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Re: [TLS] Diffie-Hellman: value of Z - the shared secret - without leading zero octets

Peter Gutmann <pgut001@cs.auckland.ac.nz> Tue, 29 March 2016 11:55 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Bill Cox <waywardgeek@google.com>, Maarten Bodewes <maarten.bodewes@gmail.com>
Thread-Topic: [TLS] Diffie-Hellman: value of Z - the shared secret - without leading zero octets
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Date: Tue, 29 Mar 2016 11:55:37 +0000
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Subject: Re: [TLS] Diffie-Hellman: value of Z - the shared secret - without leading zero octets
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Bill Cox <waywardgeek@google.com> writes:

>I spent 2 weeks last year tracking down a flaky bug that only occurs once in
>every 256 connection: the leading 0 byte was no longer being stripped in a
>code change I ported from OpenSSL master, and only Maria DB ran random tests
>enough to trigger this condition.

My self-test code includes 1K iterations of various PKC ops in mechanisms that
are sensitive to leading-zero truncation (PGP springs to mind) to detect this.
Without this, it's mostly blind luck as to whether your self-test hits the
1/256 one that triggers the problem.  Having to design coverage tests is hard
enough, but when you need to iterate them to find problems...

Peter.

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