Introduction
In november 1940 in France, the Vichy Government installs the Service des Contrôles Techniques in charge of watching French people through their letter correspondences. Eve, an employee of the SCT opens an envelop and finds a letter with a music sheet attached.
My Dearest Bob,
I would be glad if you could share with me your thoughts about the music I wrote.
Yours always,
Alice
Figure 1 : The five first modified measures of the piece Liebesträume composed Liszt.
She quickly glances at it, closes it and classifies it as non sensitive letter, and stacks it on the corresponding pile in order to send it to Bob as Alice expected.
Once Bob receives it, he follows the rule that he agreed with Alice. He reads all notes preceded by a sharp or flat symbol, then converts it to a letter of the latin alphabet according to the following decoding table:
Figure 2 : The key to decode the message.
Stacking all those letters in the order of appearance in the sheet (from left to right, both of staves at the same time) leads to the decoded message attack leave. As soon as Bob discovers it, he understands that his security is at stake and immediately leaves for another shelter.
This fictional example illustrates the concept of steganography and steganalysis. Since the formal definition of the prisoners’ problem by Simmons in 1983, steganography and steganalysis have been considered as a hide and seek game.
The prisoners’ problem tells the story of Alice and Bob, two prisoners working on an escape plan. They are allowed to communicate, but their communications pass through the warden, Eve. Eve will attempt to find any hidden communication between Alice and Bob, and if she finds one, she will cut the communication channel.
Definition of concepts
Steganography is the art of dissimulation. When Alice wants to communicate a secret message to Bob, steganography consists in hiding a message in a content, while Alice and Bob previously shared the decoding method. There are illimited possibilities for the nature of the content as long as it looks legitimate: it could be a music sheet or a poem, an image…
Steganalysis is the adversarial act of detecting if a message has been hidden in a content. Eve, as in the expression eavesdropping, wants to stop Alice and Bob to keep secrets from her. She has the power to cut the communication channel between the two of them, which encourages them to be the least suspicious as possible.
History is full of amusing anecdotes about past usages of steganography. To our knowledge, the first utilization goes back to ancient Greece. In the book Histories written by Herodotus, Histiaeus tatooes a secret message on the skull of a slave. The messenger goes undercover when his hair grow back, and the message is decoded by Aristogoras by simply shaving him.
Another famous use of steganography is writing using invisible ink on regular paper. The ink can be made of lemon juice, and the message can be read by exposing the paper to a source of heat.