Cryptographic hash function

A cryptographic hash function at work. Even small changes in the source input (here in the word "over") drastically change the resulting output, by the so-called avalanche effect

A cryptographic hash function is a hash function which takes an input (or'message') and returns a fixed-size alphanumeric string, which is called the hash value (sometimes called a message digest, a digital fingerprint, a digest or a checksum).

The ideal hash function has three main properties:

1. It is extremely easy to calculate a hash for any given data.
2. It is extremely computationally difficult to calculate an alphanumeric text that has a given hash.

A hash function takes a string of any length as input and produces a fixed length string which acts as a kind of "signature" for the data provided. In this way, a person knowing the "hash value" is unable to know the original message, but only the person who knows the original message can prove the "hash value" is created from that message.

Other pages

• Avalanche effect

Further reading

• Bruce Schneier. Applied Cryptography. John Wiley & Sons, 1996. ISBN 0-471-11709-9.

Other websites

• Hash'em all! – free online text and file hashing with different algorithms
• The Hash function lounge – a list of hash functions and known attacks
• Helger Lipmaa's links on hash functions
• Diagrams explaining cryptographic hash functions
• An Illustrated Guide to Cryptographic Hashes by Steve Friedl
• Cryptanalysis of MD5 and SHA: Time for a New Standard by Bruce Schneier
• Hash collision Q&A
• Attacking hash functions by poisoned messages (construction of multiple sensible Postscript messages with the same hash function)
• What is a hash function? from RSA Laboratories
• Password Hashing in PHP by James McGlinn at the PHP Security Consortium
• The code monkey's guide to cryptographic hashes by Val Henson, "in language that any programmer (and even some managers) can understand."