Abstract
"Attribute-based Signatures (ABS) are a powerful tool allowing users with attributes issued by authorities to sign messages while also proving that their attributes satisfy some policy. ABS schemes provide a flexible and privacy-preserving approach to authentication since the signer's identity and attributes remain hidden within the set of users sharing policy-conform attributes. Current ABS schemes exhibit some limitations when it comes to the management and issuance of attributes. In this thesis we address the lack of support for hierarchical attribute management, a property that is prevalent in traditional PKIs where certification authorities are organised into hierarchies and signatures are verified along roots of trust.
Hierarchical Attribute-based Signatures (HABS) introduced in this work support delegation of attributes along paths from a top-level authority down to the users while also ensuring that signatures produced by these users do not leak their delegation paths, thus extending the original privacy guarantees of ABS schemes. Our HABS security properties ensure unforgeability of signatures in the presence of collusion attacks and contains an extended traceability property allowing a dedicated tracing authority to identify the signer and reveal its attribute delegation paths. We include a public verification procedure for the accountability of the tracing authority. We propose two HABS constructions in the bilinear group setting, the first is generic utilising standard cryptographic building blocks and the latter is a direct construction. We formally prove their security in the standard and generic group model respectively.
An important yet challenging property for privacy-preserving ABS is revocation, which may be applied to signers or some of the attributes they possess. Existing ABS schemes lack efficient revocation of either signers or their attributes, relying on generic costly proofs. Moreover, in HABS there is a further need to support efficient revocation of authorities on the delegation paths, which is not provided by our previous HABS constructions.
Our final chapter proposes a HABS construction with a Verifier-Local Revocation (VLR) property. We extend the original HABS security model to address revocation and develop a new attribute delegation technique with appropriate VLR mechanism, which also implies the first non-hierarchical ABS scheme to support VLR. Moreover, our scheme supports inner-product signing policies, offering a wider class of attribute relations than previous HABS schemes, and is the first to be based on lattices, which are thought to offer post-quantum security.
We anticipate that HABS will be useful for privacy-preserving authentication in applications requiring hierarchical delegation of attribute-issuing rights and where knowledge of delegation paths might leak information about signers and their attributes, e.g., in intelligent transport systems where vehicles may require certain attributes to authenticate themselves to the infrastructure but remain untrackable by the latter."