Re: [openpgp] "SHA-1 is a Shambles" and forging PGP WoT signatures

Michael Richardson <> Fri, 24 January 2020 17:00 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 7798A120AE8 for <>; Fri, 24 Jan 2020 09:00:54 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -4.2
X-Spam-Status: No, score=-4.2 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=unavailable autolearn_force=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id DaY2o38HsCdO for <>; Fri, 24 Jan 2020 09:00:52 -0800 (PST)
Received: from ( []) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 14F90120AE9 for <>; Fri, 24 Jan 2020 09:00:52 -0800 (PST)
Received: from ( [IPv6:2607:f0b0:f:2::247]) by (Postfix) with ESMTP id 18DA03897F; Fri, 24 Jan 2020 12:00:17 -0500 (EST)
Received: from localhost (localhost [IPv6:::1]) by (Postfix) with ESMTP id 2E21DB56; Fri, 24 Jan 2020 12:00:51 -0500 (EST)
From: Michael Richardson <>
To: Marcus Brinkmann <>
cc: IETF OpenPGP <>
In-Reply-To: <>
References: <> <> <>
X-Mailer: MH-E 8.6; nmh 1.7+dev; GNU Emacs 25.1.1
X-Face: $\n1pF)h^`}$H>Hk{L"x@)JS7<%Az}5RyS@k9X%29-lHB$Ti.V>2bi.~ehC0; <'$9xN5Ub# z!G,p`nR&p7Fz@^UXIn156S8.~^@MJ*mMsD7=QFeq%AL4m<nPbLgmtKK-5dC@#:k
MIME-Version: 1.0
Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature"
Date: Fri, 24 Jan 2020 12:00:51 -0500
Message-ID: <25922.1579885251@localhost>
Archived-At: <>
Subject: Re: [openpgp] "SHA-1 is a Shambles" and forging PGP WoT signatures
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "Ongoing discussion of OpenPGP issues." <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Fri, 24 Jan 2020 17:00:58 -0000

Marcus Brinkmann <> wrote:
    >> Does this mean, comparing a 20 bytes (40 hex digits) fingerprint, as
    >> printed by e.g. GnuPG 2.2.x, is no longer a reliable way to verify you
    >> have obtained the correct key?

    > The answer to this would formally be "yes", because after creating two
    > such keys, the attacker could first show you one key, and, later on show
    > you the other key and if the only thing you remember about the first key
    > is the fingerprint, you have no way to notice the swap.

Would the attacker have to control the private keys of both generated keys to
accomplish this?  I don't entirely see why.

Clearly the signatures generated by the two keys (with identical
fingerprints) would also be different (assume that the signatures were
calculated on a SHA256 hash, to remove an attack from that side).

    > The question if this is an actual problem (i.e.: violates a security
    > goal that the user is actually interested in) is more difficult to
    > answer and depends on many details.  Figuring this out would require a
    > careful review of OpenPGP implementations and applications using OpenPGP.

]               Never tell me the odds!                 | ipv6 mesh networks [
]   Michael Richardson, Sandelman Software Works        |    IoT architect   [
]        |   ruby on rails    [