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

Thanks Hannes.

One question: we are recommending 32 octetes=256 bits to fill the sha256
range. Is there a reason for asking for a lesser entropy?
On 2014年12月3日(水) at 20:37 John Bradley <ve7jtb@ve7jtb.com> wrote:

> 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 <hannes.tschofenig@gmx.net>
> 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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