Cryptographic hash function

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A cryptographic hash function, also sometimes called a digest function, is a {{glossary("cryptography", "cryptographic")}}  primitive transforming a message of arbitrary size into a message of fixed size, called a {{glossary("digest")}} . Cryptographic hash functions are used for authentication, {{Glossary("digital signature", "digital signatures")}} , and {{Glossary("HMAC", "message authentication codes")}} .

To be used for cryptography, a hash function must have these qualities:

• quick to compute (because they are generated frequently)
• not invertible (each digest could come from a very large number of messages, and only brute-force can generate a message that leads to a given digest)
• tamper-resistant (any change to a message leads to a different digest)
• collision-resistant (it should be impossible to find two different messages that produce the same digest)

Cryptographic hash functions such as MD5 and SHA-1 are considered broken, as attacks have been found that significantly reduce their collision resistance.

See also

• Cryptographic hash function on Wikipedia
• Related glossary terms:
  • {{Glossary("Symmetric-key cryptography")}}