# Hash filter

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Hash_filter
> Markdown URL: https://mediated.wiki/source/Hash_filter.md
> Source: https://en.wikipedia.org/wiki/Hash_filter
> Source revision: 1291686392
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

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A '''hash filter''' creates a [hash sum](/source/hash_sum){{clarify|date=May 2016}} from data{{clarify|date=May 2016}}, typically [e-mail](/source/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](/source/function_(computer_science)) 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](/source/e-mail_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](/source/image_spam)).

==See also==
* [Bloom filter](/source/Bloom_filter)
* [Hash buster](/source/Hash_buster)
* [Locality-sensitive hashing](/source/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

{{security-software-stub}}

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Adapted from the Wikipedia article [Hash filter](https://en.wikipedia.org/wiki/Hash_filter) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Hash_filter?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.