A method of scrambling a stream of data includes obtaining from the stream a succession of first sequences of blocks of data. The order of the blocks is reversed in each of the first sequences of blocks to form respective second sequences of blocks of data. The blocks in each second sequence of blocks are encrypted using a cipher in block chaining mode, initialized with a respective initialization vector for each second sequence of blocks. For a succession of first sequences of blocks included in a unit of data within the stream, at least one initialization vector for encrypting a second sequence of blocks formed from a first sequence of blocks in the unit is generated in dependence on at least one block in a preceding first sequence of blocks of the unit.