Some Unix systems use a very simple cipher called _________.

ROT13 (rotate by 13 places) is a well-known, simple letter substitution cipher often found or referenced in Unix environments, primarily for obscuring text in online forums or email to prevent casual reading, rather than for strong cryptographic security. It shifts each letter 13 places forward in the alphabet.

SOT14 is not a recognized simple cipher used in Unix systems.

DES (Data Encryption Standard) is a block cipher that is much more complex and computationally intensive than a simple substitution cipher like ROT13, and while used on Unix, it's not described as a 'very simple cipher' in the same vein.

'Block' is a category of ciphers (block cipher), not a specific simple cipher itself.

'Stream' is a category of ciphers (stream cipher), not a specific simple cipher itself.