Deciphering and verification of the checksum of enciphered and encapsulated UDP datagrams, particularly those which enclose a tunneling protocol such as L2TP, are achieved by the provision of a checksum verifier in parallel with a decipher block. Checksum logic creates a pseudo UDP header needed for checksum verification using fields that would occur at the start of the packet that encapsulates the UDP datagram. The first part of the packet to be deciphered is the UDP header; checksum logic can latch the checksum field into a local register. As the rest of the packet is deciphered the checksum verifier processes the data at the same time. Eventually the checksum logic will acquire a complete checksum which can be compared with the checksum that had been previously latched, so as to verify the checksum.