From: edgar@spectrx.Saigon.COM (Edgar W. Swank)
Date: Tue, 20 Jul 93 01:15:41 PDT
To: Cypherpunks          <cypherpunks@toad.com>
Subject: PGP: "Stealth" mode
Message-ID: <c8ew7B5w165w@spectrx.saigon.com>
MIME-Version: 1.0
Content-Type: text/plain


-----BEGIN PGP SIGNED MESSAGE-----

J. Michael Diehl wrote:

    Many encryption tools such as ripem, pgp, and dolphin can
    recognize their own output...which indicates that there is a
    footprint to that particular implimentation.

There has been discussion among us PGP developers (I guess I am one
now; see my recent posts to alt.security.pgp) of implementation of
a "stealth" option, which would remove all the identifying footprints
from encrypted output.

RSA-Encrypted files would start with the (random) IDEA key followed
by IDEA-encrypted data.

Conventionally encrypted files (-c) would contain only encrypted data.

Possibly the armor format wouldn't change, since PGP can be used
to convert any binary file to armored form (-a). But it would probably
be better to convert PGP binary output to e-mailable form using a
common external utility like UUENCODE to make it look even more
ordinary, especially if you change the UUENCODE BEGIN statement
to specify, say, xxx.ZIP instead of XXX.PGP.

Of course, the UUDECODED file wouldn't be recognized by PKUNZIP. "Oh,
what a shame, the file must have got corrupted somewhere on the net".

That's the easy part.

The hard part is designing decryption procedures. PGP would have
to prompt something like:

"unrecognized input file. May be stealth mode. Select procedure

   1. Assume RSA file with your default secret key
   2. "     "       " Prompt for userid of secret key to try.
   3. "     "       " try all your secret keys (may be impractical on
                      slower CPU's)
   4. Assume conventional file. prompt for pass phrase."

Also note that for each secret key tried, PGP must prompt for its
pass phrase. Of course, once decryption is successful, then the
usual footprints can be there for signatures and compression.

AFAIK, this idea is still just in the talking stage.

-----BEGIN PGP SIGNATURE-----
Version: 2.3a.1/EWS

iQCVAgUBLElab94nNf3ah8DHAQFIEwP/RR1+oUMpJL75smnHJCfP+8e8b4+P6uEm
uFpyN1LOpVbuKwNG73tu2c/wvdmABDH39xDDs5C29rOj/RFjpGWj40wTXJcvJ878
dSI/Dmj1pAZXCay9qSOldxxrtXes/wsCuQtHL/PX9y+tcXGIaduP4TYlxMSCqXvr
rwNTH1jeM5I=
=t5A8
-----END PGP SIGNATURE-----

--
edgar@spectrx.saigon.com (Edgar W. Swank)
SPECTROX SYSTEMS +1.408.252.1005  Cupertino, Ca