Re: Comments on draft-ietf-kitten-krb5-gssapi-prf-03.txt

On Mon, May 23, 2005 at 06:11:56PM -0400, Blumenthal, Uri wrote:
> Darn... You start giving advices, and now they demand the actual text
> from you! Next you know they'll ask for an implementation! :-)

Text will do, thanks :)

> How about:
> 
> Pseudorandom functions by their nature are capable of producing only
> limited amount of cryptographically secure output. The exact amount of
> output that one can safely use, unfortunately varies from one PRF to
> another (which prevents us from recommending specific numbers). Because
> of this, we recommend that unless you really know what you are doing
> (i.e. you are a cryptographer and are qualified to pass judgement on
> cryptographic functions in areas of period, presence of short cycles,
> etc) - you limit the amount of the PRF output to the necessary minimum. 
> 
> Don't forget to edit my English! :-)

Ok, edited:

   Pseudo-random functions, by their very nature, are capable of
   producing only limited amounts of cryptographically secure output.
   The exact amount of output that one can safely use will vary by
   mechanism, therefore we cannot recommend specific limits on
   applications' uses of GSS_Pseudo_random().  Because of this we
   recommend that applications that are not mechanism-specific should
   limit consumption of GSS_Pseudo_random() output to the minimum they
   may need.

To which I'd add:

   It is expected that callers of GSS_Pseudo_random() will use its
   output to key other cryptosystems; for these applications the minimum
   GSS_Pseudo_random() output needed by such applications can be
   expected to be well within the margin of safety of
   GSS_Pseudo_random() output.

Are we happy? :)

Nico
-- 

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