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

[John Bradley <[e-mail]>]

We have been thinking that new hash functions and new mechanisms (eg ECDH-ES) would be treated the same and require a spec and review. 

(At some point I will do a asymmetric extension so that the key can also be used to encrypt POP keys back to the client,  but I don't want to make this spec look to complicated)

Any mechanism using a challenge & verifier approach could fit in to the defined parameters,  a different hash like SHA3 is simple in that it just be a replacement of the hash function.

I think guidance id fine as long as it doesn't limit future options.

John B.


> On Dec 3, 2014, at 8:46 AM, Hannes Tschofenig <[e-mail]> wrote:
> 
> Hi John,
> 
> I believe it makes sense to give recommendations for extensions (if you
> envision them). Of course, I hope that we do not see a flood of
> extensions that all use different hash functions.
> 
> Changing the mechanism to something that provides even stronger security
> properties would definitely require a new specification and review.
> 
> Ciao
> Hannes
> 
> 
> On 12/03/2014 12:37 PM, John Bradley 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 <[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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>> 
>