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

[Brian Campbell <[e-mail]>]

That's pretty much in line with my on-the-fence position on it.

On Fri, Apr 20, 2018 at 4:43 PM, Justin Richer <[e-mail]> wrote:

> Additional confirmation methods can be easily defined outside of this
> draft. That said, I think those two in particular are pretty
> straightforward to add (well-known algorithms that are widely available) so
> it might make sense to just toss them in now? I think it’s fine either way.
>
>  — Justin
>
> On Apr 19, 2018, at 12:23 PM, Brian Campbell <[e-mail]>
> wrote:
>
> Okay, so I retract the idea of metadata indicating the hash alg/cnf
> method (based on John pointing out that it doesn't really make sense).
>
> That still leaves the question of whether or not to define additional
> confirmation methods in this document (and if so, what they would be
> though x5t#S384 and x5t#S512 seem the most likely).
>
> There's some reasonable rational for both adding one or two new hash alg
> confirmation methods in the doc now vs. sticking with just SHA256 for
> now. I'll note again that the doc doesn't preclude using or later defining
> other confirmation methods.
>
> I'm kind of on the fence about it, to be honest. But that doesn't really
> matter because the draft should reflect rough WG consensus. So I'm looking
> to get a rough gauge of rough consensus. At this point there's one
> comment out of WGLC asking for additional confirmation method(s). I don't
> think that makes for consensus. But I'd ask others from the WG to chime
> in, if appropriate, to help me better gauge consensus.
>
> On Fri, Apr 13, 2018 at 4:49 AM, Neil Madden <[e-mail]>
> wrote:
>
>> I’m not particularly wedded to SHA-512, just that it should be possible
>> to use something else. At the moment, the draft seems pretty wedded to
>> SHA-256. SHA-512 may be overkill, but it is fast (faster than SHA-256 on
>> many 64-bit machines) and provides a very wide security margin against any
>> future crypto advances (quantum or otherwise). I’d also be happy with
>> SHA-384, SHA3-512, Blake2 etc but SHA-512 seems pretty widely available.
>>
>> I don’t think short-lived access tokens is a help if the likelihood is
>> that certs will be reused for many access tokens.
>>
>> Public Web PKI certs tend to only use SHA-256 as it has broad support,
>> and I gather there were some compatibility issues with SHA-512 certs in
>> TLS. There are a handful of SHA-384 certs - e.g., the Comodo CA certs for
>>  https://home.cern/ are signed with SHA-384 (although with RSA-2048,
>> which NSA estimates at only ~112-bit security). SHA-512 is used on some
>> internal networks where there is more control over components being used,
>> which is also where people are mostly likely to care about security beyond
>> 128-bit level (eg government internal networks).
>>
>> By the way, I just mentioned quantum attacks as an example of something
>> that might weaken the hash in future. Obviously, quantum attacks completely
>> destroy RSA, ECDSA etc, so SHA-512 would not solve this on its own, but it
>> provides a considerable margin to hedge against future quantum *or
>> classical* advances while allowing the paranoid to pick a stronger security
>> level now.. We have customers that ask for 256-bit AES already.
>>
>>
>> (I also misremembered the quantum attack - “Serious Cryptography” by
>> Aumasson tells me the best known quantum attack against collision
>> resistance is O(2^n/3) - so ~2^85 for SHA-256 but also needs O(2^85) space
>> so is impractical. I don’t know if that is the last word though).
>>
>> As for SHA-1, doesn’t that prove the point? SHA-1 is pretty broken now
>> with practical collisions having been demonstrated. The kind of attacks on
>> CA certs demonstrated for MD5 [1] are probably not too far off for SHA-1
>> now. As far as I am aware, we’re nowhere near those kinds of attacks on
>> SHA-256, but I’d prefer that there was a backup plan in place already
>> rather than waiting to see (and waiting for everyone to have hard-coded
>> SHA-256 everywhere).
>>
>> Now I have to get back to my daughter’s birthday party… :-)
>>
>> [1] http://www.win.tue.nl/hashclash/rogue-ca/
>>
>> Neil
>>
>>
>> On Thursday, Apr 12, 2018 at 10:07 pm, John Bradley <[e-mail]>
>> wrote:
>> The WG discusses RS meta-data as part of one of Dick’s proposals.   I am
>> unclear on who is going to work on it in what draft.
>>
>> If the client is doing mtls to both the Token endpoint and RS the
>> information in the confirmation method is not relevant to the client as
>> long as the AS and RS are in agreement like with most tokens.
>>
>> The hash on the certificate and length of the signing key are equally or
>> more vulnerable to any sort of attack.
>> At least with AT the tokens are not long lived.
>>
>> Doing anything better than SHA256 only makes sense if the cert is signed
>> by something stronger like SHA512 with a 2048bit RSA key.   That seems sort
>> of unlikely in the near term.
>>
>> I prefer to wait to see what general fix is proposed before we jump the
>> gun with a bandade for a small part of the overall problem.
>>
>> People are typically using SHA1 fingerprints.  We added SHA256 for JWT
>> and got push back on that as overkill.
>> I don’t think this is the correct place to be deciding this.
>>
>> We could say that if new thumbprints are defined the AS and RS can decide
>> to use them as necessary.
>> That is sort of the situation we have now.
>>
>> The correct solution may be a thousand rounds of PBKDF2 or something like
>> that to make finding collisions more difficult rather than longer hashes.
>>
>> John B.
>>
>> > On Apr 12, 2018, at 5:20 PM, Brian Campbell <[e-mail]>
>> wrote:
>> >
>> > That's true about it being opaque to the client. I was thinking of
>> client metadata (config or registration) as a way for the AS to decide if
>> to bind the AT to a cert. And moving from a boolean to a conf method as an
>> extension of that. It would be more meaningful in RS discovery, if there
>> was such a thing.
>> >
>> > I don’t know that SHA512 would be the best choice either but it is
>> something that is stronger and could be included now. If there's consensus
>> to do more than SHA256 in this doc.
>> >
>> >
>> >
>> > On Thu, Apr 12, 2018 at 1:52 PM, John Bradley <[e-mail]>
>> wrote:
>> > Inline
>> >
>> > Snip
>> >
>> >>
>> >>
>> >> 12. The use of only SHA-256 fingerprints means that the security
>> strength of the sender-constrained access tokens is limited by the
>> collision resistance of SHA-256 - roughly “128-bit security" - without a
>> new specification for a new thumbprint algorithm. An implication of this is
>> that is is fairly pointless for the protected resource TLS stack to ever
>> negotiate cipher suites/keys with a higher level of security. In more
>> crystal ball territory, if a practical quantum computer becomes a
>> possibility within the lifetime of this spec, then the expected collision
>> resistance of SHA-256 would drop quadratically, allowing an attacker to
>> find a colliding certificate in ~2^64 effort. If we are going to pick just
>> one thumbprint hash algorithm, I would prefer we pick SHA-512.
>> >>
>> >> The idea behind haveing just one thumbprint hash algorithm was to keep
>> things simple. And SHA-256 seems good enough for the reasonably foreseeable
>> future (and space aware). Also a new little spec to register a different
>> hash algorithm, should the need arise, didn't seem particularity onerous.
>>
>> >>
>> >> That was the thinking anyway. Maybe it is too short sighted though?
>> >>
>> >> I do think SHA-256 should stay regardless.
>> >>
>> >> But the draft could also define SHA-512 (and maybe others). What do
>> you and WG folks think about that?
>> >>
>> >> *** Yes please.
>> >>
>> >> It would probably then be useful for the metadata in §3.3 and §3.4 to
>> change from just boolean values to something to convey what hash alg/cnf
>> method the client expects and the list of what the server supports. That's
>> maybe something that should be done anyway. That'd be a breaking change to
>> the metadata. But there's already another potential breaking change
>> identified earlier in this message. So maybe it's worth doing...
>> >>
>> >> How do folks feel about making this kind of change?
>> >>
>> >>
>> > The confirmation method is opaque to the client.  I don’t think adding
>> hash algs to discovery will really help.
>> > The AS selection needs to be based on what the RS can support.
>> >
>> > If anyplace it should be in RS discovery.
>> >
>> > As a practical matter you art going to find a client certificate with
>> more than a SHA256 hash anytime in the near future.
>> > So for a short lived access token 128bits of collision resistance is
>> quite good.   We are going to have issues with certificates long before
>> this becomes a problem.
>> >
>> > SHA256 is appropriate for AES128 256bit elliptic curves and 3072bit RSA
>> keys, but again that is over the long term.
>> > We are using short lived access tokens.  People should rotate the
>> certificate more often than once a year if this is a real issue.
>> >
>> > I am not against new hash for the fingerprint, but I also don’t know
>> that SHA512 would be the best choice if we are concerned about quantum
>> crypto resistance.   That is a issue beyond mtls and should be addressed by
>> CFRG etc.
>> >
>> > Regards
>> > John B.
>> >
>> >
>> >
>> >
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