On Thu, Dec 05, 2019 at 05:30:10PM +0000, Daniel Van Geest wrote:
> Hi all,
>
> I think there might be ambiguity or an interoperability issue with the TLS
> 1.3 ServerHello Random value downgrade protection and some (possibly?)
> legitimate negotiation of TLS 1.2 using the supported_versions extension.
> Looking through RFC 8446 and the mail archives I don’t see anything that
> addresses this, maybe I’ve missed something?
>
> RFC 8446 Section 4.1.3:
>
> TLS 1.3 has a downgrade protection mechanism embedded in the server's
> random value. TLS 1.3 servers which negotiate TLS 1.2 or below in
> response to a ClientHello MUST set the last 8 bytes of their Random
> value specially in their ServerHello.
>
> If negotiating TLS 1.2, TLS 1.3 servers MUST set the last 8 bytes of
> their Random value to the bytes:
>
> 44 4F 57 4E 47 52 44 01
>
> If negotiating TLS 1.1 or below, TLS 1.3 servers MUST, and TLS 1.2
> servers SHOULD, set the last 8 bytes of their ServerHello.Random
> value to the bytes:
>
> 44 4F 57 4E 47 52 44 00
>
> TLS 1.3 clients receiving a ServerHello indicating TLS 1.2 or below
> MUST check that the last 8 bytes are not equal to either of these
> values. TLS 1.2 clients SHOULD also check that the last 8 bytes are
> not equal to the second value if the ServerHello indicates TLS 1.1 or
> below. If a match is found, the client MUST abort the handshake with
> an "illegal_parameter" alert.
>
> So whenever a TLS 1.3-capable server negotiates a TLS 1.2 or lower session it
> MUST set the appropriate Random values.
>
> And a TLS 1.3 client MUST check for these values and abort if found. Future
> TLS versions aren’t mentioned so perhaps one of the use cases I’ll mention
> below is not a concern until the next version is defined and the behavior can
> be adjusted then.
>
> Consider the potential cases for the ClientHello supported_versions values:
>
>
> 1. supported_versions = { 0x0305 (TLS 1.4), 0x0303 (TLS 1.2) }
> Okay, TLS 1.4 doesn’t exist so maybe this can be addressed in the future. If
> the server supports TLS 1.2 and TLS 1.3, it will negotiate TLS 1.2 and add
> the TLS 1.2 downgrade protection Random value. The Client will see the TLS
> 1.2 version, check the downgrade protection and abort the connection. But
> this is not a downgrade issue, this is the only mutually acceptable TLS
> version.
This is analogous to the case of wanting to support TLS 1.0 and 1.2 but not
1.1, which did (IIRC) get a little bit of discussion including from Andrei
Popov. My recollection is that we didn't really feel a need to support "gaps"
in supported versions for this downgrade-protection scheme.
> 2. supported_versions = { 0x0303 (TLS 1.2), 0x0304 (TLS 1.3) }
> The supported_versions list is in order of client preference, but it’s not
> required that the server respect the preference. I’ve seen implementations
> which respect the client’s preference and ones which pick the highest
> mutually acceptable version. If a server supports TLS 1.2 and TLS 1.3 and
> respects the client’s preference it will negotiate TLS 1.2 and add the
> downgrade protection random value. The client would then abort the
> connection even though this would seem to be a legitimate non-downgrade
> situation Or can we wave our hands at this and say if a client doesn’t
> prefer TLS 1.3 above TLS 1.2 then it’s not really a true Scotsman TLS 1.3
> client and those MUSTs don’t really apply to it?.
I had forgotten that we implied a potential for client preference of supported
versions. We could perhaps debate whether it's "legitimate" for a server to
prefer 1.2 over 1.3, but I think it's clear that (assuming the negotiation is
protected by a full transcript hash, as in 1.3 or 1.2 with extended master
secret) a server negotiating 1.2 in this case should not apply the
downgrade-protection scheme.
-Ben
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