Hashcash is a proof-of-work system designed to limit email spam and denial-of-service attacks. It was proposed in March 1997 by Adam Back.[1]
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Hashcash is a method of adding a textual stamp to the header of an email to prove the sender has expended a modest amount of CPU time calculating the stamp prior to sending the email. In other words, as the sender has taken a certain amount of time to generate the stamp and send the email, it is unlikely that they are a spammer. The receiver can, at negligible computational cost, verify that the stamp is valid. However, the only known way to find a header with the necessary properties is brute force, trying random values until the answer is found; though testing an individual string is easy, if satisfactory answers are rare enough it will require a substantial number of tries to find the answer.
The theory is that spammers, whose business model relies on their ability to send large numbers of emails with very little cost per message, cannot afford this investment into each individual piece of spam they send. Receivers can verify whether a sender made such an investment and use the results to help filter email.
The header line looks something like this[2]:
X-Hashcash: 1:20:060408:adam@cypherspace.org::1QTjaYd7niiQA/sc:ePa
The header contains: the recipient's email address, the date, and information proving the required computation has been performed. The presence of the recipient's email address requires that a new header be computed for each recipient, and the date allows the recipient to record headers received recently and make sure the header is unique to this email.
The sender prepares a header and adds an initial random number. It then computes the 160 bit SHA-1 hash of the header. If the first 20 bits of the hash are zeros then this is an acceptable header. If not then the sender increments the random number and tries again. Since about 1 in 220 headers will have 20 zeros as the beginning of the hash the sender will on average have to try half that many (220 / 2 = 219 = 524288) random numbers to find a valid header. Given reasonable estimates of the time needed to compute the hash, this would take about 1 second to find. At this time no more efficient method is known to find a valid header.
A normal user on a desktop PC would not be significantly impacted by the processing time required to generate the Hashcash string. However, spammers would suffer a significant impact due to the high number of spam messages required.
Technically the system is implemented as follows:
All these tests take far less time and disk space than receiving the rest of the e-mail.
The time needed to compute such a hash collision is exponential with the number of zero bits. So one can keep adding zero bits (doubling the amount of time needed to send with each zero bit) until it is too expensive for spammers to generate valid header lines. (Confirming the header is valid always takes the same amount of time, no matter how many zero bits are required for a valid header.)
The Hashcash system has the advantage over micropayment proposals applying to legitimate email that no real money is involved. Neither the sender nor recipient need pay, thus the administrative issues involved with all micropayment systems are entirely avoided.
On the other hand, as Hashcash requires significant computational resources to be expended on each e-mail being sent, it is impractical to use with low-end embedded systems without the help of an external server.
Hashcash is also fairly simple to implement in mail user agents and spam filters. No central server is needed. Hashcash can be incrementally deployed—the extra Hashcash header is ignored when it is received by mail clients that do not understand it.
Some plausible estimates[3] come to the conclusion that you can only have one of these: Either good e-mail will get stuck due to lack of processing power of the sender, or bad e-mail is bound to still get through. The reasons for this are botnets or cluster farms with which spammers can increase their processing power enormously, or centralized e-mail-topologies like mailing lists, in which some server is to send an enormous amount of legitimate e-mails.
Most of these issues may be addressed. E.g., botnets may expire faster because users notice the high CPU load and take counter-measures, and mailing list servers can be registered in white lists on the subscribers' hosts and thus be relieved from the hashcash challenges. But they represent serious obstacles to hashcash deployment that need to be addressed somehow.
Another projected problem is that computers continue to get faster according to Moore's law. So the difficulty of the calculations required must be increased over time. However, developing countries can be expected to use older hardware, which means that they will find it increasingly difficult to participate in the email system. This also applies to lower-income individuals in developed countries who cannot afford the latest hardware.
The Penny Post software project[4] on SourceForge implements Hashcash in the Mozilla Thunderbird email client.[5] The project is named for the historical availability of conventional mailing services that cost the sender just one penny; see Penny Post for information about such mailing services in history.
Like e-mail, blogs often fall victim to comment spam. Some blog owners have used hashcash scripts written in the JavaScript language to slow down comment spammers.[6] Some scripts (such as wp-hashcash) claim to implement hashcash but instead depend on JavaScript obfuscation to force the client to generate a matching key; while this does require some processing power, it does not use the hashcash algorithm or hashcash stamps.