[gnutls-devel] Bug: gnutls_dh_get_group prepends a zero byte to prime

[Peter Williams]

Simply add comment that the value is output as an x409 integer in ber encoding (and perhaps der encoding).

In type theory, one can compare properly (according to the encoding).

When comparing certs (properly, as types), one needs formalities such as this. Historically, upon receipt of client cert during association binding, one issues a directory compare operation which, she executed by the resolver, would compare the value against the value(s) in the named directory record, to test existence of the operand in the "trust list".

Obviously a security critical operation, for which one uses formal type theory which, for certs was a little over formalized due to the any typing of the dh block.

All good fodder for structured vulnerability insertion in standards, of course, in the name of extensibility.

Sent from my Windows Phone
________________________________
From: Nikos Mavrogiannopoulos<mailto:nmav at gnutls.org>
Sent: ‎5/‎22/‎2015 12:06 AM
To: Thomas Klute<mailto:thomas2.klute at uni-dortmund.de>
Cc: bugs at gnutls.org<mailto:bugs at gnutls.org>
Subject: Re: [gnutls-devel] Bug: gnutls_dh_get_group prepends a zero byte to prime

On Fri, May 22, 2015 at 12:17 AM, Thomas Klute
<thomas2.klute at uni-dortmund.de> wrote:
> Hello,
> I believe I have found a bug in gnutls_dh_get_group: It returns the
> prime with an extra zero byte at the beginning.

Indeed, I see that the number is written as a non-negative integer
there, so it will have a leading zero if the number would have been
interpreted as negative. The intention was to assist applications who
may import that value in an mpz_t value. Would it make sense to
document the fact that there may be a leading zero in that case,
rather than eliminating? That behavior has been for quite some time
and I believe any users of this function would already use or work
around it.

regards,
Nikos

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