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 <hannes.tschofe...@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 >> >> >> _______________________________________________ >> OAuth mailing list >> OAuth@ietf.org >> https://www.ietf.org/mailman/listinfo/oauth >
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