Which of the following represents a characteristic of a hashing function?

A hashing function is designed to take an input (or 'message') and produce a fixed-size string of bytes that appears random, commonly referred to as a 'digest.' One of the essential characteristics of a hashing function is that it should produce a unique digest for unique inputs. This means that different inputs should ideally result in different hash values, making it easy to verify data integrity.

This property of producing a unique hash is crucial for many applications such as data verification, digital signatures, and integrity checks. If two different inputs produced the same hash (known as a collision), it would undermine the reliability of the hashing function. Thus, the ability to generate unique digests for unique inputs is a defining aspect of effective hashing functions and is essential for their security properties.

The other options refer to attributes associated with encryption rather than hashing, such as reversibility or the use of keys, which are irrelevant in the context of hashing. Hashing is a one-way function that does not allow for the retrieval of original data from the hash, reinforcing the notion that option B accurately encapsulates the distinguishing characteristic of a hashing function.