Application (library) securing audio files in c#

Hi! We need application (library) which will securing audio files. This securing should be based on addition data to audio, without affecting the quality.

Use case:
1. We are firing this library passing parameters: type of work (encode), path to audio file, text which should be added to audio.
2. App create new file, with additional data.
3. We are firing library passing parameters: type of work (decode) and path to file.
4. We see text with we provide in step 1.

Additional requriments:
1. It should be opportunity to decode based on small part of audio.
2. Output file size shouldn't be bigger more than +10%, in reference to original file.
3. Securing should use strong alghoritm, hard to hack.
4. Securing should be resistant for changing file: we want get providen text after changing file (frequecy, volume, etc).
5. Securing should be based on certificate or external key, whitch we will be chang.

We can pay in two milestones:
1. We will get dll or exe file with documentation and app will pass our securing tests.
2. We will get source project.

Language: C#