Ico Services Limited

Thomas Francis Johnston

A satellite mobile telecommunications system includes mobile terminals which can communicate with one another using end-to end encryption and decryption techniques. When secure end-to-end communication is required, each terminal uses a common encryption code (RAND) to encode data and decode data transmitted between the terminals. The encryption code is transmitted in a secure manner from a remote database station ( ) to the terminals. Each terminal stores a terminal key (K , K ) on its SIM card and the keys are also held in the remote station ( ). Partial keys (K , K ) comprising the pseudo random number (RAND) and the keys K , K stored at the station ( ) are produced at the station ( ) by an exclusive OR process in order to mask the keys and the random number. The partial key K &equals;K &plus;(RAND) is sent to terminal At the terminal the partial key K is exclusive OR-ed with the locally stored terminal key K on the SIM card, so as to recover (RAND). The common code (RAND) is determined by the same process at terminal from K &equals;K &plus;(RAND) and the locally stored key K . The terminals then both run a GSM encryption algorithm (A5) to encrypt and decrypt transmitted data, on the basis of the common code (RAND).

Growing community of inventors

London, United Kingdom

Thomas Francis Johnston