A method of encryption, using an encryption key K with key length k, of at least one message M comprising uniformly distributed symbols, k bits are encrypted of messages at least k bits long, while shorter messages are lengthened, e.g. by padding or concatenation, to obtain a lengthened message at least k bits long before encryption. The encryption efficiency is thus optimized while the encryption security is retained. The encryption method is particularly suitable for JPEG2000 encoded packets comprising a message M. Also provided are an encryption apparatus, a decryption method and a decryption apparatus.