{{Refimprove|date=January 2008}}
A '''hash filter''' creates a hash sum{{clarify|date=May 2016}} from data{{clarify|date=May 2016}}, typically e-mail, and compares the sum against other previously defined sums. Depending on the purpose of the filter, the data can then be included or excluded in a function based on whether it matches an existing sum.
For example, when a message is received by an e-mail server with a hash filter, the contents of the e-mail are converted into a hash sum. If this sum corresponds to the hash sum of another e-mail which has been categorized as spam, the received e-mail is prevented from being delivered. Spammers attempt to evade this by adding random strings to the text content and random pixel changes ("confetti") to image content (see image spam).
==See also== * Bloom filter * Hash buster * Locality-sensitive hashing
==References== *[http://www.ianywhere.com/developer/product_manuals/sqlanywhere/1000/en/html/dbugen10/ug-queryopt-s-bloom.html Hash Filter algorithm (SQL Anywhere Server - SQL Usage)] {{Webarchive|url=https://web.archive.org/web/20071220002412/http://www.ianywhere.com/developer/product_manuals/sqlanywhere/1000/en/html/dbugen10/ug-queryopt-s-bloom.html |date=2007-12-20 }} at iAnywhere.com
Category:Hash functions
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