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 <[email protected]> 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. > """
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