RotWord

RotWord is a permutation operation used in the AES (Rijndael) key schedule. It operates on a 4-byte word and rotates the bytes to the left by one position, with the first byte wrapping to the end. In formal terms, RotWord([b0, b1, b2, b3]) = [b1, b2, b3, b0]. It does not alter the individual byte values, only their positions.

In the AES key expansion process, RotWord is part of the transformation applied to generate round keys from the previous key material. Specifically, for the 4-byte words that form a round key, RotWord is applied to a word, then SubWord (which applies the AES S-box to each byte), and the result is combined with a round constant (Rcon) through XOR operations. This sequence helps introduce diffusion and nonlinearity into the expanded keys.

Example: RotWord([0x09, 0x0A, 0x0B, 0x0C]) yields [0x0A, 0x0B, 0x0C, 0x09].

RotWord is a fixed-width operation defined for 4-byte words in AES, and it functions alongside SubWord and Rcon as part of the key schedule. It is not a standalone cryptographic primitive, but a component that contributes to the generation of secure round keys used in encryption and decryption.