# Unkeyed Hash Functions (One-Way Hash Functions)

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## Description

A hash function, otherwise known as a one-way hash function, takes an arbitrary message of arbitrary length and creates an output (a hash) of a fixed length. The main characteristics of a cryptographic hash function are that given a message, it is easy to compute the hash; given the hash, it is difficult to compute the message; and that given a message, it is difficult to find a different message that would produce the same hash (this is known as a collision).

An unkeyed hash function takes just a message as input and outputs a fixed-length hash.

## Related Problems

Related: Keyed Hash Functions

## Parameters

n: length of message

## Table of Algorithms

Name | Year | Time | Space | Approximation Factor | Model | Reference |
---|---|---|---|---|---|---|

MD5 | 1991 | $O(n)$ | $O({1})$ auxiliary? | Exact | Deterministic | |

SHA-1 | 1993 | $O(n)$ | $O({1})$ auxiliary? | Exact | Deterministic | |

RIPEMD-160 | 1996 | $O(n)$ | $O({1})$ auxiliary? | Exact | Deterministic | |

bcrypt | 1999 | $O(n)$ | $O({1})$ auxiliary?? | Exact | Deterministic | |

Whirlpool | 2000 | $O(n)$ | $O({1})$ auxiliary? | Exact | Deterministic | |

SHA-2 | 2001 | $O(n)$ | $O({1})$ auxiliary? | Exact | Deterministic | |

SHA-3 | 2015 | $O(n)$ | $O(b+d)$ auxiliary? | Exact | Deterministic |