IETF Logo Mail Archive

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

Return-Path: <davidben@google.com>
X-Original-To: tls@ietfa.amsl.com
Delivered-To: tls@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 03BD7120824 for <tls@ietfa.amsl.com>; Wed, 12 Feb 2020 09:12:24 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -9.249
X-Spam-Level:
X-Spam-Status: No, score=-9.249 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, HEADER_FROM_DIFFERENT_DOMAINS=0.25, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001, USER_IN_DEF_SPF_WL=-7.5] autolearn=no autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (1024-bit key) header.d=chromium.org
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id F-ySedpIRdey for <tls@ietfa.amsl.com>; Wed, 12 Feb 2020 09:12:22 -0800 (PST)
Received: from mail-pf1-x42b.google.com (mail-pf1-x42b.google.com [IPv6:2607:f8b0:4864:20::42b]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id BBADE120823 for <tls@ietf.org>; Wed, 12 Feb 2020 09:12:22 -0800 (PST)
Received: by mail-pf1-x42b.google.com with SMTP id y5so1532568pfb.11 for <tls@ietf.org>; Wed, 12 Feb 2020 09:12:22 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=YA9/ObPb8OxW8vHd31o8GdkzpeTi+ayM8Tn7oiK8QVw=; b=NIX6mA2ogeMxi/yW+NAF6WARXBb1sYzSLMDEX1QuxAk5Yk3cgem/osVWIMStywlgU3 R19DZ+jusOWzF6sFCau4bLSVh+UH5asBv/mV3cz0mr5LZAaTWynGM4SQabQu5i/IoQVG MuGbplfNmW1hMqGR8jTAhiROcAV+6MS4GW+ro=
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=YA9/ObPb8OxW8vHd31o8GdkzpeTi+ayM8Tn7oiK8QVw=; b=OMdA9v/Sj44TSVhu/v+8T9fVQn5elxPucKS01aEmaP7Noy9P+iConGQbBByFvv2vz4 PwjCUC852v6s6ZB0LM3+E3+qouxmnn5WugsK/904V8QF0LQAne1QSczA4kyelXA+mpLT MyhT/w0w2SavY9vnnjpAQGLNWA8LoLcnOsXEpCGufjZgi+IxFNX4GhSxMmN1EtjPRV2r K2OjdNKki13Rz+Jf4xL9dgNzKDrIA4aYH+CLiMDIzUCtWQze/3fgwwK0fdasvrnWdVoe dCf0MQTV9K6C8rPYehFw2yZBAEGRyZOh1Z5FhOlE9pvJEGoUUoym4ONKUZHPaLjeSf5u lK9g==
X-Gm-Message-State: APjAAAVvfnXFuMmRbCdF3ZS9AkLC4lJeGGXeYyoudq7UW6arKUkx5xTp jFssYS9qN24D9DZhmyl10LjSYn9cMVzqhzazQ2XT
X-Google-Smtp-Source: APXvYqxaa9okevL7cL5BLZVstDeimbBpQxuQ7Mw8vREfKTzlsFzGz8qgBGiAnULDOTa4o7l55yfW++WLV1JP3UA/qRs=
X-Received: by 2002:aa7:9a0b:: with SMTP id w11mr12697662pfj.4.1581527541951; Wed, 12 Feb 2020 09:12:21 -0800 (PST)
MIME-Version: 1.0
References: <CAG5P2e_yELKn_ypt2cVAHtoBeNVrpKLqLwuZprq0bm=h4odrHA@mail.gmail.com> <1581523805582.16466@cs.auckland.ac.nz>
In-Reply-To: <1581523805582.16466@cs.auckland.ac.nz>
From: David Benjamin <davidben@chromium.org>
Date: Wed, 12 Feb 2020 12:12:05 -0500
Message-ID: <CAF8qwaBkhurivCAWjaF6bNGCahfaS6e_y6qtR+oF+qSDFP0b2w@mail.gmail.com>
To: Peter Gutmann <pgut001@cs.auckland.ac.nz>
Cc: M K Saravanan <mksarav@gmail.com>, "<tls@ietf.org>" <tls@ietf.org>
Content-Type: multipart/alternative; boundary="00000000000057a4f8059e64130b"
Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/lhQ8r6TF9l6RdTpzDxqt1mp9eqI>
Subject: Re: [TLS] TLS 1.2 - is it allowed to strip the leading zero byte(s) in RSA signature in ServerKeyExchange?
X-BeenThere: tls@ietf.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "This is the mailing list for the Transport Layer Security working group of the IETF." <tls.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/tls>, <mailto:tls-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/tls/>
List-Post: <mailto:tls@ietf.org>
List-Help: <mailto:tls-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/tls>, <mailto:tls-request@ietf.org?subject=subscribe>
X-List-Received-Date: Wed, 12 Feb 2020 17:12:24 -0000

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

v2.23.0 | Report a Bug | By Email