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Re: [TLS] raising ceiling vs. floor (was: New Version Notification for draft-moriarty-tls-oldversions-diediedie-00.txt)

Peter Gutmann <pgut001@cs.auckland.ac.nz> Tue, 10 July 2018 16:38 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: David Benjamin <davidben@chromium.org>
CC: Hubert Kario <hkario@redhat.com>, Eric Rescorla <ekr@rtfm.com>, "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: [TLS] raising ceiling vs. floor (was: New Version Notification for draft-moriarty-tls-oldversions-diediedie-00.txt)
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Date: Tue, 10 Jul 2018 16:38:22 +0000
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References: <152934875755.3094.4484881874912460528.idtracker@ietfa.amsl.com> <3161014.mNqxEOqjoE@pintsize.usersys.redhat.com> <CABcZeBP2tZRe96dj6BvCyHZkSyNh+RBt7H2dzO8vsXkQUb+inw@mail.gmail.com> <2926046.DTQ3PP0lUg@pintsize.usersys.redhat.com> <1531237487290.30133@cs.auckland.ac.nz>, <CAF8qwaCAaVzKKccLQKOvgSpPQe+2M8+jDdtJVy5oQM1+56G_2A@mail.gmail.com>
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Subject: Re: [TLS] raising ceiling vs. floor (was: New Version Notification for draft-moriarty-tls-oldversions-diediedie-00.txt)
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David Benjamin <davidben@chromium.org> writes:

>EMS does not fix the ServerKeyExchange signature payload. It's still just the
>randoms and not the full transcript.

Maybe we're talking about different things here, EMS hashes the full
transcript, for 1.0 and 1.1 with the dual SHA-1 and MD5 hash, for 1.2 with
whatever's negotiated, hopefully SHA-2 (even if SHA-1 is used, you've now got
two hashes you need to defeat simultaneously, not one).  So while the
ServerKeyExchange signature may not detect an attacker able to compromise
SHA-1 in real time (and that statement alone should tell you how feasible the
attack actually is), the later EMS will.

Peter.

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