Explain compression function

Question

Give a brief explanation of the term compression function.

Answer 1

Damg'rd and Merkle greatly affected cryptographic hash function design by defining the hash function in terms of what is named as compression function. A compression function acquires a fixed length input and returns out shorter, fixed-length output. Then the hash function can be defined by the means of repeated applications of compression function till the whole message has been processed. In this procedure, a message of arbitrary length is broken in blocks of a certain length which is only depends on the compression function, and ‘padded’ (for security reasons) so that the size of message is the multiple of block size. The blocks are then processed sequentially, taking as input the result of hash so far and the current message block, with final output being the hash value for message.