[TLS] External PSK with certificate-based authentication

Russ Housley <housley@vigilsec.com> Sat, 02 December 2017 18:10 UTC

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From: Russ Housley <housley@vigilsec.com>
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Subject: [TLS] External PSK with certificate-based authentication
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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.