A method for encryption and decryption of data items is provided by defining a cipher key based on variables in a Chaotic Equation. The method includes selecting a Chaotic Equation (110) from a set of Chaotic Equations, defining starting conditions of the variables of the equation (140), and applying the equation to each data item (120). The real and imaginary parts of the result of the iteration of the Chaotic Equation are combined with the data item by an arithmetic operation, for example, an XOR operation (120). Data items in a continuous stream with a rate dependency can be encrypted and decrypted on an item by item basis. The input or cipher key changes for each byte of the data encryption. Blocks of data (700, 701, 702, 703, 704) can be encrypted using the method with an identifier of the order of the blocks in the data stream. If blocks are received out of sequence, the identifiers can be used to maintain the correct decryption order. The method of encryption and decryption can be used in devices (801) to avoid the need for a session key. The continuously updating input or cipher key enables fraudulent use of devices (801) to be identified.