On Feb 16, 2009, at 7:07 AM, mathew wrote:
On Sun, Feb 15, 2009 at 17:12, John Levine <johnl(_at_)taugh(_dot_)com> wrote:
>> It is possible to defend against this threat, but not cheaply,
since
>> the defense requires a robust transaction system that can serialize
>> the thousand requests, approve one, and reject the other 999, while
>> still providing service to the rest of their customers.
>Nonsense. You just make the purchased stamp dependent upon the
address of
>the recipient, for example by hashing the To: address inside the
>cryptographic stamp when it's minted.
Aw, come on. Please don't tell me I have to explain why this model
has
the exact same problem.
Go on, indulge us. Explain why stamps that are hashed to the
recipient can still be spent on multiple spams unless a network
transaction is carried out for every one.
Let's go ahead and assume that the stamp also has an expiry date
encoded into it, mmkay?
If it's simply hashed then anyone can create them. That means it's
possible to send a large number
of messages using stamps that look entirely plausible prior to them
being looked up at the central
broker. There are obvious reasons why people would do this.
So, the next step is to use some crypto such that it's possible for
anyone to validate that the stamp
may be plausible for the recipient, but not for anyone to generate it.
Maybe you use public key
signatures - presumably with the private key held solely by the bank.
But that means that stamps are not interchangeable. You can't buy them
or generate them in
advance, or at least not in bulk, in advance. Instead you have to
purchase them (from one of a
small number of "banks") at the time you send the mail as well as
redeem them (from that very
same bank) later.
(Given that the sending machine has to contact a central server and
the receiving machine
also has to contact the same central server during the transmission of
the message there are a lot
of other things you could do with it that are simpler than epostage).
Cheers,
Steve
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