Constant-size dynamic k-times anonymous authentication

Dynamic k-times anonymous authentication (k-TAA) schemes allow members of a group to be authenticated anonymously by application providers for a bounded number of times, where application providers can independently and dynamically grant or revoke access right to members in their own group. In this paper, we construct a dynamic k-TAA scheme with space and time complexities of O(log(k)) and a variant, in which the authentication protocol only requires constant time and space complexities at the cost of O(k) -sized public key. We also describe some tradeoff issues between different system characteristics. We detail all the zero-knowledge proof-of-knowledge protocols involved and show that our construction is secure in the random oracle model under the q-strong Diffie-Hellman assumption and q-decisional Diffie-Hellman inversion assumption. We provide a proof-of-concept implementation, experiment on its performance, and show that our scheme is practical

A Revocable Group Signature Scheme with Scalability from Simple Assumptions and Its Application to Identity Management

Group signatures are signatures providing signer anonymity where signers can produce signatures on behalf of the group that they belong to. Although such anonymity is quite attractive considering privacy issues, it is not trivial to check whether a signer has been revoked or not. Thus, how to revoke the rights of signers is one of the major topics in the research on group signatures. In particular, scalability, where the signing and verification costs and the signature size are constant in terms of the number of signers N, and other costs regarding signers are at most logarithmic in N, is quite important. In this paper, we propose a revocable group signature scheme which is currently more efficient compared to previous all scalable schemes. Moreover, our revocable group signature scheme is secure under simple assumptions (in the random oracle model), whereas all scalable schemes are secure under q-type assumptions. We implemented our scheme by employing Barreto-Lynn-Scott curves of embedding degree 12 over a 455-bit prime field (BLS-12-455), and Barreto-Naehrig curves of embedding degree 12 over a 382-bit prime field (BN-12-382), respectively, by using the RELIC library. We showed that the online running times of our signing algorithm were approximately 14 msec (BLS-12-455) and 11 msec (BN-12-382), and those of our verification algorithm were approximately 20 msec (BLS-12-455) and 16 msec (BN-12-382), respectively. Finally, we showed that our scheme is applied to an identity management system proposed by Isshiki et al

One-time and Revocable Ring Signature with Logarithmic Size in Blockchain

Ring signature (RS) allows users to demonstrate to verifiers their membership within a specified group (ring) without disclosing their identities. Based on this, RS can be used as a privacy protection technology for users\u27 identities in blockchain. However, there is currently a lack of RS schemes that are fully applicable to the blockchain applications: Firstly, users can only spend a UTXO once, and the current RS schemes are not yet perfect in a one-time manner. At the same time, the current RS schemes are not sufficiently developed in terms of regulation. Secondly, the size of the current RS is mostly linearly related to the number of ring members. When there are many members, the transaction processing speed is slow. We propose a one-time and revocable ring signature with logarithmic size in blockchain based on the Sigma-Protocols. Our scheme compresses the RS size and enables users to sign in the blockchain transactions. The scheme allows two RS generated with the same private key for a same UTXO to be linked together. Additionally, it allows regulatory authority to recover the signer\u27s identity at any time. A security model was presented, and its security properties, namely, unforgeability, anonymity, one-time, revocability, and non-slanderability were proven in the random oracle model. Our scheme compresses the RS size to where is the number of ring users, enabling blockchain transactions to have better processing speeds. And it can prevent double-spending attacks in blockchain and allows regulatory authority to recover the identity of the signer

Privacy Preserving Cryptographic Protocols for Secure Heterogeneous Networks

Disertační práce se zabývá kryptografickými protokoly poskytující ochranu soukromí, které jsou určeny pro zabezpečení komunikačních a informačních systémů tvořících heterogenní sítě. Práce se zaměřuje především na možnosti využití nekonvenčních kryptografických prostředků, které poskytují rozšířené bezpečnostní požadavky, jako je například ochrana soukromí uživatelů komunikačního systému. V práci je stanovena výpočetní náročnost kryptografických a matematických primitiv na různých zařízeních, které se podílí na zabezpečení heterogenní sítě. Hlavní cíle práce se zaměřují na návrh pokročilých kryptografických protokolů poskytujících ochranu soukromí. V práci jsou navrženy celkově tři protokoly, které využívají skupinových podpisů založených na bilineárním párování pro zajištění ochrany soukromí uživatelů. Tyto navržené protokoly zajišťují ochranu soukromí a nepopiratelnost po celou dobu datové komunikace spolu s autentizací a integritou přenášených zpráv. Pro navýšení výkonnosti navržených protokolů je využito optimalizačních technik, např. dávkového ověřování, tak aby protokoly byly praktické i pro heterogenní sítě.The dissertation thesis deals with privacy-preserving cryptographic protocols for secure communication and information systems forming heterogeneous networks. The thesis focuses on the possibilities of using non-conventional cryptographic primitives that provide enhanced security features, such as the protection of user privacy in communication systems. In the dissertation, the performance of cryptographic and mathematic primitives on various devices that participate in the security of heterogeneous networks is evaluated. The main objectives of the thesis focus on the design of advanced privacy-preserving cryptographic protocols. There are three designed protocols which use pairing-based group signatures to ensure user privacy. These proposals ensure the protection of user privacy together with the authentication, integrity and non-repudiation of transmitted messages during communication. The protocols employ the optimization techniques such as batch verification to increase their performance and become more practical in heterogeneous networks.

Oblivious Accumulators

A cryptographic accumulator is a succinct set commitment scheme with efficient (non-)membership proofs that typically supports updates (additions and deletions) on the accumulated set. When elements are added to or deleted from the set, an update message is issued. The collection of all the update messages essentially leaks the underlying accumulated set which in certain applications is not desirable. In this work, we define oblivious accumulators, a set commitment with concise membership proofs that hides the elements and the set size from every entity: an outsider, a verifier or other element holders. We formalize this notion of privacy via two properties: element hiding and add-delete indistinguishability. We also define almost-oblivious accumulators, that only achieve a weaker notion of privacy called add-delete unlinkability. Such accumulators hide the elements but not the set size. We consider the trapdoorless, decentralized setting where different users can add and delete elements from the accumulator and compute membership proofs. We then give a generic construction of an oblivious accumulator based on key-value commitments (KVC). We also show a generic way to construct KVCs from an accumulator and a vector commitment scheme. Finally, we give lower bounds on the communication (size of update messages) required for oblivious accumulators and almost-oblivious accumulators

Identity and identification in an information society: Augmenting formal systems of identification with technological artefacts

Information and Communication Technology (ICT) are transforming society’s information flows. These new interactive environments decouple agents, information and actions from their original contexts and this introduces challenges when evaluating trustworthiness and intelligently placing trust.This thesis develops methods that can extend institutional trust into digitally enhanced interactive settings. By applying privacy-preserving cryptographic protocols within a technical architecture, this thesis demonstrates how existing human systems of identification that support institutional trust can be augmented with ICT in ways that distribute trust, respect privacy and limit the potential for abuse. Importantly, identification systems are located within a sociologically informed framework of interaction where identity is more than a collection of static attributes.A synthesis of the evolution and systematisation of cryptographic knowledge is presented and this is juxtaposed against the ideas developed within the digital identity community. The credential mechanism, first conceptualised by David Chaum, has matured into a number of well specified mathematical protocols. This thesis focuses on CL-RSA and BBS+, which are both signature schemes with efficient protocols that can instantiate a credential mechanism with strong privacy-preserving properties.The processes of managing the identification of healthcare professionals as they navigate their careers within the Scottish Healthcare Ecosystem provide a concrete case study for this work. The proposed architecture mediates the exchange of verifiable, integrity-assured evidence that has been cryptographically signed by relevant healthcare institutions, but is stored, managed and presented by the healthcare professionals to whom the evidence pertains.An evaluation of the integrity-assured transaction data produced by this architecture demonstrates how it could be integrated into digitally augmented identification processes, increasing the assurance that can be placed in these processes. The technical architecture is shown to be practical through a series of experiments run under realistic production-like settings.This work demonstrates that designing decentralised, standards-based, privacy-preserving identification systems for trusted professionals within highly assured social contexts can distribute institutionalised trust to trustworthy individuals and empower these individuals to interface with society’s increasingly socio-technical systems

Design, modeling, and simulation of secure X.509 certificate revocation

TLS communication over the internet has risen rapidly in the last seven years (2015--2022), and there were over 156M active SSL certificates in 2022. The state-of-the-art Public Key Infrastructure (PKI), encompassing protocols, computational resources, and digital certificates, has evolved for 24 years to become the de-facto choice for encrypted communication over the Internet even on newer platforms such as mobile devices and Internet-of-Things (IoT) (despite being low powered with computational constraints). However, certificate revocation is one sub-protocol in TLS communication that fails to meet the rising scalability demands and remains open to exploitation. In this dissertation, the standard for X.509 revocation is systematically reviewed and critically evaluated to identify its limitations and assess their impact on internet security. Because of fragmented revocation information and limited scalability, even the latest version of the X.509 revocation standard is susceptible to Man-in-the-Middle (MiTM) attacks. Blockchain technology can provide a decentralized and peer-to-peer distributed ledger to enable a unified, tamper-proof platform for X.509 certificate authorities to collaborate securely in a trustless environment. To understand blockchain technology\u27s capabilities and limitations in distributing X.509 revocation information, different blockchain platforms are explored and compared in terms of scalability, degree of decentralization, and cost of operation. Moreover, the unification of the revocation lists leads to a massive expansion in the number of revoked certificates to query by a verifying client thus increasing the latency during revocation lookup. And, to minimize revocation-status lookup times, cryptographic constructions and approximate set-membership data structures are prototyped and analyzed. The key contributions of this dissertation are twofold: 1) the novel design of a secure and robust system for distributing X.509 certificate revocation information; and, 2) the prototype, experimentation, and optimization of cascading XOR filter, fuse filter, and cuckoo filter for quick lookup with zero false positives (and zero false negatives). The Secure Certificate Revocation as a Peer Service (SCRaaPS) is designed using the Lightweight Mining consensus algorithm-based Scrybe blockchain protocol to store and distribute certificate revocation lists. And, the cascading fuse filter (demonstrating the highest space efficiency and fastest build time) is applied to minimize the revocation lookup time with zero false positives

SoK: Privacy-Preserving Signatures

Modern security systems depend fundamentally on the ability of users to authenticate their communications to other parties in a network. Unfortunately, cryptographic authentication can substantially undermine the privacy of users. One possible solution to this problem is to use privacy-preserving cryptographic authentication. These protocols allow users to authenticate their communications without revealing their identity to the verifier. In the non-interactive setting, the most common protocols include blind, ring, and group signatures, each of which has been the subject of enormous research in the security and cryptography literature. These primitives are now being deployed at scale in major applications, including Intel\u27s SGX software attestation framework. The depth of the research literature and the prospect of large-scale deployment motivate us to systematize our understanding of the research in this area. This work provides an overview of these techniques, focusing on applications and efficiency