In order for a digital watermark to be considered up to specifications, it must meet four criteria: (1) it must appear to be invisible, (2) it must have a small probability of false detection, (3) it must be able to be embedded with a degree of flexibility, and (4) it must be difficult, if not impossible, to remove. We will consider each one of these requirements in turn.
Unobtrusive: Should be invisible or if it is visible, should not interfere with what is being protected.
Robust: The watermark must be difficult, or impossible, to remove.
Common signal processing: The watermark should be retrievable if signal processing is applied. Signal processing includes digital-to-analog, analog-to-digital, resampling, and other signal enhancements.
Common geometric distortions: Watermarks used in images and video should not be affected by scaling, rotation, cropping, or format translation.
Subterfuge attacks: The watermark should be robust against the technique of combining several copies of the same data set for the purpose of destroying the watermark.
The following list expands on the previous list a little differently:
Imperceptibility: The watermark should be indistinguishable from the original signal.
Information capacity: The payload bit rate must be compatible with the limits imposed by the system.
Robustness: The watermark must be recoverable, even after filtering, cropping, or the addition of noise to the signal.
Low complexity: Needed primarily for use with real-time applications.
Survive multiple encode-decode generations.
Tamper resistant or tamper evident: It should be possible to recognize when a watermark has been modified.
Difficult to create or extract watermark without proper credentials.