From: Richard Barnes [mailto:rlb(_at_)ipv(_dot_)sx]
Sent: Friday, September 19, 2014 2:32 PM
To: Mike Jones
Cc: Tero Kivinen; iesg(_at_)ietf(_dot_)org; secdir(_at_)ietf(_dot_)org;
ietf(_at_)ietf(_dot_)org;
draft-ietf-jose-json-web-signature(_dot_)all(_at_)tools(_dot_)ietf(_dot_)org;
jose(_at_)ietf(_dot_)org
Subject: Re: Secdir review of draft-ietf-jose-json-web-signature-31
"""
# 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 "PS1" and "PS256" as
"alg" values, and a signer creates a signature using "PS256". If the attacker
can craft a payload that has the same SHA-1 digest has as the SHA-256 digest of
the legitimate payload, then the "PS1" signature over the bogus payload will be
the same as the "PS256" signature over the legitimate payload.
There are several ways for an application using JOSE to mitigate algorithm
substitution attacks:
* Don't accept signatures using vulnerable algorithms: Algorithm substitution
attacks do not arise for all signature algorithms.
* Signatures using RSA PKCS#1 v1.5 ("RS1", "RS256", etc.) are not subject to
substitution attacks because the signature value itself encodes the hash
function used.
* Signatures with HMAC algorithms ("HS1", "HS256", etc.) cannot be
substituted because the signature values have different lengths Likewise for
signatures with ECDSA algorithms ("ES256", "ES384", etc.).
[JLS] This is not a true statement. If you support both SHA256 and SHA512/256
then the signature values have the same length. This will also be an issue if
you support both the SHA2 and the SHA3 algorithm sets as they have results of
the same length.
* The only algorithms defined in JWA {{I-D.ietf-jose-json-web-algorithms}}
that is vulnerable to algorithm substitution attacks is RSA-PSS ("PS1",
"PS256", etc.). An implementation that does not support RSA-PSS is not
vulnerable to algorithm substitution attacks.
[JLS] ECDSA is open to this attack if you support both SHA256 and SHA512/256
the hash lengths are the same. This will also be an issue if you support both
the SHA2 and the SHA3 algorithm sets as they have results of the same length.
* Require that the "alg" parameter be carried in the protected header. (This
is the approach taken by RFC 6211.)
* 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}}.)
[JLS] RSA-PKCS#1.5 is open to this attack if the suggestion of Mike in a
message prior to this is put into the text. That is to allow for the hash
inside of the signature and that outside of the signature to differ in value
because you only enforce one of the two values.
Of these mitigations, the only sure solution is the first. 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.
"""
On Fri, Sep 19, 2014 at 2:49 PM, Mike Jones
<Michael(_dot_)Jones(_at_)microsoft(_dot_)com> wrote:
I would appreciate it if you would write a draft of the proposed security
considerations text, Richard. Perhaps title the section “Unsecured Algorithm
Values”.
Thanks!
-- Mike
From: Richard Barnes [mailto:rlb(_at_)ipv(_dot_)sx]
Sent: Wednesday, September 17, 2014 6:24 AM
To: Tero Kivinen
Cc: Mike Jones; iesg(_at_)ietf(_dot_)org; secdir(_at_)ietf(_dot_)org;
ietf(_at_)ietf(_dot_)org;
draft-ietf-jose-json-web-signature(_dot_)all(_at_)tools(_dot_)ietf(_dot_)org;
jose(_at_)ietf(_dot_)org
Subject: Re: Secdir review of draft-ietf-jose-json-web-signature-31
On Wednesday, September 17, 2014, Tero Kivinen <kivinen(_at_)iki(_dot_)fi>
wrote:
Richard Barnes writes:
Perhaps, but is there benefits for leaving the alg without protection?
Simplicity (if you omit protected headers altogether), and
compatibility with other signed things. In the sense that you could
transform one of them into a JWS without re-signing. This would
apply, for example, to an X.509 certificate -- just parse the outer
SEQUENCE, and re-assemble into a JWS with the tbsCertificate as
payload. Same security properties that X.509 already has.
Ok, having this kind of information somewhere in the draft would help
to understand the reason. Also having text explaining that is
possible, and that the security properties of this option (i.e. no
problem with PKCS#1, etc... the text you had in the other email).
It's also completely unnecessary for PKCS#1 signatures, which are
the dominant use case today.
I agree.
In general, I'm opposed to protocols baking in more
application-specific logic than they need to. The point of JOSE is
to describe the cryptographic operation that was performed, and
carry the relevant bits around. Its job is not to fix all the
weaknesses that every algorithm has.
Yes, but this property might have security issues, so they should be
covered by the security considerations section.
I'm perfectly happy to have it documented in the Security Considerations.
Mike: Should I generate some text, or do you want to take a stab?
--
kivinen(_at_)iki(_dot_)fi