patches/fallback_scsv.patch

diff --git a/third_party/tlslite/tlslite/constants.py b/third_party/tlslite/tlslite/constants.py
index 8720de6..69e6067 100644
--- a/third_party/tlslite/tlslite/constants.py
+++ b/third_party/tlslite/tlslite/constants.py
@@ -107,6 +107,7 @@ class AlertDescription:
     protocol_version = 70
     insufficient_security = 71
     internal_error = 80
+    inappropriate_fallback = 86
     user_canceled = 90
     no_renegotiation = 100
     unknown_psk_identity = 115
@@ -118,6 +119,9 @@ class CipherSuite:
     # We actually don't do any renegotiation, but this
     # prevents renegotiation attacks
     TLS_EMPTY_RENEGOTIATION_INFO_SCSV = 0x00FF
+
+    # draft-ietf-tls-downgrade-scsv-03
+    TLS_FALLBACK_SCSV = 0x5600
     
     TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA  = 0xC01A
     TLS_SRP_SHA_WITH_AES_128_CBC_SHA = 0xC01D
diff --git a/third_party/tlslite/tlslite/errors.py b/third_party/tlslite/tlslite/errors.py
index 22c298c..001ef33 100644
--- a/third_party/tlslite/tlslite/errors.py
+++ b/third_party/tlslite/tlslite/errors.py
@@ -63,6 +63,7 @@ class TLSAlert(TLSError):
         AlertDescription.protocol_version: "protocol_version",\
         AlertDescription.insufficient_security: "insufficient_security",\
         AlertDescription.internal_error: "internal_error",\
+        AlertDescription.inappropriate_fallback: "inappropriate_fallback",\
         AlertDescription.user_canceled: "user_canceled",\
         AlertDescription.no_renegotiation: "no_renegotiation",\
         AlertDescription.unknown_psk_identity: "unknown_psk_identity"}
diff --git a/third_party/tlslite/tlslite/tlsconnection.py b/third_party/tlslite/tlslite/tlsconnection.py
index 4dedc5f..0563fb5f 100644
--- a/third_party/tlslite/tlslite/tlsconnection.py
+++ b/third_party/tlslite/tlslite/tlsconnection.py
@@ -969,7 +969,8 @@ class TLSConnection(TLSRecordLayer):
                         reqCAs = None, 
                         tacks=None, activationFlags=0,
                         nextProtos=None, anon=False,
-                        tlsIntolerant=None, signedCertTimestamps=None):
+                        tlsIntolerant=None, signedCertTimestamps=None,
+                        fallbackSCSV=False):
         """Perform a handshake in the role of server.
 
         This function performs an SSL or TLS handshake.  Depending on
@@ -1048,6 +1049,11 @@ class TLSConnection(TLSRecordLayer):
         binary 8-bit string) that will be sent as a TLS extension whenever
         the client announces support for the extension.
 
+        @type fallbackSCSV: bool
+        @param fallbackSCSV: if true, the server will implement
+        TLS_FALLBACK_SCSV and thus reject connections using less than the
+        server's maximum TLS version that include this cipher suite.
+
         @raise socket.error: If a socket error occurs.
         @raise tlslite.errors.TLSAbruptCloseError: If the socket is closed
         without a preceding alert.
@@ -1060,7 +1066,8 @@ class TLSConnection(TLSRecordLayer):
                 checker, reqCAs, 
                 tacks=tacks, activationFlags=activationFlags, 
                 nextProtos=nextProtos, anon=anon, tlsIntolerant=tlsIntolerant,
-                signedCertTimestamps=signedCertTimestamps):
+                signedCertTimestamps=signedCertTimestamps,
+                fallbackSCSV=fallbackSCSV):
             pass
 
 
@@ -1071,7 +1078,8 @@ class TLSConnection(TLSRecordLayer):
                              tacks=None, activationFlags=0,
                              nextProtos=None, anon=False,
                              tlsIntolerant=None,
-                             signedCertTimestamps=None
+                             signedCertTimestamps=None,
+                             fallbackSCSV=False
                              ):
         """Start a server handshake operation on the TLS connection.
 
@@ -1092,7 +1100,8 @@ class TLSConnection(TLSRecordLayer):
             tacks=tacks, activationFlags=activationFlags, 
             nextProtos=nextProtos, anon=anon,
             tlsIntolerant=tlsIntolerant,
-            signedCertTimestamps=signedCertTimestamps)
+            signedCertTimestamps=signedCertTimestamps,
+            fallbackSCSV=fallbackSCSV)
         for result in self._handshakeWrapperAsync(handshaker, checker):
             yield result
 
@@ -1102,7 +1111,7 @@ class TLSConnection(TLSRecordLayer):
                              settings, reqCAs, 
                              tacks, activationFlags, 
                              nextProtos, anon,
-                             tlsIntolerant, signedCertTimestamps):
+                             tlsIntolerant, signedCertTimestamps, fallbackSCSV):
 
         self._handshakeStart(client=False)
 
@@ -1137,7 +1146,7 @@ class TLSConnection(TLSRecordLayer):
         # Handle ClientHello and resumption
         for result in self._serverGetClientHello(settings, certChain,\
                                             verifierDB, sessionCache,
-                                            anon, tlsIntolerant):
+                                            anon, tlsIntolerant, fallbackSCSV):
             if result in (0,1): yield result
             elif result == None:
                 self._handshakeDone(resumed=True)                
@@ -1237,7 +1246,7 @@ class TLSConnection(TLSRecordLayer):
 
 
     def _serverGetClientHello(self, settings, certChain, verifierDB,
-                                sessionCache, anon, tlsIntolerant):
+                                sessionCache, anon, tlsIntolerant, fallbackSCSV):
         #Initialize acceptable cipher suites
         cipherSuites = []
         if verifierDB:
@@ -1283,6 +1292,14 @@ class TLSConnection(TLSRecordLayer):
         elif clientHello.client_version > settings.maxVersion:
             self.version = settings.maxVersion
 
+        #Detect if the client performed an inappropriate fallback.
+        elif fallbackSCSV and clientHello.client_version < settings.maxVersion:
+            self.version = clientHello.client_version
+            if CipherSuite.TLS_FALLBACK_SCSV in clientHello.cipher_suites:
+                for result in self._sendError(\
+                        AlertDescription.inappropriate_fallback):
+                    yield result
+
         else:
             #Set the version to the client's version
             self.version = clientHello.client_version