## 2 Principles for Developing a Suitable Watermark  
The underlying principles of a suitable watermark for authenticating and checking the integrity of audio recordings are [18]:  
1. The watermark should be embedded at the time of recording--embedding the watermark in a recording at a later moment can be considered a malicious operation and contested. This principle imposes computational complexity limits on the embedding method because it should work in real time;The watermark should be captured by all the recording devices that are near to one another--all the participants in a discussion can carry their own recording device and, if the watermark is embedded only in some of the recordings, a dispute about what recording is authentic can start;
3. The embedding of the watermark should not be noticeable--if people know that a watermarking process is running, they could be reserved in their declarations and the flow of the conversation could be affected by it. Thus, it should be kept secret and the embedding operations should not rise suspicion;
4. The watermark should be secure--only authorized persons should be able to extract the watermark.  
All the principles presented here are found in the case of the ENF criterion scenario. The security principle is found because, considering the fact that the picked-up power line signal is very small, intense signal processing knowledge is needed to reliably extract the ENF from an audio recording. In the process of developing an audio watermarking method compatible with new recording devices, the first two principles are met if sonic watermarking is used. Let us stress that the third principle does not necessary imposes that the watermark should be inaudible. It only needs to avoid raising the suspicion of the participants. The embedding methods should be chosen to satisfy the fourth principle. In [19], the authors propose a sonic watermarking scheme compliant with the four principles defined above, in which ticking sounds are used to mask one low frequency chirp signal. The watermark is embedded as the temporal distance between ticking sounds. Some disadvantages of the proposed system are the small value of the time-bandwidth product of the chosen chirp signal, thus affecting the detection process, the requirement to use speakers with large dimensions to play the low frequency chirp signal in the room making the system hard to conceal, and the placement of the chirp signal outside the audio bandwidth (starting from 10 Hz), thus risking its filtering by the recording device. The effects of the sound propagation are not considered, and a security issue is present in the form of using a periodic time delay sequence between the ticking sounds (therefore a cut operation equal to the length of the sequence cannot be detected and, similarly, a well-placed copy-paste operation can pass undetected). The system was improved in [20], where the advantages of using a high frequency chirp signal instead of the low frequency one are highlighted, the effects of the sound propagation are investigated, resulting that the high frequency chirp version gives much better results than the original, allows the reduction of the physical dimensions of the system (because smaller speakers are required to play high frequency sounds), thus improving the concealment, and also solving the aforementioned security problem by setting the time delays between ticking sounds using a pseudorandom sequence. The system presented in this paper is further improved based on the advantages brought by embedding the ENF.