The Next Level in E-Book Watermarking

Last week’s announcement of a partnership between Digimarc and Erudition Digital represents an interesting next step in the evolution of watermarking as a copyright protection technology, in this case for e-books.  Erudition Digital is an e-book distributor based in the UK that has incorporated watermarking technology from Custos Technologies of South Africa. According to last week’s announcement, Custos is now using Digimarc’s e-book watermarking scheme in addition to its own.

The overall scheme has three pieces, which form a unique combination when used together.  First is the digital watermark itself: it’s a set of user information that gets embedded into e-book files at download time.  E-book watermarking schemes differ (among other things) in their levels of robustness. Simpler e-book watermarks aren’t robust: they are easy to remove completely while keeping the e-book content intact. Completely robust watermarks are designed such that removing them damages the content in material ways. Such watermarks exist for audio, image, and video content, but they’re impossible for e-books: at the worst case, a hacker can strip out the plain text and simple formatting elements, and eliminate the watermark while leaving the e-book readable. In the middle are more robust watermarks, which incorporate multiple ways of embedding data (such as user IDs or email addresses) into the content, so that a hacker who finds and removes one set of watermark information can’t be sure that he has removed all of them. Both Digimarc and Custos have robust e-book watermarks.

The second piece to the overall scheme is a web crawler that finds e-book files with watermarks in them and flags them as potentially infringing. Digimarc operates a service (Digimarc Guardian) that does this using Digimarc’s own watermarking scheme.  (Digimarc Guardian works more commonly without watermarking, instead examining e-book metadata and context to determine potential infringements.)

The third and newest piece is Custos’s bounty-hunting technology. Custos embeds a code in each e-book file that enables bounty hunters to claim a reward in the form of Bitcoin cryptocurrency. As I’ve explained before, Custos provides a website that enables people to claim bounties anonymously, and it includes many safeguards to prevent abuse of the system. The technology debuted early this year.

The great advantage of the bounty-hunting system over web crawling is that it can go where web crawlers can’t, such as into cyberlocker accounts, on BitTorrent, on campus networks, and so on. It’s especially advantageous on BitTorrent over existing BitTorrent monitoring schemes, which have to trade off conspicuousness (BitTorrent trackers can find out who is monitoring and blacklist them) against quality of evidence of potential infringement.

Custos decided to work with Digimarc because Digimarc’s e-book watermark is established in the market, and it has the web crawling scheme that complements its bounty-hunting technology. The overall result is a piracy monitoring scheme that can penetrate many areas of the “dark web” while not interfering with legitimate users’ uses of e-books. It’s another step forward as e-book watermarking continues its grass-roots rise to importance in book publishing.


  1. Pirates and bounty hunters will work in concert.

  2. The great disadvantage of Custos’s system is that it incentivizes cheating. A “pirate” can report themselves and collect the reward.

  3. Hi Nate – thanks for the comment, and taking the time to write up a longer response! This is something we manage very carefully, to make sure that there is no perceived benefit to deliberate self-claiming.
    The main principle is that we should not create a moral hazard by making the bounties larger than the perceived downside risk. You never want a bounty to be greater than the perceived risk to the recipient. If you have a bounty of $100, and the recipients know the punishment for piracy is only $50 fine, or a 2-week ban from buying ebooks, we can’t be surprised if there is self-claiming happening, even if the recipients are 100% certain that they will be caught. We have good estimates of the monetary equivalent of the perceived risk of being caught leaking content for various groups of potential recipients from different jurisdictions, and ensure that the bounties remain below a reasonable threshold for the intended recipients.
    For ebooks, we’ve found that bounties as low as $5 provide ample incentive for our bounty hunters to quickly claim the rewards when piracy occurs. An ebook buyer won’t risk a loss of an account or other sanctions by self-claiming a $5-reward on a $10 ebook. It certainly doesn’t make the creation of throwaway accounts profitable.
    This is a technology that we’ve used very successfully to prevent movie screener piracy for a number of years, but only started offering for ebooks recently.

  4. […] those who remain concerned about piracy, an interesting development is a new form of digital watermarking, which embeds a key that will enable bounty hunters who report pirated copies of ebooks to claim a […]

  5. […] DRM System Offers Readers Bitcoin Bounty to Snitch on Pirate Sites 14 September (Copyright and Technology) Combines bounty hunting, multi-layer watermarking and web-crawling to sniff out infringing ebooks […]

  6. […] The Next Level in E-Book Watermarking (Copyright and Technology) […]

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