Which of the following is not a requirement for a hash function?

A hash function is designed to transform input data of arbitrary length into a fixed-size output, commonly referred to as a hash or digest. This transformation has certain characteristics that define a good hash function, but it does not necessitate a minimum key length of 256 bits.

The requirement of a minimum key length primarily relates to symmetric and asymmetric encryption algorithms rather than hash functions. Hash functions such as SHA-256 produce a fixed-length output (256 bits in this case) regardless of the length of the input data, but they do not inherently depend on keys in the same manner that encryption algorithms do.

In contrast, a good hash function does aim to have variable-length input and produces a consistent fixed-length output, ideally avoids collisions (two different inputs creating the same hash), and ensures that the process is non-reversible, meaning that one should not be able to derive the original input from the hash value. These characteristics are essential for ensuring the security and functionality of hash functions in various applications, such as integrity verification and digital signatures.