An embodiment of a method is disclosed for protecting a key from discovery during decryption of a data stream. This embodiment of the method includes decrypting the data stream with the key. Before completing decryption of the data stream, the method checks consistency between a decrypted portion of the data stream and expected data using a circuit arrangement. In response to an inconsistency between the decrypted portion and the expected data, a tampering signal is generated to indicate tampering is suspected.