From: loki@nately.UCSD.EDU (Lance Cottrell)
Date: Tue, 1 Feb 94 15:05:48 PST
To: cypherpunks@toad.com
Subject: SASE Suggestion
Message-ID: <9402012306.AA09568@nately.UCSD.EDU>
MIME-Version: 1.0
Content-Type: text/plain


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I have been meditating on this problem of return
addresses, and have a proposal. The remailers
can not be allowed to choose the return path,
as any corrupted remailer will corrupt the rest
of the path. I suggest the following SASE packet
format.

Notation:
A(foo) = foo encrypted to remailer A
P = some sort of one use postage token.
end is a flag indicating the final destination.
x,y,z,b are large random integers.
n is a large prime.

Packet:
This will rout reply from A -> B -> C -> Bob

A(P,x,B,B(P,y,C,C(P,z,Bob,end))),A(b,n,message)

Upon receiving the packet, A does the following:

A decrypts the packet (both parts separately).

A calculates a new b' = b^x mod n and encrypts
B(b',n,message)

So B receives B(P,y,C,C(P,z,Bob,end)),B(b',n,message)

C receives C(P,z,Bob,end),C(b'',n,message)

Analysis:
The message, which would normally be encrypted to Bob,
is never transmitted in the clear. Bob can easily
compute b'' to confirm that the message was correctly
routed, but this reveals no information about the path
the message has taken. The first remailer will refuse
to deliver the message twice, because of the expired
postage token, so the same path will not be reused.

So, what do you think? It does require some work from
the remailers, but not too much more than now.

- ----------------------------------------------------------
Lance Cottrell	who does not speak for CASS/UCSD
loki@nately.ucsd.edu
PGP 2.3 key available by finger or server.

"Love is a snowmobile racing across the tundra.  Suddenly
it flips over, pinning you underneath.  At night the ice
weasels come."
			--Nietzsche
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