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

Peter Gutmann <pgut001@cs.auckland.ac.nz> Wed, 12 February 2020 16:10 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: M K Saravanan <mksarav@gmail.com>, "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: [TLS] TLS 1.2 - is it allowed to strip the leading zero byte(s) in RSA signature in ServerKeyExchange?
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Date: Wed, 12 Feb 2020 16:10:00 +0000
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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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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.

Peter.