In this chapter, we focus on how a watermark can be used to assist in maintaining and verifying the integrity of its associated cover Work. This is in contrast to the previous chapter, which is concerned with maintaining the integrity of the watermark itself.

The ease of tampering with photographs was demonstrated in Chapter 2 (Figure 2.3). Video and audio signals can also be easily altered. In many circumstances, alterations to content serve legitimate purposes. However, in other cases, the changes may be intentionally malicious or may inadvertently affect the interpretation of the Work. For example, an inadvertent change to an X-ray image might result in a misdiagnosis, whereas malicious tampering of photographic evidence in a criminal trial can result in either a wrong conviction or acquittal. Thus, in applications for which we must be certain a Work has not been altered, there is a need for verification or authentication of the integrity of the content. Specifically, we are interested in methods for answering the following questions:

• Has the Work been altered in any way whatsoever?

• Has the Work been significantly altered?

• What parts of the Work have been altered?

• Can an altered Work be restored?

Many non-watermarking methods exist for answering these questions. For example, one method for answering the first question (i.e., has the Work been altered at all) is to append a cryptographic signature to it, as discussed in Appendix A. Even without cryptographic signatures, forensic specialists can often detect modifications to images and...