A transmitting device (100) transmits a first packet, comprising a second encryption vector, that is encrypted using a first encryption vector. The transmitting device transmits a second packet that is encrypted using the second encryption vector if an acknowledgement message is received within a predetermined time after transmitting the first packet; otherwise, the first packet is re-transmitted. Upon receipt of the first packet, a receiving device (102) decrypts the first packet using the first encryption vector and transmits the acknowledgement message. Upon receipt of the second packet; the receiving device attempts to decrypt a portion of the second packet using the first and second encryption vectors. If the portion of the second packet was successfully decrypted using the first encryption vector, the receiving device re-transmits the acknowledgement message for the first packet; otherwise, it transmits an acknowledgement message for the second packet.