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

Eric Rescorla <ekr@rtfm.com> Sat, 02 December 2017 18:52 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sat, 2 Dec 2017 10:51:45 -0800
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To: Russ Housley <housley@vigilsec.com>
Cc: IETF TLS <tls@ietf.org>
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Subject: Re: [TLS] External PSK with certificate-based authentication
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On Sat, Dec 2, 2017 at 10:10 AM, Russ Housley <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.

-Ekr



Russ
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