What function transforms an input into a fixed-length hexadecimal output, also known as a digest?

The function that transforms an input into a fixed-length hexadecimal output, known as a digest, is hashing. Hashing algorithms take data of arbitrary length and produce a hashed value that is typically much shorter and of a consistent size. This process is essential in various applications, such as ensuring data integrity, creating digital signatures, and storing passwords securely.

The output, or digest, generated through hashing is unique to the specific input data to a certain extent; even slight changes in the input will yield a vastly different hash. This property makes hashing widely used for verifying data integrity and authenticity. Examples of commonly used hashing algorithms include SHA-256 and MD5, each producing a fixed number of bits in their output, regardless of the input size.

Other options, while relevant in the field of cryptography, do not perform the hashing function. Block ciphers and stream ciphers are methods of encrypting data to secure it from unauthorized access, but they do not produce a hash or digest. Modes of operation refer to how block ciphers are applied in practice, which again relates to encryption rather than hashing specifically.