An apparatus for installing a decryption key is initially arranged so that a decryption algorithm received by a control processor is passed via a first interface path to a decryption processor. The algorithm is installed in the decryption processor together with a program decryption key. The apparatus is subsequently arranged so that encrypted working decryption keys received by the control processor are passed on to the decryption processor over the first interface path. The decryption processor decrypts the encrypted keys using the program decryption key. Decryption keys are thus obtained and are transferred via a second interface path to decryptors for use in decrypting encrypted program signals input to the decryptors.