Subject: RE: FW: Updated I-Ds location Re: Extended One Week: WGLC GSS PRF and GSSKRB5 PRF drafts

From: "Blumenthal, Uri" <[email-masked]>
Date: Fri, 6 May 2005 12:08:02 -0400

Blumenthal,> Handbook of Applied Cryptography, by Menezes, van
    Blumenthal,> Oorschot and Vanstone, CRC Press, ISBN:
    Blumenthal,> 0-8493-8523-7. 5th printing in 2001.

    Blumenthal,> Chapter 5 deals with Pseudorandom sequences. Still
    Blumenthal,> enough math, but with a practical bent, usable by
    Blumenthal,> somebody who wants to design one.

> A pseudo-random sequence isn't really the same thing as a PRF is it?

No, a pseudo-random sequence is an OUTPUT of a pseudo-random Generator
or a pseudo-random Function, doesn't matter.

Note that ANSI standardized PRG (ANSI X9.17), not PRF.

PRG takes input S (seed or key). PRF takes inputs S (seed or key) and I
(index), which allows direct access to "bits of potentially huge
pseudorandom sequence which is not feasible to scan bit-by-bit" (quoting
from Goldreich's "Foundations of Cryptography").

PRFs can be constructed from any PRGs (this theorem and its proof are in
Goldreich's book).

Texts dealing with PRF and PRG would normally end up talking about PR
sequences, because that's what determines whether what you've
constructed is in fact a PR*.

> Or does their definition of sequence include arbitrary indexing which
> I guess would be close enough.

A PR sequence doesn't have to be indexed. It's a matter of convenience
and usability - some applications require it and some don't.

> This seems like an informative not normative reference.

This comment probably applies to both GGM paper and this book reference.

BTW, 'twould probably be good to add Oded Goldreich's book "Foundations
of Cryptography".

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