New directions in anonymity (needed)

Mike Ingle MIKEINGLE at
Sun Feb 5 22:17:10 PST 1995

There has been a lot of talk about anonymity and the best ways of providing
it recently. Anonymity now is about where cryptography was before Diffie and
Hellman's paper, or untraceable transactions before Chaum. To use it, you
have to trust someone, or at least a group of people.

All of our anonymous systems boil down to only two techniques: indirection
and broadcast. Indirection is sending a message through one or more
intermediate nodes to conceal its point of origin. Broadcast is sending a
message to multiple recipients to conceal the intended recipient.

Indirection is somewhat secure for anonymous-to-known messaging. Properly
done, all intermediate nodes must be compromised to reveal the anonymous
sender. It is entirely broken for allowing replies to an anonymous person.
To allow a reply, a traceable path must exist back to the anonymous person.
Having obtained whatever piece of data is used to send a reply, the attacker
can trace this back one node at a time. Kludge all you want, but there is no
good solution to this. You can make the path disappear when it is used, but
if the attacker gets it first, there is nothing you can do.

Broadcast is exactly as secure as it is nonscalable. If you broadcast to 100
people, an attacker's uncertainty is one in 100. The security grows linearly
with the overall bandwidth. For cryptographic-level security, it would need
to grow exponentially with bandwidth.

Anonymity needs something fundamentally new, something comparable to public
key for cryptography or blind signatures for digital cash. Suppose a server
has a large file. A message comes in and is combined into this file. Another
message comes in with a key to retrieve data. The server processes the
retrieval key against this large file and comes up with an output, which it
returns to the person doing the retrieving. This output contains the input
message, transformed in such a way that even the server cannot match it
to the input that produced it. This is what we need.

It would require three keys: an encryption key, a selection key, and a
decryption key. The sender uses the recipient's encryption key to encrypt
the message, then sends it to the server. The server mixes the message into
its file, but cannot identify the encrypted file as destined for a
particular recipient. The recipient's selecting key extracts the data, but
the data extracted does not resemble the data the sender sent. The
decryption key, kept private by the recipient, is necessary to return the
data to its original form. The server must be unable to link the data from
the sender to the data sent to the recipient, and this is the hard part.

A method like this would permit anonymity without trust, much as public key
allows secrecy without trust and digicash allows transaction privacy without
trust. Is there any way to do it? There are functions like the Fourier
transform that can distribute data over a large file, but the inverse
function gives you back the same data. We need the not-so-inverse function
to give back a different piece of data, but one that can be converted, by a
private key, into the original, which the server never sees.


P.S. Have any of the "beta testers" actually tested RSAREF for Perl?
RSA gave me permission to call the random number and DES routines, so I am
ready to add that in and release the code.

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