Which type of function distill the information contained within a file (small or large) into a single fixed-length number, typically between 128 and 256 bits in length?

a. message-digest

b. private-key

c. random permutation

d. public-key?

As far as I remember, message-digest functions distill the information contained within a file (small or large) into a single fixed-length number, typically between 128 and 256 bits in length. This function is designed to protect the integrity of a piece of data to define changes to any part of a message.