I need to re read Kaliski's paper from CT-RSA 2002 On Hash Function Firewalls in Signature Schemes to see how difficult is difficult. http://bookzz.org/book/2302665/6f0670 This is not exactly a new issue. I can see other ISEG discussions over the years on this and related topics. There seems to be a padding consideration for PSS that also acts as a mitigation to prevent the recipient from interpreting the hash to be shorter than was sent. Having the correct security considerations will help implementers. The paper above is referenced by a number of RFC, but I see some people did not consider the mitigation effective as the other uses of PSS do not tie the MGF to the signing hash as we did in JWA. I am just getting on a flight, and will have a look at it tomorrow. John B. On Sep 21, 2014, at 9:31 PM, Richard Barnes <rlb@xxxxxx> wrote: > > > On Sun, Sep 21, 2014 at 8:59 PM, John Bradley <ve7jtb@xxxxxxxxxx> wrote: > Also with PSS the attack is largely mitigated if the mask function uses the same hash as the message. http://tools.ietf.org/html/rfc3447#page-29 > > 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 function" > > ... 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". > > --Richard > > > > Sorry I just remembers that there was that mitigation for PSS. > > John B. > > On Sep 21, 2014, at 8:47 PM, John Bradley <ve7jtb@xxxxxxxxxx> 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 <rlb@xxxxxx> 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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