Digital watermarking and uses

Steganography is often confused with digital watermarking. Both insert invisible information into a carrier, but their goals are opposite.

Two opposing objectives

Steganography aims at secrecy: no one must suspect the existence of the message. If an observer detects anything at all, it is a failure.

Digital watermarking aims at robustness and traceability: you mark a piece of content (image, video, music) to prove its ownership or to detect copies. Here, it does not matter whether it is known that a mark exists — what matters is that it withstands manipulations: compression, cropping, format change.

The comparison table

+---------------------+---------------------------+---------------------------+
|                     |    Stéganographie         |    Tatouage numérique     |
+---------------------+---------------------------+---------------------------+
| Objectif principal  | le SECRET                 | la ROBUSTESSE / traçage   |
| On doit ignorer...  | l'existence de la marque  | (peu importe si visible)  |
| Doit résister à...  | à la détection            | compression, recadrage    |
| Message inséré      | grand (le vrai message)   | petit (identifiant)       |
| Exemple d'usage     | communication clandestine | anti-piratage, copyright  |
+---------------------+---------------------------+---------------------------+

What watermarking is used for

Digital watermarking has very concrete uses:

• anti-piracy watermarks: marking a film or a photo to prove who its author is;
• leak tracing: giving each recipient a copy bearing a different mark; if the document leaks, the mark reveals who disclosed it;
• authentication: verifying that a piece of content has not been altered.

Unlike the naive LSB of steganography, a good watermark is designed to survive a recompression or a cropping — this is even its foremost requirement.

The uses of steganography

For its part, steganography serves above all clandestine communication: journalists, dissidents or intelligence services concealing the very existence of an exchange, where an encrypted message would attract attention or be forbidden.

Good practice: encrypt before hiding

One rule dominates all others: always encrypt the message before hiding it.

Indeed, steganography alone offers no guarantee once discovered. If an attacker spots the hidden data but it is encrypted, they come across an unreadable string: they may know there is a message, but they cannot read it.

Message clair --[ on CHIFFRE ]--> message illisible
              --[ on CACHE   ]--> image anodine

Défense en profondeur :
  couche 1 (stégano) : cacher l'existence
  couche 2 (crypto)  : protéger le contenu si la couche 1 tombe

Combining the two is a defence in depth: two independent barriers are better than one.

In summary

• Digital watermarking aims at robustness and traceability, not secrecy.
• Steganography aims at secrecy: not being detected.
• A watermark must withstand compression and cropping; the stego LSB does not.
• Uses of watermarking: anti-piracy, leak tracing, authentication.
• Absolute good practice: encrypt before hiding.