AddRoundKey

AddRoundKey is a fundamental operation in the Advanced Encryption Standard (AES) cipher. In AES, the 128-bit state is mixed with a 128-bit round key by a bitwise exclusive OR (XOR). This operation is simple and highly efficient in both software and hardware.

In the AES encryption process, AddRoundKey is applied multiple times. An initial AddRoundKey is performed before the first round using the first round key derived from the cipher key. Then, for each of the Nr−1 rounds, the algorithm applies SubBytes, ShiftRows, and, if not the last round, MixColumns, followed by AddRoundKey. In the final round, MixColumns is omitted, and SubBytes, ShiftRows, and AddRoundKey are executed. The round keys used in these steps come from the Rijndael key schedule, which expands the original key into a series of round keys.

Key properties of AddRoundKey include its linearity and invertibility. Since it is implemented as XOR, it is reversible simply by applying the same operation again with the same round key. The security contribution of AddRoundKey lies in combining the current state with the corresponding round key at each round, ensuring that the effect of the key material propagates through the cipher alongside the nonlinear SubBytes operation and the diffusion steps.

In practice, AddRoundKey is a core building block of AES and is widely supported by hardware accelerators and software implementations. While essential for security, it is the combination of AddRoundKey with the other AES transformations that provides the overall cryptographic strength of the algorithm.