We propose an ideal functionalityFCDand a construction ΠCDfor oblivious and updatable committed databases.FCDallows a proverPto read, write, and update values in a database and to prove to a verifierVin zero-knowledge (ZK) that a value is read from or written into a certain position. The following properties must hold: (1) values stored in the database remain hidden fromV; (2) a value read from a certain position is equal to the value previously written into that position; (3) (obliviousness) both the value read or written and its position remain hidden fromV.ΠCDis based on vector commitments. After the initialization phase, the cost of read and write operations is independent of the database size, outperforming other techniques that achieve cost sublinear in the dataset size for prover and/or verifier. Therefore, our construction is especially appealing for large datasets. In existing “commit-and-prove” two-party protocols, the task of maintaining a committed database betweenPandVand reading and writing values into it is not separated from the task of proving statements about the values read or written.FCDallows us to improve modularity in protocol design by separating those tasks. In comparison to simply using a commitment scheme to maintain a committed database,FCDallowsPto hide efficiently the positions read or written fromV. Thanks to this property, we design protocols for e.g. privacy-preserving e-commerce and location-based services whereVgathers aggregate statistics about the statements thatPproves in ZK.

Token payment systems were the first application of blockchain technology and are still the most widely used one. Early implementations of such systems, like Bitcoin or Ethereum, provide virtually no privacy beyond basic pseudonymity: all transactions are written in plain to the blockchain, which makes them linkable and traceable.

One important use case for (business) process technology is process compliance. We propose that managing security objects such as cryptographic keys is an application domain that can greatly benefit from using process technology because of a critical compliance requirement. To this end, we present a BPMN extension and a design of a support system for key management processes. Both were driven by a case study on a distributed application using a standard hierarchical public key infrastructure, which we also present. Our prototypical implementation of the support system shows that process compliance for security object management processes can be substantially supported with moderate effort by extending the standard process technology.

Certification of keys and attributes is in practice typically realized by a hierarchy of issuers. Revealing the full chain of issuers for certificate verification, however, can be a privacy issue since it can leak sensitive information about the issuer's organizational structure or about the certificate owner. Delegatable anonymous credentials solve this problem and allow one to hide the full delegation (issuance) chain, providing privacy during both delegation and presentation of certificates. However, the existing delegatable credentials schemes are not efficient enough for practical use.

Membership revocation is essential for cryptographic applications, from traditional PKIs to group signatures and anonymous credentials. Of the various solutions for the revocation problem that have been explored, dynamic accumulators are one of the most promising. We propose Braavos, a new, RSA-based, dynamic accumulator. It has optimal communication complexity and, when combined with efficient zero-knowledge proofs, provides an ideal solution for anonymous revocation. For the construction of Braavos we use a modular approach: we show how to build an accumulator with better functionality and security from accumulators with fewer features and weaker security guarantees. We then describe an anonymous revocation component (ARC) that can be instantiated using any dynamic accumulator. ARC can be added to any anonymous system, such as anonymous credentials or group signatures, in order to equip it with a revocation functionality. Finally, we implement ARC with Braavos and plug it into Idemix, the leading implementation of anonymous credentials. This work resolves, for the first time, the problem of practical revocation for anonymous credential systems.

We present a protocol for anonymous access to a database where the different records have different access control permissions. These permissions could be attributes, roles, or rights that the user needs to have in order to access the record. Our protocol offers maximal security guarantees for both the database and the user, namely (1) only authorized users can access the record; (2) the database provider does not learn which record the user accesses; and (3) the database provider does not learn which attributes or roles the user has when she accesses the database.