1. A cryptography apparatus, comprising: multiple multiplication units, which are arranged in two or more multiplication levels, and which are configured to operate in accordance with Galois-Field (GF) arithmetic over respective Galois fields; and logic circuitry, which is configured to receive input data whose word-size exceeds a maximal input word-size among the multiplication units, to hold a cryptographic key comprising multiple sub-keys whose number does not exceed a number of the multiplication units, and to perform a cryptographic operation on the input data by applying the sub-keys to the multiplication units.