Re: [TLS] TLS 1.2 - is it allowed to strip the leading zero byte(s) in RSA signature in ServerKeyExchange?

David Benjamin <davidben@chromium.org> Wed, 12 February 2020 17:12 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Wed, 12 Feb 2020 12:12:05 -0500
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To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
Cc: M K Saravanan <mksarav@gmail.com>, "<tls@ietf.org>" <tls@ietf.org>
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Subject: Re: [TLS] TLS 1.2 - is it allowed to strip the leading zero byte(s) in RSA signature in ServerKeyExchange?
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On Wed, Feb 12, 2020 at 11:10 AM Peter Gutmann <pgut001@cs.auckland.ac.nz>;
wrote:

> M K Saravanan <mksarav@gmail.com>; writes:
>
> >Is this allowed?  i.e. stripping the leading zero of the RSA signature and
> >marking the length as 255?   It is not clear to me from the RFC5246
> whether
> >it is allowed or not.
>
> It's not allowed according to the spec but a number of implementations do
> it
> because their underlying bignum libraries perform leading-zero truncation,
> so
> you're better off allowing it to avoid breakage.
>

For web use cases, this does not appear to be necessary. BoringSSL and
Chrome do not accept such signatures and have not for around five years
now. (Possibly longer. I do not know off-hand what Chrome's behavior was
when it used NSS.) I don't think I've ever seen a report of problems with a
website, and the specification quite clearly says to reject those
signatures. The robustness principle sounds plausible at face value, but I
think we now have the experience to know otherwise.
https://tools.ietf.org/html/draft-iab-protocol-maintenance-04

Note that bignum libraries that perform leading-zero truncation are
unlikely to be suitable for cryptography anyway. Signatures are public
values, but if you're implementing RSA decryption and care about side
channel attacks, fixed-width in-memory representations and serialization
functions are mandatory.
http://archiv.infsec.ethz.ch/education/fs08/secsem/Manger01.pdf