Stored Hashcash

Why?

1) The supermajority of spam is sent through systems which can be operated by an attacker but which are not actually owned by them. This includes, most prominently, botted-up residential PCs. Prior to wide availability of always-on high speed Internet it included email servers which were insufficiently locked-down. Hashcash does not meaningfully affected the economics of botnets.

2) The most successful anti-spam measure in history makes direct use of a consequence of #1: since mail servers do not move in nature that much, and legitimate mail systems do not frequently mix outgoing spam and outgoing ham, you can make IP-based reputational systems. If a residential IP starts sending email in large amounts, assume spam. If a novel IP starts sending email, treat as suspiciously smelling ham until they've demonstrated sufficient history, but flip the spam bit if/when they get aggressive. If a new MSA springs up, ensure their industry veterans at the helm understand the importance of their anti-abuse team, and tell them explicitly that their IPs are dead if they don't.

This worked fantastically well. It's the primary anti-spam measure which protects your inbox. (No. I know you think Bayesian filters are. They're more expensive to operate at scale, are virtually unusable by the common-denominator email user, and don't solve any problem better than IP reputation does.)

3) Hashcash never caught on in part because the people who care most about spam also care most about sending billions of emails. "We make it economically unattractive to send billions of emails" is a non-starter for them. You can guess who I'm talking about by taking a look at any user database and observing what percentage of email addresses in it terminate with the top, oh, five domain names. (Interestingly, email is a P2P protocol at the server level which is best described as "All peers are equal, in that they will be equally squashed beneath the boots of our governing oligopoly if they misbehave.")

Note that none of hashcash's problems are solved by "And now it can be stored for later."

Source: My first engineering job was as an anti-spam researcher.

Illegitimate activities are often carried out in a parasitic manner using infected and hijacked equipment for which the spammer and attacker don't have to pay. This will do little to hinder those tactics.

It depends on the rules of the system. If the system arbitrarily limits the creation of hashcash to 2.5 per minute and limits the total in circulation to 21 M then it could be worth a lot (although with high volatility). Without such limits each unit of hashcash would be worth almost nothing, but the overall system might be more efficient and thus worth more.

Also, if the system limits itself to 7 transactions per second it may not be useful for anti-spam or bandwidth accounting.

I've long been a fan of a "bounty" system -- you attach digital coins to a message, with the recipient specifying how many coins must be attached (maybe higher for voice, lower for email, mid level for SMS). If the message was unwanted (not merely spam, but even just a stupid request), the recipient collects the coins; otherwise, the sender gets them back.

This nicely aligns incentives. If the recipient takes coins improperly, you stop sending him mail.

We already see this kind of thing in CAPTCHA farms.

(X) technical ( ) legislative (X) market-based ( ) vigilante

approach to fighting spam. Your idea will not work. Here is why it won't work. (One or more of the following may apply to your particular idea, and it may have other flaws which used to vary from state to state before a bad federal law was passed.)

( ) Spammers can easily use it to harvest email addresses

(X) Mailing lists and other legitimate email uses would be affected

( ) No one will be able to find the guy or collect the money

( ) It is defenseless against brute force attacks

( ) It will stop spam for two weeks and then we'll be stuck with it

( ) Users of email will not put up with it

( ) Microsoft will not put up with it

( ) The police will not put up with it

( ) Requires too much cooperation from spammers

(X) Requires immediate total cooperation from everybody at once

(X) Many email users cannot afford to lose business or alienate potential employers

( ) Spammers don't care about invalid addresses in their lists

( ) Anyone could anonymously destroy anyone else's career or business

Specifically, your plan fails to account for

( ) Laws expressly prohibiting it

( ) Lack of centrally controlling authority for email

( ) Open relays in foreign countries

( ) Ease of searching tiny alphanumeric address space of all email addresses

(X) Asshats

( ) Jurisdictional problems

(X) Unpopularity of weird new taxes

(X) Public reluctance to accept weird new forms of money

(X) Huge existing software investment in SMTP

( ) Susceptibility of protocols other than SMTP to attack

(X) Willingness of users to install OS patches received by email

(X) Armies of worm riddled broadband-connected Windows boxes

(X) Eternal arms race involved in all filtering approaches

(X) Extreme profitability of spam

( ) Joe jobs and/or identity theft

( ) Technically illiterate politicians

( ) Extreme stupidity on the part of people who do business with spammers

( ) Dishonesty on the part of spammers themselves

( ) Bandwidth costs that are unaffected by client filtering

(X) Outlook

and the following philosophical objections may also apply:

(X) Ideas similar to yours are easy to come up with, yet none have ever been shown practical

( ) Any scheme based on opt-out is unacceptable

( ) SMTP headers should not be the subject of legislation

( ) Blacklists suck

( ) Whitelists suck

( ) We should be able to talk about Viagra without being censored

( ) Countermeasures should not involve wire fraud or credit card fraud

( ) Countermeasures should not involve sabotage of public networks

( ) Countermeasures must work if phased in gradually

(X) Sending email should be free

( ) Why should we have to trust you and your servers?

( ) Incompatiblity with open source or open source licenses

( ) Feel-good measures do nothing to solve the problem

( ) Temporary/one-time email addresses are cumbersome

( ) I don't want the government reading my email

(X) Killing them that way is not slow and painful enough

Furthermore, this is what I think about you:

(X) Sorry dude, but I don't think it would work.

( ) This is a stupid idea, and you're a stupid person for suggesting it.

( ) Nice try, assh0le! I'm going to find out where you live and burn your house down!

We abstract out the computational requirements into a sub-penny but not insignificant payment.

Then again, how much would spammers be willing to pay?