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Towards transferable watermarks in buyer-seller watermarking protocols
KTH, School of Computer Science and Communication (CSC), Theoretical Computer Science, TCS.
2013 (English)In: Proceedings of the 2013 IEEE International Workshop on Information Forensics and Security, WIFS 2013, IEEE conference proceedings, 2013, 197-202 p.Conference paper (Refereed)
Abstract [en]

Buyer-seller watermarking protocols are protocols that let a seller embed a watermark which uniquely identifies the buyer of each sold copy of some work without allowing the seller to learn the watermark. The purpose of such protocols is to deter buyers from illegally redistributing the work while protecting the buyer from being framed by dishonest sellers. Existing buyer-seller watermarking schemes require that every buyer receives his or her copy directly from the seller. We consider the problem of extending buyer-seller watermarking to allow (controlled) redistribution between buyers while maintaining a watermark that uniquely identifies each recipient. An efficient and secure protocol of this type could allow distribution of digital content in peer-to-peer networks while protecting the owner's copyright. We give a proof-of-concept protocol which only requires limited interaction with the original seller to change the watermark.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2013. 197-202 p.
National Category
Other Computer and Information Science
URN: urn:nbn:se:kth:diva-147493DOI: 10.1109/WIFS.2013.6707818ISI: 000346136700034ScopusID: 2-s2.0-84894129214ISBN: 978-146735593-3OAI: diva2:730298
2013 5th IEEE International Workshop on Information Forensics and Security, WIFS 2013, 18 November 2013 through 21 November 2013, Guangzhou, China

QC 20140627

Available from: 2014-06-27 Created: 2014-06-27 Last updated: 2015-12-03Bibliographically approved
In thesis
1. Some aspects of cryptographic protocols: with applications in electronic voting and digital watermarking
Open this publication in new window or tab >>Some aspects of cryptographic protocols: with applications in electronic voting and digital watermarking
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cryptographic protocols are widely used on the internet, from relatively simple tasks such as key-agreement and authentication to much more complex problems like digital cash and electronic voting. Electronic voting in particular is a problem we investigate in this thesis.

In a typical election, the main goals are to ensure that the votes are counted correctly and that the voters remain anonymous, i.e. that nobody, not even the election authorities, can trace a particular vote back to the voter. There are several ways to achieve these properties, the most general being a mix-net with a proof of a shuffle to ensure correctness. We propose a new, conceptually simple, proof of a shuffle. We also investigate a mix-net which omits the proof of a shuffle in favor of a faster, heuristically secure verification. We demonstrate that this mix-net is susceptible to both attacks on correctness and anonymity. A version of this mix-net was tested in the 2011 elections in Norway.

We also look at a simple and widely used proof of knowledge of a discrete logarithm in groups of prime order. While the requirement of prime order is well known, we give a precise characterization of what the protocol proves in a group of composite order. Furthermore, we present attacks against a class of protocols of the same form, which shows that the protocol cannot easily be extended to groups where the order is composite or unknown.

We finally look at the problem of music and video piracy. Using a buyer-seller watermark to embed a unique watermark in each sold copy has been proposed as a deterrent since it allows a seller who discovers a pirated copy to extract the watermark and find out which buyer released it. Existing buyer-seller watermarking schemes assume that all copies are downloaded directly from the seller. In practice, however, the seller wants to save bandwidth by allowing a paying customer to download most of the content from other buyers. We introduce this as an interesting open research problem and present a proof-of-concept protocol which allows transfer of content between buyers while keeping the seller's communication proportional to the size of the watermark rather than the size of the content.

Abstract [sv]

Kryptografiska protokoll används i stor omfattning på internet, för att lösa allt från enkla uppgifter som nyckelutbyte och autentisering till komplexa problem som digitala pengar och elektroniska val. I den här avhandlingen är elektroniska val av speciellt intresse.

I ett typiskt valsystem är målen att garantera att rösterna räknas korrekt och att ingen, inte ens valförrättaren, kan spåra en röst tillbaka till den röstande. Det finns flera metoder för att åstadkomma detta, men den mest generella är mixnät med ett så kallat ``proof of a shuffle'' för att garantera korrekthet. Vi föreslår i avhandlingen ett nytt, konceptuellt enkelt, ``proof of a shuffle''. Vi undersöker också ett mixnät som använder ett snabbt heuristiskt argument för korrekthet istället för ett ``proof of a shuffle''. Vi demonstrerar att både korrekthet och anonymitet kan angripas i det mixnätet. En version av samma mixnät användes i valet 2011 i Norge.

Vi undersöker också ett enkelt bevis av kunskap om en diskret logaritm i en grupp. Det är sedan länge välkänt att just det protokollet kräver att gruppen har primtalsordning, men vi ger en karaktärisering av vad som händer i en grupp av sammansatt ordning. Vidare presenterar vi attacker mot en klass av protokoll med samma struktur, vilket visar att protokollet inte enkelt kan utvidgas till grupper av okänd eller sammansatt ordning.

Slutligen studerar vi problemet med piratkopiering av film och musik. Det har föreslagits att vattenmärkning kan användas för att bädda in ett unikt vattenmärke i varje såld kopia. En säljare som upptäcker en piratkopia kan extrahera vattenmärket och därmed avslöja vilken köpare som läckte kopian. Existerande vattenmärkningssystem förutsätter att alla kopior laddas ner direkt från säljaren, men i praktiken vill säljaren ofta reducera mängden datatrafik genom att låta en ny betalande kund ladda ner kopian från tidigare köpare. Vi introducerar detta som ett intressant öppet problem och presenterar ett protokoll som tillåter överföring av data mellan köpare och som bara kräver att säljaren kommunicerar data proportionellt mot storleken på vattenmärket istället för mot storleken på filen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. ix, 67 p.
TRITA-CSC-A, ISSN 1653-5723 ; 2015:08
National Category
Computer Science
urn:nbn:se:kth:diva-165436 (URN)978-91-7595-545-2 (ISBN)
Public defence
2015-05-22, Kollegiesalen, Valhallavägen 79, KTH, STOCKHOLM, 14:00 (English)

QC 20150428

Available from: 2015-04-28 Created: 2015-04-28 Last updated: 2015-04-28Bibliographically approved

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