A system for encryption, and subsequent decryption, of encoded data allows for transcoding of the encrypted data. The data is encoded in such a way that different packets have different importance levels, so that some or all of the packets at the lower importance levels can be discarded or truncated in order to reduce the data rate. This is achieved by introducing dependencies into the encoding process. The packets at the highest importance level are encoded with reference only to other packets at the highest importance level, while the encoding of packets at lower importance levels also depend on the encoding of the packets at the highest importance level. The encoded data is then encrypted in such a way that the encryption process has dependencies that correspond to the dependencies in the encoding process. Packets at the highest importance level are encrypted with reference to other encrypted packets at the highest importance level, while the encryption of packets at lower importance levels also uses the results of encrypting the packets at the highest importance level.