forward secrecy in mixmaster

[Adam Back <aba@dcs.ex.ac.uk>]

Since Peter Allen's discussion of mixmaster, I started doing something
I'd been thinking of for a while, since noticing that it was on the
mixmaster to-do list months ago (ie there is unfinished source to do
this): direct socket connections and diffie-hellman key exchange for
forward secrecy.

I wrote the socket stuff yesterday evening, didn't take too long as
socket programming is something I've done lots of.

Now comes what do you actually send down the sockets.

Question for Lance, and any others who were involved in mixmasters
implementation: what did you have in mind as a way of negotiating the
DH keys?

I notice that mixmaster generates a DH key and stores it in file
`DH.mix', but that this is not (as far as I can see from the source)
included in the remailers public key block.

(A couple of comments as an aside: I think that you should be able to
have a much smaller generator without loss of security, this should
reduce the overhead of a DH key exchange.  Using 3 even I think is
safe, without any extra precautions on prime generation.  You can even
go to 2, with a few precautions (PGPfone does this).  Comment #2 I
think 1024 may be a bit small, I don't have any figures handy for
relative security of DH key lengths, but PGPfone offers 4096 bit DH
for instance.  Does rsaref have limits on prime lengths for DH, the
same as it does for RSA?).

There are lots of options for DH public key negotiation.

First option is whether you have a common prime and generator for all
remailers or not.  If you have a common prime, accusations of the
prime being `cooked' (chosen to have a weakness) can be mitigated by
using a deterministic generation method based on the hash of a known
phrase (a Jefferson quote perhaps), or PI or whatever.

A common modulus may offer a fatter target for attack (for some
precomputation attacks), but with large enough keys this probably
isn't that bad, as there aren't that many mixmasters anyway.

With a common modulus there is DH key negotiation needed, just include
it with the source.

For different modulii for each remailer, there are more options:

a) include the DH key signed by the RSA key in the remailers public key
   (may break backwards compatibility with existing versions of
   mixmaster)

b) send the DH public key at the start of each session

c) send the DH public key on request

There is also a question of which key do you use, the sender remailers
or the recipient remailers.

Negotiating DH public keys during execution also opens the possibility
for periodic re-keying.

Thats the end of my thoughts on direct socket mixmaster.

Next message is some thoughts on non-interactive forward secrecy
protocols.

Adam
--
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