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Re: [TLS] Triple Handshake Fix.

Geoffrey Keating <geoffk@geoffk.org> Tue, 29 April 2014 23:16 UTC

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Subject: Re: [TLS] Triple Handshake Fix.
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Adam Langley <agl@google.com> writes:

> [2] https://secure-resumption.com/draft-bhargavan-tls-session-hash-00.txt

I read this and couldn't work out how it interacted with
renegotiation.  I'm surprised there's no discussion of renegotiation
at all in section 3.  Does 'handshake_messages' include:

a) Only the first handshake; or

b) Only the last handshake; or

c) All messages from all handshakes, up to the Client Key Exchange
message in each handshake; or

d) All messages from all handshakes, up to the Client Key Exchange
message in the last handshake, up to the Finished message in previous
handshakes, including any HelloRequest messages; or

e) Like d but without HelloRequest messages; or

f) Other?

I'm fairly sure a and b lead to security vulnerabilities.  I'm not
sure whether c does so or not.

d and e present some challenges for implementations with TLS 1.2,
because you could renegotiate to a different hash algorithm, so you
need to keep the contents of the handshake(s), or all possible partial
hashes, for the duration of the connection.

I also note that the draft leaves out the (last?) 'certificate verify'
message, is that intentional?  Are there any security consequences?

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