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

Bill Cox <waywardgeek@google.com> Mon, 28 March 2016 16:07 UTC

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Date: Mon, 28 Mar 2016 09:07:09 -0700
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From: Bill Cox <waywardgeek@google.com>
To: Maarten Bodewes <maarten.bodewes@gmail.com>
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Subject: Re: [TLS] Diffie-Hellman: value of Z - the shared secret - without leading zero octets
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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.

I guess I would prefer that this particular weird goober in TLS would go
away, but it would be incompatible.

Bill

On Mon, Mar 28, 2016 at 8:56 AM, Maarten Bodewes <maarten.bodewes@gmail.com>
wrote:

> Hi all,
>
> I see that the leading zero is stripped off of the value of Z (the shared
> secret) before it is used as input to HKDF. This seems to be compatible
> with TLS 1.2. Then again, it is not compatible with e.g. NISP800-56A which
> uses the value of Z with the same size of the prime in octets. Furthermore,
> it is also different with regards to handling the coordinate X as used in
> ECDH.
>
> Was this a conscious decision to keep compatibility with TLS? Has the use
> of the value of Z including zero octets been considered?
>
> Regards,
> Maarten
>
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