A cryptographic hash function, also sometimes called a digest function, is a cryptographic primitive transforming a message of arbitrary size into a message of fixed size, called a digest. Cryptographic hash functions are used for authentication, digital signatures, and 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.