[AFS3-std] rxgk and the RFC 4402 PRF+ function

[Michael Meffie]

On Wed, 19 Feb 2014 16:23:36 -0500
Benjamin Kaduk <kaduk@MIT.EDU> wrote:

> Hi all,
> 
> The core rxgk document (which just had a last call period) has a normative 
> reference to RFC 4402 for the PRF+ construction, which is an algorithm to 
> get variable-length pseudo-random bytestrings from the RFC 3961 enctype's 
> pseudo_random() function.  The construction is basically just to invoke 
> the underlying pseudo_random() function in counter mode.
> 
> However, there is an erratum [1] filed against RFC 4402, which notes that 
> the implementors of that specification for krb5 gss_pseudo_random() 
> started the counter at 0, even though the text of RFC 4402 mandates that 
> the counter start at 1.
> 
> Because of this ambiguity about what value the counter starts at, in order 
> to ensure interoperability of rxgk implementations, we should note/clarify 
> what behavior rxgk expects.  It's probably easiest to do this by noting 
> directly in the document, i.e., issue a new I-D with just this change. 
> It's my understanding that if we have agreement on the list for the 
> clarification, no additional last call period is necessary.

Thanks Ben,

So if I understand; This is not a change (or errata), but a clarification?

The clarification is to say the RFC 4402 mandate of starting the counter
at 1 is correct for afs3-rxgk (even though other impementations of 4402
start at 0)?

Can you suggest the correct wording?


> 
> It doesn't really matter whether rxgk starts at 0 or 1, it's just an 
> arbitrary choice and we have to pick one to ensure interoperability. 
> There's some argument for starting at 0, in that we'll be doing the same 
> thing as krb5.  However, the krb5 libraries only export the RFC 4402 
> functionality as the gss routine, which requires a GSS security context in 
> order to be called.  Thus, rxgk implementations would not be able to 
> easily share code with krb5 implementations for this matter.  I have an 
> implementation of this PRF+ in OpenAFS gerrit for review; it's about 70 
> lines of code.  The argument for starting at 1 is that we're citing RFC 
> 4402 for the construction, and the actual text of RFC 4402 is to start at 
> 1.  Entries in the RFC series should never change, so citing RFC 4402 as a 
> normative reference means just that -- use the text that is there, not 
> some edited version from an erratum.
> 
> Does anyone have an opinion that might sway us to one side or the other?
> 
> -Ben
> 
> [1] http://www.rfc-editor.org/errata_search.php?rfc=4402
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-- 
Michael Meffie <mmeffie@sinenomine.net>