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Challenge-response systems In so-called challenge-response systems, the sender of a message must prove she is a human (as opposed to a computer program) before the message is delivered. Like micropayment systems, challenge-response systems impose a cost on senders, but unlike micropayment systems, the cost is the human attention necessary to complete the challenge, rather than money. Although challenge-response systems can limit the amount of unwanted commu- nication, these systems have several disadvantages. One disadvantage is that they eliminate all automatically generated email messages, even when such messages are wanted. The typical way that this issue is handled is by having users specify a special email address to avoid false positives. Moreover, the need to complete a challenge may annoy and discourage some legitimate senders, as has been observed with captchas today [166]. Finally, if challenge-response systems were widely deployed, their de- signers could face an arms race to develop challenges that can be easily answered by most human users, yet cannot be answered by a program. Legislation Some countries have passed legislation that mandates a penalty for those sending unwanted communication. There is little evidence that it has significantly reduced the level of email spam received by users. There are at least two reasons why such laws have had limited impact. First, spammers are using technical means to obscure 52PDF Image | Online Social Networks: Measurement, Analysis, and Applications to Distributed Information Systems
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