In a method for testing a transport packet decrypting module of a client device, a first decryption operation of the transport packet decrypting module is implemented on a test encrypted control word using a content decryption key ladder to derive a test control word, a second decryption operation of the transport packet decrypting module is implemented on one or more test transport packets using the test control word via a predetermined content decryption algorithm, the KIV is derived from the decrypted transport packets, and the derived KIV is compared with a value stored in the client device to verify whether the transport packet decrypting module of the client device is functioning properly.