Proposals Re: DoS and Replay protection for message signatures

[Amir Herzberg <herzbea@xxxxxxxxxxxxxx>]

Hallam-Baker, Phillip wrote:
> Surely what an optimized implementation would do is look to see if the
> reputation is in the cache, if so and the reputation is bad then throw
> out the message and stop processing.
>
> Otherwise verify the signature and only look up the reputation if it
> verifies.
Would you reduce reputation if you get multiple signature verification 
failures? Up to throwing messages without validating signatures? I think 
that's what you (and Tony) suggest.

If so... can't an attacker abuse this to perform DoS on _senders_, by 
thrashing their reputation by sending malsigned messages (also hitting 
some recipients at the same time)?

If not... then the DoS attack of sending mal-signatures remains.

We can solve this by placing the burden on the senders. Add a `recipient 
policy` record for the _recipient_ (in a DNS record). This record 
identifies what the recipient requires from incoming messages, to 
prevent them being thrashed as DoS. Some proposed options:

-- No DoS protection required (current DMIK)

-- `Anti-DoS computation cookie`: specifies a hash-cash like challenge 
<x,k> s.t. the sender must find string y s.t. h(m, x, y)=z||0^k, where m 
is the (digest of?) message and h is a hash function e.g. SHA1.

-- `Anti-DoS IP-based cookie`: specifies an IP address of a `cookie 
server`, to which the sending MTA should send a request, and get back a 
cookie to be added to the message (like in IKE).

-- `Anti-DoS 3rd-party cookie`: specifies one or more third party 
DoS-prevention servers, which share a key with the recipient. Sender can 
contact any of them, send the the signature, and get back a cookie 
(which is a MAC over the signature and/or original message, and the 
identity of recipient). This may require some long term relationship 
with 3rd party and/or payment to 3rd party. Notice that with this 
option, it may sometimes be possible to avoid validating the signature 
at all in the recipient.
--
Best regards,

Amir Herzberg

Associate Professor
Department of Computer Science
Bar Ilan University
http://AmirHerzberg.com
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