From: Hal <hfinney@shell.portal.com>
Date: Tue, 13 Sep 94 21:02:52 PDT
To: cypherpunks@toad.com
Subject: Re: alleged-RC4
In-Reply-To: <199409131449.KAA00544@orchard.medford.ma.us>
Message-ID: <199409140402.VAA26572@jobe.shell.portal.com>
MIME-Version: 1.0
Content-Type: text/plain


Another thing that is pretty obvious is that this kind of cypher is not
suitable for certain applications.  For example, if you wanted to encrypt
individually a lot of different files on your disk, all using the same
key, this kind of stream cypher would be totally unsuitable.  Any success
in guessing the plaintext which corresponds to a given cyphertext reveals
the XOR stream that the key generates, and that is the same stream that
would be XOR'd to encrypt any other file with the same key.  Doing this 
would be similar to re-using a "one time" pad for many encryptions.  This
kind of cypher is more appropriate for a communications channel where the
key is never re-used, and the two sides can keep persistent and
synchronized state.

Hal