One Time Reply Blocks (was Re: strengthening remailer protocols)

[frantz@netcom.com (Bill Frantz)]

On Sun, 8 Sep 1996, Lance Cottrell wrote:

> Mixmaster prevents replay, so flooding multiple copies of a single message
> will not work. This is the reason Mixmaster has no reply block feature. I
> can see two ways in which replies can work safely.

To paraphrase John Von Neumann, any system which uses reply blocks is in a
state of sin.  By this I mean that if there is a chain pointing at you, a
sufficiently powerful attacker can walk down that chain and find you.

Given that, I will join the state of sin by proposing a mechanism which
will allow Alice to receive a reply from Bob, but change her mind at any
time.  The basic idea is to have a one-time reply block which either Bob or
Alice can send to.  If Alice thinks that too much time has elapsed, and
powerful enemies are walking down her reply block chain, she can send
herself a reply and break the chain.  (She might send a reply thru each
link in the chain to break all the links.)

It also occurs to me that since email addresses are about the same size as
secure symmetric keys, it would be attractive to use real one time pads
instead of symmetric key cyphers for encrypting them.


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