Couple of weeks ago Microsoft released an update to the SSL/TLS stack to implement secure renegotiation as described in RFC 5746. The Microsoft KB article describes the three settings controlling the behavior of the patch, but a bit more detail can be useful.

A bit of background first. TLS extensions are a method of extending the TLS protocol without having to change the specification of the core protocol and are described in RFC 4366. It is defined as arbitrary extra data that can be appended to the ClientHello and/or ServerHello messages (which are the first messages sent by each side). Servers are supposed to ignore data following the ClientHello if they don’t understand it.

Since the TLS extensions were not a formal RFC in the past, some server implementations were written to fail requests which have more data following the ClientHello message, which makes them non-interoperable with clients that send TLS extensions. This is the precise reason why RFC 5746 has adopted the idea of Signaling Cipher Suite Value (SCSV) to avoid breaking interoperability with servers not accepting TLS extensions. The recommended approach, though, is to use the TLS extension defined by the RFC.

Now here are the important details. By default, any version of Windows prior to Vista did not send TLS extensions when using the TLSv1.0 protocol. With the new update, this has changed and if TLSv1.0 is enabled, then the renegotiation indication extension will be sent as part of the TLS handshake, as recommended. So the small set of servers (no one that I know of knows the actual percentage of such servers) which do not tolerate this behavior will cause interoperability problems. This is where the UseScsvForTls setting described in the Microsoft KB comes in. Setting the registry key to non-zero value will cause the SSL/TLS stack to generate TLS ClientHello messages containing SCSV and without extensions, so interoperability with such servers can be restored. As far as the other two keys, AllowInsecureRenegoClients and AllowInsecureRenegoServers, they control the compatible vs strict mode and will not make any difference on the structure of the messages on the wire. The only effect is whether communication is allowed to continue with unpatched party or not.