Q (cipher)

This article is about the block cipher. For other uses, see Q (disambiguation).

In cryptography, Q is a block cipher invented by Leslie McBride. It was submitted to the NESSIE project, but was not selected.

Q

General
Designers	Leslie McBride
First published	November 2000
Derived from	AES, Serpent
Cipher detail
Key sizes	128, 192, or 256 bits
Block sizes	128 bits
Structure	Substitution–permutation network
Rounds	8 or 9
Best public cryptanalysis
A linear attack succeeds with 98.4% probability using 297 known plaintexts.[1]

The algorithm uses a key size of 128, 192, or 256 bits. It operates on blocks of 128 bits using a substitution–permutation network structure. There are 8 rounds for a 128-bit key and 9 rounds for a longer key. Q uses S-boxes adapted from Rijndael (also known as AES) and Serpent. It combines the nonlinear operations from these ciphers, but leaves out all the linear transformations except the permutation.^[2] Q also uses a constant derived from the golden ratio as a source of "nothing up my sleeve numbers".

Q is vulnerable to linear cryptanalysis; Keliher, Meijer, and Tavares have an attack that succeeds with 98.4% probability using 2⁹⁷ known plaintexts.^[1]