1. A receiver for securely obtaining a control word, the receiver comprising: a first memory configured for storing a transform function configured to receive a transformed control word and a seed and to migrate the transformed control word from an input transform space to an output transform space to obtain the control word using a mathematical transformation under control of the seed, the receiver further comprising a cache memory and a cache control module, wherein the cache control module is configured to: intercept the transformed control word and the seed; search in the cache memory for the control word matching the transform function, the transformed control word and the seed; if the control word is found in the cache memory, provide the control word to an output of the transform function thereby bypassing the transform function; and if the control word is not found in the cache memory, provide the control word and the seed to the transform function, obtain the control word from the transform function and store the control word associatively with the transform function, the transformed control word and the seed in the cache memory.