Re: [jose] Secdir review of draft-ietf-jose-json-web-signature-31

Richard Barnes <> Mon, 22 September 2014 01:31 UTC

Return-Path: <>
Received: from localhost ( []) by (Postfix) with ESMTP id 5FEE91A03F9 for <>; Sun, 21 Sep 2014 18:31:58 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -1.977
X-Spam-Status: No, score=-1.977 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, FM_FORGED_GMAIL=0.622, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_LOW=-0.7] autolearn=unavailable
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id WTDfw2XTUpKb for <>; Sun, 21 Sep 2014 18:31:55 -0700 (PDT)
Received: from ( []) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 266F31A03A3 for <>; Sun, 21 Sep 2014 18:31:54 -0700 (PDT)
Received: by with SMTP id b12so5744983lbj.25 for <>; Sun, 21 Sep 2014 18:31:53 -0700 (PDT)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from:to:cc:content-type; bh=hbQtFlkRhtDk8RpUYSpKC4Kl1TATV4Cr4U8ACz1LsFQ=; b=V6tw0ENVJoJkuXdG8sVJWySF84PCI+oYwvPbZVEW7JO8R3RIy+oJCr6j38VTAxGfqj x0rUTjSs4A0jb2nqghHh3PDDtCvDJ/Ltd1K2eTa9cp8/FZzlidX7Mbj4UBJGiojL3dFc ONBIkOujgblR21VgxFig7dM1hzWLAZv7feWGzSAhzvDYnMNI8uoR2V4iJD2lr4sodg/C JMgd9Ri/HY1RS5oGRWfDT9s6HzkpIlPIRPRR2zpM1qE/2kThei7QAiPeTWi3h9KDZbeq EyTNRgMtcXEcZ0ecIeYU5Cjg0h3U0m9CrEWfp4niVXDBNVLuvQzY8khIP6ms/3rAuQWt OKng==
X-Gm-Message-State: ALoCoQmnoFWlG3Hz1CyqMW2JlKFYDhNSXE4yzeWfIhfQIgZGc10rLws64yg4EZ2NzYflmUAoN5Vy
MIME-Version: 1.0
X-Received: by with SMTP id tt10mr21503385lbb.61.1411349513437; Sun, 21 Sep 2014 18:31:53 -0700 (PDT)
Received: by with HTTP; Sun, 21 Sep 2014 18:31:53 -0700 (PDT)
In-Reply-To: <>
References: <> <> <> <> <> <> <> <> <> <03a601cfd460$f8d71d70$ea855850$> <> <03c701cfd483$d1cf45e0$756dd1a0$> <> <043a01cfd4f2$3df75ff0$b9e61fd0$> <> <> <> <>
Date: Sun, 21 Sep 2014 21:31:53 -0400
Message-ID: <>
From: Richard Barnes <>
To: John Bradley <>
Content-Type: multipart/alternative; boundary="047d7b3a7d8e67085c05039d6a4e"
Cc: "" <>, secdir <>, Jim Schaad <>, Tero Kivinen <>, Michael Jones <>, IESG <>, "" <>, "" <>
Subject: Re: [jose] Secdir review of draft-ietf-jose-json-web-signature-31
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: Javascript Object Signing and Encryption <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Mon, 22 Sep 2014 01:31:58 -0000

On Sun, Sep 21, 2014 at 8:59 PM, John Bradley <> wrote:

> Also with PSS the attack is largely mitigated if the mask function uses
> the same hash as the message.
> JWA sec 3.5  requires SHA2 MGF functions for SHA2 message hashes with the
> equivalent length.
> If the same is done when SHA3 is added then I think PSS is not as
> susceptible to a substitution attack as it might look on the surface.

Certainly, having to change the MGF makes things much harder for the
attacker.  However, in principle an attacker with a sufficiently broken
hash algorithm could compute a preimage for the message *and* the MGF.  As
RFC 3447 says:

"it will be difficult for an opponent to substitute a different hash

... using "difficult" in the sense of "way harder than if we hadn't", not
in the sense of "just as hard as forging the signature".


> Sorry I just remembers that there was that mitigation for PSS.
> John B.
> On Sep 21, 2014, at 8:47 PM, John Bradley <> wrote:
> I like the general direction.
> One question,  wouldn't the recipient of a PSS signature detect the
> substitution of SHA-284 with SHA-256 due to the different key length.
> I was under the perhaps mistaken impression that the key lengths needed to
> be the same and just the alg different eg SHA3 and SHA2 keys of the same
> length.
> If that is the case we probably have not defined any algs currently that
> may be subject to this.  That is not to say that we shouldn't warn people
> as new algs are defined.
> John B.
> On Sep 21, 2014, at 8:32 PM, Richard Barnes <> wrote:
> I think I may have erred by trying to write a treatise on which algorithms
> are vulnerable :)  Here's some updated text, trying to be more concise.
> Jim: Your points about SHA-256 vs. SHA-512/256 and SHA-256 vs. SHA-3 don't
> really apply, since JOSE hasn't defined algorithm identifiers for
> SHA-512/256 or SHA-3.
> """
> # Signature Algorithm Protection
> In some usages of JWS, there is a risk of algorithm substitution attacks,
> in which an attacker can use an existing signature value with a different
> signature algorithm to make it appear that a signer has signed something
> that he actually has not.  These attacks have been discussed in detail in
> the context of CMS {{RFC 6211}}.  The risk arises when all of the following
> are true:
> * Verifiers of a signature support multiple algorithms of different
> strengths
> * Given an existing signature, an attacker can find another payload that
> produces the same signature value with a weaker algorithm
> * In particular, the payload crafted by the attacker is valid in a given
> application-layer context
> For example, suppose a verifier is willing to accept both "PS256" and
> "PS384" as "alg" values, and a signer creates a signature using "PS256".
> If the attacker can craft a payload that results in the same signature with
> SHA-256 as the signature with SHA-384 of the legitimate payload, then the
> "PS256" signature over the bogus payload will be the same as the "PS384"
> signature over the legitimate payload.
> There are several ways for an application using JOSE to mitigate algorithm
> substitution attacks
> The simplest mitigation is to not accept signatures using vulnerable
> algorithms: Algorithm substitution attacks do not arise for all signature
> algorithms.  The only algorithms defined in JWA
> {{I-D.ietf-jose-json-web-algorithms}} that may be vulnerable to algorithm
> substitution attacks is RSA-PSS ("PS256", etc.).  An implementation that
> does not support RSA-PSS is not vulnerable to algorithm substitution
> attacks.  (Obviously, if other algorithms are added, then they may
> introduce new risks.)
> In addition, substitution attacks are only feasible if an attacker can
> compute pre-images for the weakest hash function accepted by the
> recipient.  All JOSE algorithms use SHA-2 hashes, for which there are no
> known pre-image attacks as of this writing.  Until there begin to be
> attacks against SHA-2 hashes, even a JOSE implementation that supports PSS
> is safe from substitution attacks.
> Without restricting algorithms, there are also mitigations at the JOSE and
> application layer: At the level of JOSE, an application could require that
> the "alg" parameter be carried in the protected header.  (This is the
> approach taken by RFC 6211.)  The application could also include a field
> reflecting the algorithm in the application payload, and require that it be
> matched with the "alg" parameter during verification. (This is the approach
> taken by PKIX {{RFC5280}}.)
> Of these mitigations, the only sure solution is the first, not to accept
> vulnerable algorithms.  Signing over the "alg" parameter (directly or
> indirectly) only makes the attacker's work more difficult, by requiring
> that the bogus payload also contain bogus information about the signing
> algorithm.  They do not prevent attack by a sufficiently powerful attacker.
> """