A device and method of encrypting a sequence. The method (300) encrypts data (D.sub.i) over a sequence of encryption periods (T.sub.i) by generating (315) a sequence of forward encryption keys (GK.sup.f.sub.i) each associated with a respective encryption period (T.sub.i). Each forward encryption key is generated recursively by applying a forward one way function (F.sup.F.sub.i-1) to the forward encryption key (GK.sup.F.sub.i-1) associated with the preceding encryption period (T.sub.i-1). Next the method (300) generates (320) a sequence of reverse encryption keys (GK.sup.R.sub.i), each associated with a respective encryption period (T.sub.i), each reverse encryption key being generated recursively by applying a reverse one way function (F.sup.R.sub.i-1) to the reverse encryption key (GK.sup.R.sub.i+1) associated with the subsequent encryption period (T.sub.i+1). Encrypting (325) the data (D.sub.i) for each encryption period (T.sub.i) with a respective forward encryption key (GK.sup.F.sub.i) and a respective reverse encryption key (GK.sup.F.sub.i) is then performed.