HashCash 1.09 Description:

Hashcash is a software solution that acts like a denial-of-service counter measure tool. Its main current use is to help hashcash users avoid losing email due to content based and blacklist based anti-spam systems.

A hashcash stamp constitutes a proof-of-work which takes a parameterizable amount of work to compute for the sender. The recipient can verify received hashcash stamps efficiently

Hashcash is a technological approach to reducing the impact of spam. Hashcash aims to make email more reliable. It is a companion technology which should be used with any anti-spam technology to avoid that anti-spam technology adversely affecting email reliability.

Whatever anti-spam technology you are using, you want it to be configured so that hashcash can bypass what ever filters and blocks it puts in place so that other hashcash users will be able to still reliably send you mail. Similarly as a sender you want to send hashcash to bypass such filters so that you can make your email as reliable as possible.

What does hashcash do?
Hashcash comes in the form of plugin software for mailers which adds hashcash stamps to sent email.

The hashcash plugin software inserts a X-Hashcash: header into the email headers section of the email the user sends.

What stops a spammer using hashcash?
Spammers can use hashcash too, however hashcash is bad news for spammers because the hashcash stamp takes your CPU some work to compute. To you as a normal user, with an entry level desktop or laptop class machine the CPU overhead per mail is negligible because you don't send that many mails; at worst your mail is delayed a few seconds before being sent on slow old hardware.

However to spammers this is a show-stopper: they want to send 10,000+ emails per minute down a DSL line bought with a stolen credit card quick before the account gets cancelled.


But won't spammers steal CPU time?
Spammers already compromise security on many users machines to make so-called "Zombie" armies to send spam from. However currently the rate at which spammers can send mail on a zombie machine is limited purely by the speed of those machine's internet links.

A typical DSL user might be able to send 25 unique messages per second each of size 1KB (assumes 256kbit uplink). Or many more messages per second if the messages are delivered to multiple users at once (using multiple Cc or Bcc recipients).

Even a 20-bit stamp takes 1/2 second per recipient on the highest end pc hardware at time of writing. This would slow spammers down by a factor of 10-100 or more per compromised machine (depending on whether the messages sent are sent individually or to many users at once).


But won't spammers deliver to many recipients at once?
Spammers commonly optimize the amount of spam they can send over a given link speed by delivering messages to 100s or 1000s of Bcc recipients at once directly to an end-site, or to an ISP mail-hub.

In this way they can consume just 3.5KB of bandwidth in sending messages to 100 recipients compared to the 100KB which would be used to send each message separately. This would allow a spammer to send 700 messages per second (assumes DSL with 256kbit uplink).

Delivering in batches reduces the degree of customization the spammer can make because all of the message bodies in a batch have to be the same, but never-the-less is a trick spammers commonly use to increase the number of mails per second they can send.

However with hashcash a separate stamp is required for each individual recipient, which stops this spammer trick. If the spammer has to put a hashcash stamp for each recipient, even a 3Ghz Pentium 4 can only generate 2 stamps per second, compared to 700 per second with no hashcash, so using hashcash in this scenario slows the number of mails the spammer can send by 350x.

How does this make my mail more reliable?
ISPs and recipients who use anti-spam technologies such as keyword filtering, known spammer blacklists, missing reverse-DNS checks, etc are starting to use hashcash as an anti-spam exemption mechanism.

Your mail has a form of postage on it -- the hashcash stamp -- and sails through anti-spam check-points. This helps reliability because the spam-detectors are busy and error prone and frequently block lots of non-spam.

Which anti-spam technologies currently exempt mails with hashcash postage?
Hashcash is supported in SpamAssassin as of version 2.70. SpamAssassin is a popular user and ISP anti-spam tool to add hashcash support. SpamAssassin supports keyword filtering (and other techniques) to weed out spam. If you look in your mail headers for X-Spam-Checker-Version: SpamAssassin then mail you receive is being examined by SpamAssassin.

Hashcash is also supported by TMDA and CAMRAM. This means by sending hashcash on your mails you can virtually eliminate your chances of getting a false positive and hence of the mail you send not getting delivered, or getting delivered into a junk folder where the receiving ISP or user is using SpamAssassin, TMDA or CAMRAM. The number of hashcash supporting systems is growing.

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HashCash security information

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