Answer 6—The key attributes for a watermark to be used in chain-of-evidence applications relate to security and robustness (and indirectly to bandwidth).

First of all, the watermark must be secure, in the sense that it must be impossible for a different person to attach the same watermark to a different item. Secondly, the watermark needs to be able to be used to detect modifications to the marked item. This can be accomplished by making the watermark relatively fragile (deliberately non-robust), or by having enough room in the watermark to carry enough data to describe the marked item in detail. The detectability of the watermark isn’t as important, as long as the watermark doesn’t obscure the item in terms of its value as evidence.

Contributor: David Tweed