Re: Sun setting on stream ciphers?

From: Nicol So (anonymous_at_no.spam.please)
Date: 02/29/04 

Date: Sun, 29 Feb 2004 14:37:09 GMT

Tom St Denis wrote:

> It's expected that a block cipher act like a PRP. A
> stream cipher may be a PRF [e.g. it's time dependent too].

A stream cipher does not behave like a pseudorandom function. Let m and
m||m' be two plaintexts. Starting from identical initial states, a
deterministic stream cipher E has the property that E(m, K) is a prefix
of E(m||m', K). A random function generally doesn't have this property. 

-- 
Nicol So
Disclaimer: Views expressed here are casual comments and should
not be relied upon as the basis for decisions of consequence.

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