Selection key crypto protocol trial balloon

[rishab@dxm.ernet.in]

Before replying to Tim's comments on Mike Ingle's new directions for anonymity
which, I think, are based on a misunderstanding of what Mike was essentially
positing, here's my (slightly lengthy) take on the Selection Key Crypto 
protocol. Please read it before criticising, as I've already done that...

Mp is the plaintext message. Alice encrypts it with Bob's public key:
Mc = E.kpub (M)                                 [1]

She sends Mc to Hermes, the Hoarder of Messages. Hermes also receives 
Alice's identity string (Ia) which will probably be anonymous, but let's 
assume it's not. Now's the tricky bit. Hermes adds Mc to its message store, S:
S' = G (S, Mc)                                  [2]

The key here is G, the Digest function, which has to spread M over S (with a 
variation of the Fourier function, perhaps), and not just append it, so that
the _only_ way it can be extracted is with Bob's selection key, which Bob
transmits to Hermes along with an involuntary identity string (Ib) to get:
Mx = H.ksel (S')                                [3]

where H.ksel is the Regurgitate function applied with Bob's selection key,
and Mx is _not_ the original encrypted message, Mc, nor the plaintext Mp.

Finally Bob decrypts Ms with his _private_ key (or possibly a variation, which
we'll call v(kpri)), to get the plaintext:
Mp = D.v(kpri) (Ms)                                [4]

So the possible crypto operations are:
Mp = D.kpri (E.kpub(Mp))
Mp = D.v(kpri) (Mx)  where v(x) may be = x
Mx = H.ksel (S')
And the Digest function, S' = G (S, E.kpub(Mp))
Note that only the private key kpri, and its variation v(kpri) are private. 
The other two are public.

Now for the obvious criticisms:
1. after [1] Hermes can store Alice's identity (Ia) in a dark grey dossier,
   and match it to Bob's identity (Ib) and the extracted message (Mx). Nope.
   The premise of this scheme is that Mx doesn't look like Mc at all, and that
   G (in [2]) dissolves the original message into a sludge, from which Mx
   somehow emerges, NOT that it chops Mc into bits with Ia on a little nametag
2. after [1] Hermes can store Ia along with the message and feed [Mc, Ia]
   to [2] in the hope that it will reappear in [3] as [Mx, Ia']. Well, the
   digest function has prevent this, possibly by making the regurgitated
   thing so different from the input that matching Ia to Ia' will be 
   impossible. Of course, Bob probably won't grok [Mx, Ia'] in [4]
3. Hermes gets Bob's identity string (Ib) in [3]. Et alors? As that is 
   incidental, not required to extract the message (unlike in a remailer, where
   that identity string - e-mail address, return block, whatever is _precisely_
   what is required for a message to reach), it's irrelevant. The _real_
   identity is the selection key, which does not correlate to any cyberspatial
   location. See further on traffic analysis.
4. ksel, Bob's selection key, is obviously not private, at least not to Hermes,
   and may as well be public. So anyone can extract the message intended for
   Bob (which reinforces my previous point that Ib is irrelevant). But what
   they extract is Mx, which is not the plaintext. To get the plaintext you
   need not the selection key but Bob's private key, which only Bob has.
5. The Digest function [2-3] has to be robust, i.e. it shouldn't puke if
   it's got lots of messages, and should be able to extract the right one
   in any state. Well, yes, that's tough.

Traffic analysis
The difference between the SKC model and remailers
  Alice --> (Raven, the remailer net) --> Bob   .... multiplied ad infinitum
  Alice --> (Hermes) --> Bob
                     --> Carol, David, the whole world as the 
                         selection key is public
The remailer net is essentially one-to-one, making traffic analysis easier
and limiting Bob's deniability - if the last remailer says it's gone to Bob,
it was _for_ Bob.

The broadcast model essentially increases deniability - they all got it,
any of them could have seen it if they had the key, and you'll only know that
I had the private key with thumb-screws. But they all _get_ it, so bandwidth
is huge.

The SKC model is a compromise. It increases deniability - anyone could have
got it, they all have the selection key, they could only have seen it with the
private key etc. It cuts bandwidth. But it's not a data haven, at least not
as earlier discussed.

The data haven in its latest guise has the same deniability as SKC, and the
same prerequisites that aid recipient protection (anon anon ftp etc). But it
doesn't protect the sender. If the haven is in collusion with remailers that
Alice used to post, it can link her to the message and possibly to Bob. With
SKC, even with the collusion of remailers the link gets lost in digestion.
Also, Bob (or anyone) can frequently extract stuff with the selection key,
perhaps garbage if Hermes is misbehaving or without messages for Bob.

Of course when (and how) Hermes is to expire messages is part of the problem
of the various functions involved.

Comments?


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Rishab Aiyer Ghosh          rishab@dxm.ernet.in           rishab@arbornet.org
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