Spammers usually charge you $US 250 to bombard about 10 million email addresses with your ad. (See: The Cost of Spam ) We'd like to increase this cost, by making them do some computation every time they change their "From" address. This proposal will make it cheap to be anonymous, or cheap to mass-email, but not cheap to do both at the same time.
When spammers send out spam, they fill the "From" address with lots of different bogus email addresses - even addresses that don't exist. If they only used a single email address, it would be much easier to detect them (and everyone could blacklist spam addresses). In contrast, normal people don't usually need to send from more than one (or, to be conservative, five) addresses. This is the key to eliminating spam.
We propose that all emails contain a moderately-hard-to-compute (1 minute, say?), but easy-to-verify token that is a function of the sender's and receiver's email addresses. For the spammer, this means a cost of 1 minute of computation per person every time they change email address. For the normal human, 1 minute of computation is incurred "in the background" each time the human makes a new friend.
So, how do the costs for normal users and spammers stack up?
Firstly, how much computation does US$0.01 buy? Assuming a computer is up-to-date for 1 year, and costs US$500, US$0.01 buys (0.01 / 500) year worth of computation, which is about 10 minutes worth. This isn't including real-estate, power and maintenance costs.
Spammers get to choose how many times to reuse an email address. Let's suppose that they can send you 5 different spams before you notice and blacklist them. That means they need to do 1 minute / 5 = 10 seconds of computation per spam they send you. That's 125 days for 1 million people. That costs about US$150 per spam-out. Unfortunately, this isn't very expensive, relative to the price spammers are prepared to pay :( (Are my estimates wrong?)
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