We allow for new thumbprint algorithms to be defined and used with this spec. I think that we all agree that is a good thing.
The question is if we should define them here or as part of JWT/CWT based on broader demand. Including them in this document may be a distraction in my opinion. There is no attack against SHA256 with a short duration token/key (days) that is better solved by using a long duration token/key (years) with a longer hash. That said it woiulden't like me. I just think it will distract people in the wrong direction. John B. > On Apr 30, 2018, at 7:23 PM, Neil Madden <neil.mad...@forgerock.com> wrote: > > Responses inline again. > > On Mon, 30 Apr 2018 at 19:44, John Bradley <ve7...@ve7jtb.com > <mailto:ve7...@ve7jtb.com>> wrote: > Inline. > > >> On Apr 30, 2018, at 12:57 PM, Neil Madden <neil.mad...@forgerock.com >> <mailto:neil.mad...@forgerock.com>> wrote: >> >> Hi John, >> >>> On 30 Apr 2018, at 15:07, John Bradley <ve7...@ve7jtb.com >>> <mailto:ve7...@ve7jtb.com>> wrote: >>> >>> I lean towards letting new certificate thumbprints be defined someplace >>> else. >>> >>> With SHA256, it is really second preimage resistance that we care about for >>> a certificate thumbprint, rather than simple collision resistance. >> >> That’s not true if you consider a malicious client. If I can find any pair >> of certificates c1 and c2 such that SHA256(c1) == SHA256(c2) then I can >> present c1 to the AS when I request an access token and later present c2 to >> the protected resource when I use it. I don’t know if there is an actual >> practical attack based on this, but a successful attack would violate the >> security goal implied by the draft: that that requests made to the protected >> resource "MUST be made […] using the same certificate that was used for >> mutual TLS at the token endpoint.” >> >> NB: this is obviously easier if the client gets to choose its own client_id, >> as it can find the colliding certificates and then sign up with whatever >> subject ended up in c1. >> > > Both C1 and C2 need to be valid certificates, so not just any collision will > do. > > That doesn’t help much. There’s still enough you can vary in a certificate to > generate collisions. > > If the client produces C1 and C2 and has the private keys for them, I have a > hard time seeing what advantage it could get by having colliding certificate > hashes. > > Me too. But if the security goal is proof of possession, then this attack > (assuming practical collisions) would break that goal. > > > If the AS is trusting a CA, the attacker producing a certificate that matches > the hash of another certificate so that it seems like the fake certificate > was issued by the CA, is an attack that worked on MD5 given some > predictability. That is why we now have entropy requirements for certificate > serial numbers, that reduce known prefix attacks. > > The draft allows for self-signed certificates. > > Second-preimage Resistance is being computationaly infusible to find a second > preimage that has the same output as the first preimage. The second > preimage strength for SHA256 is 201-256bits and collision resistance strength > is 128 bits. See Appendix A of > https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-107r1.pdf > > <https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-107r1.pdf> > if you want to understand the relationship between message length and second > Preimage resistance. > > RFC 4270 is old but still has some relevant info. > https://tools.ietf.org/html/rfc4270 <https://tools.ietf.org/html/rfc4270> > > Think of the confirmation method as the out of band integrity check for the > certificate that is presented in the TLS session. > > This is all largely irrelevant. > >>> MD5 failed quite badly with chosen prefix collision attacks against >>> certificates (Thanks to some X.509 extensions). >>> SHA1 has also been shown to be vulnerable to a PDF chosen prefix attack >>> (http://shattered.io <http://shattered.io/>) >>> >>> The reason NIST pushed for development of SHA3 was concern that a preimage >>> attack might eventually be found agains the SHA2 family of hash algorithms. >>> >>> While SHA512 may have double the number of bytes it may not help much >>> against a SHA2 preimage attack,. (Some papers suggest that the double word >>> size of SHA512 it may be more vulnerable than SHA256 to some attacks) >> >> This is really something where the input of a cryptographer would be >> welcome. As far as I am aware, the collision resistance of SHA-256 is still >> considered at around the 128-bit level, while it is considered at around the >> 256-bit level for SHA-512. Absent a total break of SHA2, it is likely that >> SHA-512 will remain at a higher security level than SHA-256 even if both are >> weakened by cryptanalytic advances. They are based on the same algorithm, >> with different parameters and word/block sizes. >> > > SHA512 uses double words and more rounds, true. It also has more rounds > broken by known attacks than SHA256 https://en.wikipedia.org/wiki/SHA-2 > <https://en.wikipedia.org/wiki/SHA-2>.. So it is slightly more complex than > doubling the output size doubles the strength. > > SHA-512 also has more rounds (80) than SHA-256 (64), so still has more rounds > left to go... > > >>> >>> It is currently believed that SHA256 has 256 bits of second preimage >>> strength. That could always turn out to be wrong as SHA2 has some >>> similarities to SHA1, and yes post quantum that is reduced to 128bits. >>> >>> To have a safe future option we would probably want to go with SHA3-512. >>> However I don’t see that getting much traction in the near term. >> >> SHA3 is also slower than SHA2 in software. > > Yes roughly half the speed in software but generally faster in hardware. > > I am not necessarily arguing for SHA3, rather I think this issue is larger > than this spec and selecting alternate hashing algorithms for security should > be separate from this spec. > > I am for agility, but I don’t want to accidentally have people doing > something that is just theatre. > > Rotating certificates, and having the lifetime of a certificates validity is > as useful as doubling the hash size. > > Why not allow both? > > > I don’t think the confirmation hash length is the weakest link. > > Shouldn’t we allow all the parts to be as strong as possible? > > > John B. > >> >>> >>> Practical things people should do run more along the lines of: >>> 1: Put at least 64 bits of entropy into the certificate serial number if >>> using self signed or a local CA. Commercial CA need to do that now. >>> 2: Rotate certificates on a regular basis, using a registered JWKS URI >>> >>> My concern is that people will see a bigger number and decide it is better >>> if we define it in the spec. >>> We may be getting people to do additional work and increasing token size >>> without a good reason by putting it in the spec directly. >> >> I’m not sure why this is a concern. As previously pointed out, SHA-512 is >> often *faster* than SHA-256, and an extra 32 bytes doesn’t seem worth >> worrying about. >> >> [snip] >> >> — Neil > — Neil
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