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

[Hannes Tschofenig <[e-mail]>]

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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>