Re: [OAUTH-WG] WGLC on draft-ietf-oauth-mtls-07

[Brian Campbell <[e-mail]>]

Wearing my editor's hat here (did I do that right?) and looking to bring
this to a close so the draft can proceed - I don't see a consensus for
additional confirmation methods in this draft.

On Tue, May 1, 2018 at 3:08 AM, Neil Madden <[e-mail]>
wrote:

> JOSE and many other specs have allowed algorithms to be specified at
> multiple security levels: a baseline 128-bit level, and then usually 192-
> and 256-bit levels too. It seems odd that a draft that is ostensibly for
> high security assurance environments would choose to only specify the
> lowest acceptable security level, especially when the 256-bit level has
> essentially negligible overhead. (OK, ~60 bytes additional overhead in a
> JWT - I’d be surprised if that was a deal breaker though).
>
> Still, if the consensus of the WG is that this is not worth it, then I
> don’t want to delay the draft any further. I can always submit a 2 line RFC
> in future to add a SHA-512 confirmation method.
>
> — Neil
>
> > On 30 Apr 2018, at 23:58, John Bradley <[e-mail]> wrote:
> >
> > 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 <[e-mail]>
> wrote:
> >>
> >> Responses inline again.
> >>
> >> On Mon, 30 Apr 2018 at 19:44, John Bradley <[e-mail]> wrote:
> >> Inline.
> >>
> >>
> >>> On Apr 30, 2018, at 12:57 PM, Neil Madden <[e-mail]>
> wrote:
> >>>
> >>> Hi John,
> >>>
> >>>> On 30 Apr 2018, at 15:07, John Bradley <[e-mail]> 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 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
> >>
> >> 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)
> >>>>
> >>>> 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.. 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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