+
#define USAGE \
"\n usage: ssl_client2 param=<>...\n" \
"\n acceptable parameters:\n" \
@@ -313,6 +322,7 @@
USAGE_MAX_FRAG_LEN \
USAGE_TRUNC_HMAC \
USAGE_ALPN \
+ USAGE_FALLBACK \
"\n" \
" min_version=%%s default: \"\" (ssl3)\n" \
" max_version=%%s default: \"\" (tls1_2)\n" \
@@ -413,6 +423,7 @@
opt.reco_delay = DFL_RECO_DELAY;
opt.tickets = DFL_TICKETS;
opt.alpn_string = DFL_ALPN_STRING;
+ opt.fallback = DFL_FALLBACK;
for( i = 1; i < argc; i++ )
{
@@ -519,6 +530,15 @@
{
opt.alpn_string = q;
}
+ else if( strcmp( p, "fallback" ) == 0 )
+ {
+ switch( atoi( q ) )
+ {
+ case 0: opt.fallback = SSL_IS_NOT_FALLBACK; break;
+ case 1: opt.fallback = SSL_IS_FALLBACK; break;
+ default: goto usage;
+ }
+ }
else if( strcmp( p, "min_version" ) == 0 )
{
if( strcmp( q, "ssl3" ) == 0 )
@@ -952,6 +972,10 @@
ssl_set_min_version( &ssl, SSL_MAJOR_VERSION_3, opt.min_version );
if( opt.max_version != -1 )
ssl_set_max_version( &ssl, SSL_MAJOR_VERSION_3, opt.max_version );
+#if defined(POLARSSL_SSL_FALLBACK_SCSV)
+ if( opt.fallback != DFL_FALLBACK )
+ ssl_set_fallback( &ssl, opt.fallback );
+#endif
/*
* 4. Handshake
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 4eafed4..ab7793a 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -80,6 +80,21 @@
fi
}
+# skip next test if OpenSSL doesn't support FALLBACK_SCSV
+requires_openssl_with_fallback_scsv() {
+ if [ -z "${OPENSSL_HAS_FBSCSV:-}" ]; then
+ if $OPENSSL_CMD s_client -help 2>&1 | grep fallback_scsv >/dev/null
+ then
+ OPENSSL_HAS_FBSCSV="YES"
+ else
+ OPENSSL_HAS_FBSCSV="NO"
+ fi
+ fi
+ if [ "$OPENSSL_HAS_FBSCSV" = "NO" ]; then
+ SKIP_NEXT="YES"
+ fi
+}
+
# skip next test if GnuTLS isn't available
requires_gnutls() {
if [ -z "${GNUTLS_AVAILABLE:-}" ]; then
@@ -425,6 +440,45 @@
0 \
-s "dumping 'computed mac' (10 bytes)"
+# Tests for FALLBACK_SCSV
+
+run_test "Fallback SCSV: default" \
+ "$P_SRV" \
+ "$P_CLI debug_level=3 force_version=tls1_1" \
+ 0 \
+ -C "adding FALLBACK_SCSV" \
+ -C "is a fatal alert message (msg 86)"
+
+run_test "Fallback SCSV: explicitly disabled" \
+ "$P_SRV" \
+ "$P_CLI debug_level=3 force_version=tls1_1 fallback=0" \
+ 0 \
+ -C "adding FALLBACK_SCSV" \
+ -C "is a fatal alert message (msg 86)"
+
+run_test "Fallback SCSV: enabled" \
+ "$P_SRV" \
+ "$P_CLI debug_level=3 force_version=tls1_1 fallback=1" \
+ 0 \
+ -c "adding FALLBACK_SCSV" \
+ -C "is a fatal alert message (msg 86)"
+
+requires_openssl_with_fallback_scsv
+run_test "Fallback SCSV: default, openssl server" \
+ "$O_SRV" \
+ "$P_CLI debug_level=3 force_version=tls1_1 fallback=0" \
+ 0 \
+ -C "adding FALLBACK_SCSV" \
+ -C "is a fatal alert message (msg 86)"
+
+requires_openssl_with_fallback_scsv
+run_test "Fallback SCSV: enabled, openssl server" \
+ "$O_SRV" \
+ "$P_CLI debug_level=3 force_version=tls1_1 fallback=1" \
+ 1 \
+ -c "adding FALLBACK_SCSV" \
+ -c "is a fatal alert message (msg 86)"
+
# Tests for Session Tickets
run_test "Session resume using tickets: basic" \