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General objectives

The general aim of BOWS-2 contest is to investigate how and when an image watermarking system can be broken though preserving the highest possible quality of the content. It is not intended to prove how well-performing a specific watermarking technique is. But it is expected that, thanks to this contest, the watermarking community will have a better understanding of the impact of the disparate attacks, perhaps unknown at the moment, the BOWS-2 participants will carry out, and hence have a better estimate of how robust watermarking (in general) is when attacked by a large set of attackers.

BOWS-2 contest is divided into 3 sequential challenges, hereafter named episodes, to study the robustness and security of watermarking under three different scenarios. For each episode the goal is the same:
- remove the watermark on 3 different images that are watermarked with the same key using the same system,
- minimise the average distortion.
Only the conditions to attack the system are different.

Note also that the images have been watermarked with a PSNR between 42.5dB and 42.6dB.

Episode 1: Robustness against image processing

- Period:
Jul 17, 2007 - Oct 17, 2007

- Objectives:
The objectives of the first challenge are to evaluate the basic robustness of the proposed watermarking scheme to image processing operations during 3 months. During this period no information about the watermarking algorithm will be provided and the numbers of trials per day will be limited. Participants that might want to attack the system by providing numerous trials have to wait for "Episode 2: sensitivity".

- Rules:
R1.1: Only 30 attempts/day per participant are allowed (if you want to do more, please wait for Episode 2).
R1.2: Conditions to enter in the Hall of Fame:
- no watermark has to be detected on the 3 uploaded PGM images.
- the average PSNR between the watermarked and the attacked images must be greater than 20dB (note that the average PSNR is computed as the PSNR of the mean of the distortions for the 3 images and not the mean of the 3 PSNRs).
- only the 20 highest average PSNRs are listed. Hence, people might go out of the Hall of Fame as new participants succeed to hack the three images with a higher average PSNR.
R1.3: The three first participants in the Hall of Fame at the end of the episode wins a prize .
R1.4: The winner will have to send the attacked and successful images to the organisers and to provide a description of the attack.

Episode 2: Sensitivity against oracle attacks

- Period:
Oct 17, 2007 - Jan 17, 2008

- Objectives:
The objective of Episode 2 is to assess the robustness of the watermarking technique when the attacker has access to a sealed black box detector. The watermarking technique is not disclosed, but, we offer the possibility to perform a very large number of trials, as required by so-called "sensitivity" or "oracle" attacks.

- Rules:
R2.1: The number of attempts/day is not restricted as far as our server can handle it.
R2.2: In order to decrease to load of the BOWS-2 server, a binary executable performing detection will be provided on request (a licence usage agreement between the two parties has to be signed first). This executable only works on a typical unix based platform (linux, osX.4). It uses a different secret key than the one of the contest. 3 watermarked images with this key are also provided. With this software, participants can fine-tune their attacks, and when ready, they can lead it on the BOWS-2 images.
R2.3: Conditions to enter in the Hall of Fame and to win: same as R1.2 and R1.4 respectively.


Episode 3: Security against information leakages

- Period:
Jan 17, 2008 - Apr 17, 2007

- Objectives:
The objective of the last episode is to assess the security level against attackers who estimate the watermark or the secret key used by the system. In order to do this, BOWS-2 provides a database of 10000 images (watermarked using the same key), the source-code of the embedding and the detection algorithms and a description of the system. The attackers try to get information of the secret key by analysing both the algorithms and the database. Note that providing the source code and the description of the watermarking system fits with the Kerckhoffs principle in cryptanalysis.

- Rules:
R3.1: Only 10 attempts/day per participant are allowed.
R3.2: The source code of the embedding and detection scheme is provided to the participants under the CeCILL license
R3.3: A database of 10000 watermarked images will be available for download to participants (see the licence of usage)
R3.4: Conditions to enter in the Hall of Fame and to win: same as R1.2 and R1.4 respectively.