Re: [TLS] External PSK with certificate-based authentication

Russ Housley <housley@vigilsec.com> Thu, 07 December 2017 15:35 UTC

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From: Russ Housley <housley@vigilsec.com>
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Date: Thu, 7 Dec 2017 10:34:59 -0500
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To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] External PSK with certificate-based authentication
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> On Dec 2, 2017, at 1:51 PM, Eric Rescorla <ekr@rtfm.com> wrote:
> 
> On Sat, Dec 2, 2017 at 10:10 AM, Russ Housley <housley@vigilsec.com <mailto:housley@vigilsec.com>> wrote:
> At the bottom of page 136, the current draft says:
> 
>    Note: TLS does not currently permit the server to send a
>    certificate_request message in non-certificate-based handshakes
>    (e.g., PSK).  If this restriction were to be relaxed in future, the
>    client's signature would not cover the server's certificate directly.
>    However, if the PSK was established through a NewSessionTicket, the
>    client's signature would transitively cover the server's certificate
>    through the PSK binder.  [PSK-FINISHED] describes a concrete attack
>    on constructions that do not bind to the server's certificate (see
>    also [Kraw16]).  It is unsafe to use certificate-based client
>    authentication when the client might potentially share the same PSK/
>    key-id pair with two different endpoints.  Implementations MUST NOT
>    combine external PSKs with certificate-based authentication of either
>    the client or the server.
> 
> [PSK-FINISHED] tells why it is not safe to do client authentication after resumption.
> 
> [Kraw16] says two things: (1) using a PSK from a previous handshake and adding client authentication is not secure; and (2)does not work; and the client signature must cover the public key.
> 
> So, the final sentence in the quoted paragraph seems to be too broad.  I do not see why we forbid an external PSK and certificate-based authentication in an initial handshake.  I acknowledge that TLS 1.3 does not support it, but I have been expecting an extension to be specified to do just that once the TLS 1.3 base specification is finished.
> 
> My view on this is that that's not a currently specified configuration. A future specification could of course relax that. If you wanted to submit a PR that said "absent some extension to the contrary" that would be fine, though I think that's implicit.

Please see https://github.com/tlswg/tls13-spec/pull/1117 <https://github.com/tlswg/tls13-spec/pull/1117>

Russ