A digital recording medium such as an optical disc is recorded using an encoding process this is modified to cause a peak or rapid change in the digital sum value in the encoded data for part of the recording data. This provides the corresponding part of the recorded medium with a modified DC characteristic that affects an operational characteristic of the recording medium. This modified DC characteristic will not be present in unauthorized copies of the recording medium because the normal encoding process will encode the recording data so as to minimize DC imbalance. Profile data is obtained from operational data acquired during operation of the recording medium. This profile data is compared with expected profile data that is the profile data that would be expected to be obtained if the modified DC characteristic is present. The results of this comparison are used to determine whether or not the recording medium is authentic.