“…On the contrary, distortionfree techniques aim to preserve the integrity of the protected data [53], [54]. Usually, distortion-free techniques are defined as fragile, while distortion-based as robust approaches, to the extent that the embedded information survives at malicious or accidental attempts to remove it [1], [55]. Relational watermarking techniques can also be classified by their (i) cover-type, defining the type of data of the attribute in R selected to embed the marks; (ii) intent, i.e., ownership protection [1]- [3], [56], data tampering detection [4], [5], [57], traitor tracing [58]- [62], among others; (iii) watermark source, which can be meaningless such as a random binary stream [19], [31], or meaningful, i.e., a source for watermark generation presenting a meaning that does not depend on the watermarking technique [17], [64], [65].…”