SECDIR Review: Section 10.3 - Indication on "large entropy" and "short lifetime" should be indicated

[Nat Sakimura]

Section 10.3 finally mandates authentication of the request source, something that was ambiguous in earlier sections of this document. There are some ambiguous statement here, e.g. “Since Request Object URI can be replayed, the lifetime of the Request Object URI MUST be short and preferably one-time use. The entropy of the Request Object URI MUST be sufficiently large.” The lack of guidance of what constitutes a “short” lifetime or a “sufficiently large” amount of entropy (in a short URI) is worrisome.

[Nat Sakimura]

As an input to your discussion with Sascha.

Simple way to resolve:

• Short = 1 min.
• Sufficiently large = 128 bits of cryptographic nonce.

Long answer:

• Short = t sec.
• Regulated access frequency to the URI = n / sec.
• Entropy of the request_uri = y bits
• Accepted probability of success of the attack = p

Then, if ( n * t / 2 ^y) / 2 < p, then the combination (n, y) is compliant to the spec.

[Nat Sakimura]

I added

        The adequate shortness of the validity and 
        the entropy of the Request Object URI depends 
        on the risk calculation based on the value  
        of the resource being protected. A general guidance 
        for the validity time would be less than a minute 
        and the Request Object URI is to include a cryptographic 
        random value of 128bit or more at the time of the 
        writing of this specification.

[Nat Sakimura]

Re: #28

→ <<cset 357af0c886a9>>