Abstract: A device and method of encrypting a sequence. The method (300) encrypts data (Di) over a sequence of encryption periods (Ti) by generating (315) a sequence of forward encryption keys (GKfi) each associated with a respective encryption period (Ti). Each forward encryption key is generated recursively by applying a forward one way function (FFi?1) to the forward encryption key (GKFi?1) associated with the preceding encryption period (Ti?1). Next the method (300) generates (320) a sequence of reverse encryption keys (GKRi), each associated with a respective encryption period (Ti), each reverse encryption key being generated recursively by applying a reverse one way function (FRi?1) to the reverse encryption key (GKRi+1) associated with the subsequent encryption period (Ti+1). Encrypting (325) the data (Di) for each encryption period (Ti) with a respective forward encryption key (GKFi) and a respective reverse encryption key (GKFi) is then performed.