Which cipher encrypts the plain text digit (bit or byte) one by one?

Classical ciphers (e.g., Caesar, Vigenere) use substitution or transposition on characters but are not defined by bit-by-bit or byte-by-byte processing in the modern cryptographic sense.

Block ciphers encrypt data in fixed-size blocks (e.g., 128 bits for AES), not one digit at a time.

Modern cipher is a broad category encompassing both block and stream ciphers and is not a specific cipher type defined by per-digit encryption.

Stream ciphers operate by generating a pseudorandom keystream and XORing it with each individual plaintext bit or byte sequentially. This one-at-a-time processing makes stream ciphers well suited for real-time or continuous data transmission where low latency is required. RC4 is a classic example of a stream cipher that operates on bytes individually.