Daniel Gardham

Dr Daniel Gardham

Postgraduate Research Student

My research project

My qualifications

University of Bath

My teaching

Courses I teach on


My publications


CC Dragan, D Gardham, M Manulis. (2018). Hierarchical Attribute-based Signatures
View abstract View full publication
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 anonymity set of users sharing policy-conform attributes. Current ABS schemes exhibit some limitations when it comes to the management and issue of attributes. In this paper 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 the 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 generic HABS construction also ensures 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 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
D Gardham, M Manulis (2019). Hierarchical Attribute-based Signatures: Short Keys and Optimal Key Length
View abstract View full publication
With Attribute-based Signatures (ABS) users can simultaneously sign messages and prove compliance of their attributes, issued by designated attribute authorities, with some verification policy. Neither signer’s identity nor possessed attributes are leaked during the verification process, making ABS schemes a handy tool for applications requiring privacy-preserving authentication. Earlier ABS schemes lacked support for hierarchical delegation of attributes (across tiers of attribute authorities down to the signers), a distinct property that has made traditional PKIs more scalable and widely adoptable. This changed recently with the introduction of Hierarchical ABS (HABS) schemes, where support for attribute delegation was proposed in combination with stronger privacy guarantees for the delegation paths (path anonymity) and new accountability mechanisms allowing a dedicated tracing authority to identify these paths (path traceability) and the signer, along with delegated attributes, if needed. Yet, current HABS construction is generic with inefficient delegation process resulting in sub-optimal signature lengths of order O(k 2 |Ψ|) where Ψ is the policy size and k the height of the hierarchy. This paper proposes a direct HABS construction in bilinear groups that significantly improves on these bounds and satisfies the original security and privacy requirements. At the core of our HABS scheme is a new delegation process based on the length-reducing homomorphic trapdoor commitments to group elements for which we introduce a new delegation technique allowing step-wise commitments to additional elements without changing the length of the original commitment and its opening. While also being of independent interest, this technique results in shorter HABS keys and achieves the signature-length growth of O(k|Ψ|) which is optimal due to the path-traceability requirement.
D Gardham, M Manulis, CC Dragan (2020). Biometric-Authenticated Searchable Encryption
View abstract View full publication
We introduce Biometric-Authenticated Keyword Search (BAKS), a novel searchable encryption scheme that relieves clients from managing cryptographic keys and relies purely on client’s biometric data for authenticated outsourcing and retrieval of files indexed by encrypted keywords. BAKS utilises distributed trust across two servers and the liveness assumption which models physical presence of the client; in particular, BAKS security is guaranteed even if clients’ biometric data, which often has low entropy, becomes public. We formalise two security properties, Authentication and Indistinguishability against Chosen Keyword Attacks, which ensure that only a client with a biometric input sufficiently close to the registered template is considered legitimate and that neither of the two servers involved can learn any information about the encrypted keywords. Our BAKS construction further supports outsourcing and retrieval of files using multiple keywords and flexible search queries (e.g., conjunction, disjunction and subset-type queries). An additional update mechanism allows clients to replace their registered biometrics without requiring re-encryption of outsourced keywords, which enables smooth user migration across devices supporting different types of biometrics.