Still more on the Digicash protocol

[daw@guaymas.CS.Berkeley.EDU (David A Wagner)]

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Ian & I were talking about the Digicash protocol some more.

Hal has pointed out that payments & deposits with a wildcard in the
payment_hdr should NOT be sent in the clear, since they can be stolen
by any passive eavesdropper.

Ian has pointed out the same problem with cancellations: the payer_code
should NOT be sent in the clear, since any passive eavesdropper can
grab this info and steal the corresponding payment.

Sadly, the current Digicash client DOES send those items in the clear
(without even warning users to avoid wildcards).  Why?


Anyhow, the obvious solution is encryption.  Our new observation is
that encrypting deposits & cancellations with the mint's public key
is not enough to solve the problem.

To see this, recall how the public key encryption works: a session
key for a bulk cipher is encrypted under the public key, and the data
(e.g. userhdr, payment_hdr, coins, ...) is encrypted under the bulk
cipher.  Digicash hasn't told us yet what bulk cipher they use, but
let's assume for concreteness they use RC4.

Now I'm an attacker -- I want to modify userhdr.userID -- and then
the mint will accept the payment and deposit the coins into the wrong
account.  But this is just easy for an active attacker -- I just flip
bits in the ciphertext!  (Presumably I can guess fairly well what
plaintext value is expected for the userhdr.userID field.)

This is just a corollary of my Pet Peeve for protocol designers:

	Don't use encryption when you want message authentication;
	with encryption, you should only count on confidentiality!

Ok, so you mention that maybe Digicash uses DES-CBC or somesuch instead
of a stream cipher like RC4.  I'll briefly note that while the attack
isn't so trivial anymore, DES-CBC ciphertext is not tamper-proof -- again,
encryption does not provide message authentication.


Another nitpick from your friendly neighborhood techno-geek.  I noticed
that signatures (when used) don't cover the headers.  Since the mint uses
header information to make important decisions (e.g. the userhdr.userID
field specifies who's account a deposit should go to), shouldn't this be
signed, just as a matter of ordinary everyday paranoia?


While I'm ranting, let me also remind you of a problem Ian discovered
earlier through reverse-engineering: the payment & deposit messages aren't
encrypted, and they send
	payee (e.g. shop) identity
	payer's and payee's banks
	payer's currency-of-choice
	amount of money transferred
	description of the product
	time of payment
in the clear, accessible to any passive eavesdropper.

With traffic analysis, if payers use the default TCP connection, all
this information about them can be compiled.  If I target a payer, I'll
probably be able to record all his transactions (unless he's using
remailers or pipenet).  If I sit outside a small business, I can compile
a dossier on its buying habits.

Worse still, anonymity for the shop is worse with Digicash than with real
cash.  If I pay you real cash on a secluded street, you're fairly anonymous.
If I pay you Digicash over the Internet, any passive eavesdropper could be
recording your identy and the whole transaction.  Blech.

So Digicash's product does all sorts of neat crypto to provide you with
anonymity from the bank, but doesn't do much to provide anonymity from
eavesdroppers on the Internet.

This is exactly backwards from how I'd prioritize -- I'll trust my bank
to keep me private long before I'll trust messages sent in the clear
over the Internet (on a virtual postcard) for all to see!  Blech.


So, what should Digicash be doing to remedy these problems?

* always set up an encrypted, authenticated connection before sending
  any messages in the Digicash protocol.

  (Yes, this means shops will need to have certificates if you want
  to avoid a man-in-the-middle attack.  So be it.  Most online shops
  will be using SSL, and thus have a certificate anyhow.  You can safely
  punt on the authentication between customer <-> shop if you're not
  worried about active attacks.)

* add a big warning to the documentation: users should not use wildcards
  in payments (unless they know the dangers & are encrypting with e.g. PGP).

* sign the header stuff too.

* continue specifying the protocol at a deeper level, like you promised
  (and throw in source for security-critical modules too, eh? :-)
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