Patent ID: 7191335

Claim:
A method of encryption of data in a digital television system communicated between a first decoder and a portable security module operatively connected to the first decoder on a receiving side of the digital television system, comprising: storing a plurality of precalculated key pairs in a memory of the first decoder, each of said plurality of precalculated key pairs comprising a session key and an encrypted session key prepared using a transport key, selecting and processing at least one session key to generate a definitive session key, wherein the definitive session key is generated by repeatedly encrypting an initial session key value known to both the first decoder and the portable security module in both devices using an ordered sequence of session keys and an encryption algorithm sensitive to an order of encryption; and communicating the ordered sequence of session keys and an encrypted version of said at least one session key to the portable security module, wherein the portable security module is configured to use the ordered sequence of session keys and the transport key to decrypt the encrypted version of the at least one session key to obtain the definitive session key, wherein the transport key is stored in a memory associated with the portable security module; wherein data communicated from at least the portable security module to the first decoder may thereafter be encrypted and decrypted by the definitive session key.