Tiivistefunktion

Tiivistefunktion is a Finnish term that translates to "compression function" in English. It is a fundamental component in the field of cryptography, particularly in the construction of hash functions. A compression function takes a fixed-size input block and an internal state, and produces a new internal state. This process is typically repeated iteratively for variable-length messages.

The core purpose of a compression function is to distill a potentially large amount of data into a smaller, fixed-size output. This is achieved through a series of complex mathematical operations, often involving bitwise operations, modular arithmetic, and substitutions. The design of a compression function is critical for the security of the overall hash function. A good compression function should be:

One-way: It should be computationally infeasible to determine the input given the output.

Collision-resistant: It should be difficult to find two different inputs that produce the same output.

Pseudorandom: The output should appear random and unpredictable.

Common examples of cryptographic hash functions, such as SHA-256 and SHA-3, are built upon compression functions. The Merkle-Damgård construction is a widely used method for building hash functions from compression functions, where the message is divided into blocks and processed sequentially by the compression function, with the output of one stage serving as the input state for the next. The security properties of the hash function are directly inherited from the security properties of its underlying compression function.