An apparatus and corresponding method and computer program comprises a key mixing circuit to generate N groups of AES Blocks, each generated based upon a key, a transmitter MAC address, and a start value for a packet number. An input circuit receives N encrypted MPDUs, each comprising the transmitter address and one of N values for the packet number. Each of the N values for the packet number is greater than, or equal to, the start value for the packet number. A decryption circuit decrypts each of the N encrypted MPDUs using the respective one of the N groups of AES Output Blocks. The key mixing circuit generates each of the N groups of AES Output Blocks before the input circuit receives the respective one of the N encrypted MPDUs. The predetermined start value is initialized when the apparatus is initialized and incremented when each of the N groups of AES Output Blocks is generated.