On Cryptographic Building Blocks and Transformations

Cryptographic building blocks play a central role in cryptography, e.g., encryption or digital signatures with their security notions. Further, cryptographic building blocks might be constructed modularly, i.e., emerge out of other cryptographic building blocks. Essentially, one cryptographically transforms the underlying block(s) and their (security) properties into the emerged block and its properties. This thesis considers cryptographic building blocks and new cryptographic transformations

Scalable public-key tracing and revoking

Abstract. Traitor Tracing Schemes constitute a very useful tool against piracy in the context of digital content broadcast. In such multi-recipient encryption schemes, each decryption key is fingerprinted and when a pirate decoder is discovered, the authorities can trace the identities of the users that contributed in its construction (called traitors). Public-key traitor tracing schemes allow for a multitude of nontrusted content providers using the same set of keys, which makes the scheme “server-side scalable.” To make such schemes also “client-side scalable, ” i.e. long lived and usable for a large population of subscribers that changes dynamically over time, it is crucial to implement efficient Add-user and Remove-user operations. Previous work on public-key traitor tracing did not address this dynamic scenario thoroughly, and there is no efficient scalable public key traitor tracing scheme that allows an increasing number of Add-user and Remove-user operations. To address these issues, we introduce the model of Scalable Public-Key Traitor Tracing, and present the first construction of such a scheme. Our model mandates for deterministic traitor tracing and an unlimited number of efficient Add-user operations and Remove-user operations. A scalable system achieves an unlimited number of revocations while retaining high level of efficiency by dividing the run-time of the system into periods. Each period has a saturation level for the number of revocations. When a period becomes saturated, an efficien

Scalable public-key tracing and revoking

Traitor tracing schemes constitute a useful tool against piracy in the context of digital content distribution. They are encryption schemes that can be employed by content providers that wish to deliver content to an exclusive set of users. Each user holds a decryption key that is fingerprinted and bound to his identity. When a pirate decoder is discovered, it is possible to trace the identities of the users that contributed to its construction. In most settings, both the user population and the set of content providers are dynamic, thus scalable user management and scalable provider management are crucial. Previous work on public-key traitor tracing did not address the dynamic scenario thoroughly: no efficient scalable public-key traitor tracing scheme has been proposed, in which the populations of providers and users can change dynamically over time without incurring substantial penalty in terms of system performance and management complexity. To address these issues, we introduce a formal model for Scalable Public-Key Traitor Tracing, and present the first construction of such a scheme. Our model mandates for deterministic traitor tracing and unlimited number of efficient provider and user management operations. We present a formal adversarial model for our system and we prove our construction secure, against both adversaries that attempt to cheat the provider and user management mechanism, and adversaries that attempt to cheat the traitor tracing mechanism