From: Bill Stewart <stewarts@ix.netcom.com>
Date: Wed, 19 Feb 1997 00:42:06 -0800 (PST)
To: cypherpunks@toad.com
Subject: Re: PGP key compromise with multiple independent encryptions of same message?
In-Reply-To: <3307FA3F.7B0B@coe.woodbine.md.us>
Message-ID: <3.0.1.32.19970218233359.04736c50@popd.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


At 01:27 AM 2/17/97 -0500, David Coe wrote:
>If I were to PGP-encrypt the same exact message to a number of
>different people, each copy with that person's public key, would
>I be making it easy (or easier) for one (or a group) of those
>recipients to compromise another recipient's private key?

If you're doing it as N separate messages, you're using 
N separate IDEA session keys; IDEA's strong enough that's no risk,
especially since they'll be using separate Initialization Vectors.
If you use the multiple-recipient capability, which uses one copy
of the message encrypted with one session key, and separate headers with
the session key encrypted with each public key.  If PGP didn't take 
precautions to prevent it, there are attacks on RSA which can be used
when you encrypt the same message with different RSA keys.  However,
PGP pads the session key with different random padding for each
session-key-encrypted-with-public-key header, so they're different messages,
so there's no risk there either.

So don't keep worrying about it; use whichever is more convenient.
(But it was worth worrying about once. :-)


#			Thanks;  Bill
# Bill Stewart, +1-415-442-2215 stewarts@ix.netcom.com
# You can get PGP outside the US at ftp.ox.ac.uk/pub/crypto/pgp
#     (If this is a mailing list, please Cc: me on replies.  Thanks.)