> 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.

John B.

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