A _____ is a function that takes a variable-size input m and returns a fixed-size string.

A hash function is designed to take an input of variable length and produce a fixed-size output, known as a hash value or digest. This characteristic is essential for various applications, including data integrity verification, digital signatures, and password storage. The fixed size of the output allows for consistent storage and comparison, regardless of the size of the input data.

Hash functions utilize mathematical algorithms that are one-way, meaning they can easily convert input data into a hash output, but it is computationally infeasible to reverse the process and retrieve the original input from the hash. This property is what gives hash functions their security feature in many cryptographic applications.

The other concepts mentioned—symmetric ciphers, Feistel networks, and asymmetric ciphers—do not share the same defining feature of converting variable-sized inputs into fixed-size outputs. Symmetric and asymmetric ciphers are used for encrypting and decrypting data, while Feistel is a structure used in some symmetric ciphers but does not specifically define the function of producing a hash. Therefore, the defining nature of a hash function is what makes it the correct answer.