An efficient symmetrical-cryptographic method for using a fast but insecure host to perform encryption/decryption based on a secret key in a secure, but slow hardware token, such as a smartcard or similar device, without revealing the secret key to the host, and such that the ciphertext and plaintext are exactly the same size. The present method is suitable for use in Digital Rights Management and Software Rights Management applications which require precise interchangeability of ciphertext and plaintext in pre-allocated areas of data storage.