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