Re: Stream cipher against block cipher

From: David Hopwood (david.hopwood_at_zetnet.co.uk)
Date: 06/29/03 

Date: Sun, 29 Jun 2003 08:07:54 +0000

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"John E. Hadstate" wrote:
> "aditya" <truman@rediffmail.com> wrote:
> > You guys seem to be having an interesting conversation out here. Whats
> > a block cipher? At times it helps to get back to the basics.....so can
> > I add?
>
> > A block cipher encrypts plaintext in blocks. While a stream
> > cipher encrypts plaintext 1-bit or 1-byte at a time.
>
> The distinction is not so black and white. Some have suggested that a
> better characteristic is to look for the preservation of state across blocks
> or "encryption units". Even this is very slippery ground. How big is an
> "encryption unit"? It could be the size of the whole message. That
> point-of-view sees every "stream cipher" as a very-large-block cipher in ECB
> mode.

A block cipher is any symmetrically-keyed *permutation* (or a family of
symmetrically-keyed permutations on blocks of different lengths, in which
case it is called a "variable-length block cipher"). A stream cipher is any
symmetrically-keyed *encryption scheme*.

Thus modes, including ECB, construct stream ciphers from block ciphers.
Equivalently, they construct encryption schemes from keyed permuations.

The problem occurs when people assume that because these terms have the form
"<foo> cipher" and "<bar> cipher", they must be subclassifications of a class
of "ciphers" such that "cipher" has a consistent meaning for both cases.
"stream cipher" and "block cipher" should each be treated as indivisible
phrases, not as applications of an adjective to "cipher".

(A similar problem occurs with "dynamic typing" and "static typing",
incidentally, but that's off-topic.)

- --
David Hopwood <david.hopwood@zetnet.co.uk>

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