Systems and methods are provided for protecting and managing 
electronic data signals that are registered in accordance with a predefined encoding scheme, while allowing access to unregistered data signals. In one embodiment a relatively hard-to-remove, easy-to-detect, strong 
watermark is inserted in a 
data signal. The 
data signal is divided into a sequence of blocks, and a 
digital signature for each block is embedded in the 
signal via a 
watermark. The 
data signal is then stored and distributed on, e.g., a 
compact disc, a DVD, or the like. When a user attempts to access or use a portion of the data 
signal, the 
signal is checked for the presence of a 
watermark containing the 
digital signature for the desired portion of the signal. If the watermark is found, the 
digital signature is extracted and used to verify the authenticity of the desired portion of the signal. If the signature-containing watermark is not found, the signal is checked for the presence of the strong watermark. If the strong watermark is found, further use of the signal is inhibited, as the presence of the strong watermark, in combination with the absence or corruption of the signature-containing watermark, provides evidence that the signal has been improperly modified. If, on the other hand, the strong mark is not found, further use of the data signal can be allowed, as the absence of the strong mark indicates that the data signal was never registered with the signature-containing watermark.