nonrandom string

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> Return-Path: <@CORNELLC.cit.cornell.edu:cjl at micro.med.cornell.edu>
> Date: Mon, 20 Sep 93 17:02:49 EDT
> From: cjl at micro.med.cornell.edu (Chris Leonard)
> To: cypherpunks at toad.com
> Subject: nonrandom string
> 
> 
> I address this to the list at large hoping that one or more of you can 
> explain something unusual that I observed.  A friend of mine and I have been 
> using PGP for several months and I just recently noted that in at least three
> of the recent posts to me that the first 18 characters of the encrypted 
> message were identical.  The first words of the plaintexts were not identical
> and so I assume that these characters are something else, perhaps the stuffer
> that I have heard mention of on this list.  This repeating string is of 
> concern for obvious reasons (e.g. so much for anonymity),  and I would like to 
> understand the cause of its recurrence. 
> 
> Please post to me and I will post a precis of the most reasonable suggestions.

This is normal. PGP labels each message with a byte that defines its type
(for messages between people this is a byte that indicates that what follows
is public key encrypted) followed by type dependant information. Among the
things in the first bunch of bytes are the KeyID (low order 64 bits, so this
is eight of the bytes you see) of the destination's key. If you are always
sending to the same person (i.e., the same key) these will always be the same.