Matt Blaze posted the following to cryptography/coderpunks. Looks like potentially cool stuff. I had to use ftp://research.att.com/dist/mab/proxy.ps as a URL, but that may just have been Netcom DNS weirdnesses.

Subject: Proxy Cryptography draft available Date: Tue, 13 May 1997 18:37:10 -0400 From: Matt Blaze <mab@research.att.com> Sender: owner-cryptography@c2.net

I've put a draft of a new paper in my ftp directory. Comments and discussion welcome. In particular, I'm curious if anyone can find any real practical application for symmetric proxy functions.

ftp://ftp.research.att.com/dist/mab/proxy.ps

Proxy Cryptography

Matt Blaze Martin Strauss

AT&T Labs -- Research {mab,mstrauss}@research.att.com

Abstract:

This paper introduces {\em proxy cryptography,} in which a {\em proxy function,} in conjunction with a public {\em proxy key,} converts ciphertext (messages in a public key encryption scheme or signatures in a digital signature scheme) for one key ($k_1$) into ciphertext for another ($k_2$). Proxy keys, once generated, may be made public and proxy functions applied in untrusted environments. Various kinds of proxy functions might exist; {\em symmetric} proxy functions assume that the holder of $k_2$ unconditionally trusts the holder of $k_1$, while {\em asymmetric} proxy functions do not. It is not clear whether proxy functions exist for any previous public-key cryptosystems. Several new public-key cryptosystems with symmetric proxy functions are described: an encryption scheme, which is at least as secure as Diffie-Hellman, an identification scheme, which is at least as secure as the discrete log, and a signature scheme derived from the identification scheme via a hash function.

# 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.)