Which of the following functions provides an output which cannot be reversed and converts data into a string of characters?

Hashing algorithms such as SHA-256 produce a fixed-length digest from arbitrary input data using a one-way mathematical function. The output cannot be reversed to recover the original input, which distinguishes hashing from encryption.

Stream ciphers are reversible encryption algorithms that use a key to both encrypt and decrypt data, so the output can be reversed given the key.

Steganography is a technique for hiding data within another file or medium, not a function that converts data into a fixed-length irreversible string.

Block ciphers are symmetric encryption algorithms that process fixed-size blocks of data and are fully reversible using the appropriate decryption key.