Abstract: Encrypted data transmitted over a data network is decrypted in accordance with a cipher decryption process using a Control Word transmitted with the data. The process is arranged to automatically identify the mode by which the data has been encrypted by identifying (66) the length of the Control Word transmitted with the encrypted data. The Control Word length may be identified (64) from metadata explicitly stating the length, or by analysis of the message in which the Control Word is conveyed. Different encryption modes using Control Words of different lengths can thus be identified, allowing the receiver to identify (67) which of a plurality of decryption modes available to the receiver should be used (69) to decrypt the data.