Escalating The War On SPAM Through Practical PoW Exchange

Proof-of-work (PoW) schemes have been proposed in the past. One prominent system is HASHCASH which uses cryptographic puzzles. However, work by Laurie and Clayton (2004) has shown that for a uniform PoW scheme on email to have an impact on SPAM, it would also impact on senders of legitimate email. A targeted cost PoW scheme on email is proposed as a potential solution to this problem. By selectively applying the cost to SPAM it has the potential to limit SPAM, without unduly penalizing legitimate senders. Moreover, it is constructed using only current SPAM filter technology, and a small change to the SMTP (simple mail transfer protocol). Specifically, it is argued that such a system can make sending SPAM 1,000 times more expensive than sending legitimate email (so called HAM). Also, unlike the system proposed by Liu and Camp, it does not require the complications of maintaining a reputation system.