Re: [OAUTH-WG] SPOP: Code Challenge Discussion

[John Bradley <[e-mail]>]

Thanks Hannes.

Other methods such as different hashes need to be added via extension specs.   

Are you saying that we should set minimum recommendations for them.

It is also possible that those methods might use something other than hashing.  Key agreement might be a possibility.

Those properties would all be requirements for selecting a different hash function.   We could add that as a requirement for extensions if you think that is appropriate.

John B.

> On Dec 3, 2014, at 8:17 AM, Hannes Tschofenig <[e-mail]> wrote:
> 
> Hi all,
> 
> I am trying to figure out how to progress the SPOP document and
> therefore I read through the discussion about the code challenge, see
> 
> I wanted to share my view about this topic.
> 
> As a summary, the mechanism works as follows:
> 
> C: Compute code_verifier:=rand()
> C: Compute code_challenge:=func(code_verifier)
> 
> (For this discussion, the function func() is SHA-256.)
> 
> C: Send(Authz Request + code_challenge,S)
> 
> S: store code_challenge
> S: Send(Authz Grant,C)
> 
> C: Send(Access Token Request || code_verifier, S)
> 
> S: Compute code_challenge':=func(code_verifier)
> S: IF (code_challenge'==code_challenge) THEN SUCCESS ELSE FAIL.
> 
> The document currently does not say how much entropy the random number
> has to have.
> 
> The text only talks about the output size and SHA-256 indeed produces a
> 256 bit output.
> 
> Here is the relevant text:
> 
> "
>   NOTE: code verifier SHOULD have enough entropy to make it impractical
>   to guess the value.  It is RECOMMENDED that the output of a suitable
>   random number generator be used to create a 32-octet sequence.
> "
> 
> I suggest to recommend at least 128 bits, which is inline with the
> recommendations for symmetric ciphers in
> http://tools.ietf.org/html/draft-ietf-uta-tls-bcp-07
> 
> I would also suggest to reference RFC 4086 concerning the creation of
> random numbers.
> 
> Furthermore, since you allow other hash functions to be used as well it
> would be good to give guidance about what the properties of those hash
> functions should be. You definitely want a cryptographic hash function
> that provides pre-image resistance, second pre-image resistance, and
> collision resistance.
> 
> Given the size of the input and output it is impractical to compute a
> table that maps code_verifies to code_challenges.
> 
> This mechanism provides better properties than the "plain" mechanism
> since it deals with an attacker that can see responses as well as
> requests (but cannot modify them). It does not provide any protection
> against a true man-in-the-middle attacker.
> 
> Ciao
> Hannes
> 
> 
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