Available online public/governmental services requiring authentication by citizens have considerably expanded in recent years. This has hindered the usability and security associated with credential management by users and service providers. To address the problem, some countries have proposed nation-scale identification/authentication systems that intend to greatly reduce the burden of credential management, while seemingly offering desirable privacy benefits. In this paper we analyze two such systems: the Federal Cloud Credential Exchange (FCCX) in the United States and GOV.UK Verify in the United Kingdom, which altogether aim at serving more than a hundred million citizens. Both systems propose a brokered identification architecture, where an online central hub mediates user authentications between identity providers and service providers. We show that both FCCX and GOV.UK Verify suffer from serious privacy and security shortcomings, fail to comply with privacy-preserving guidelines they are meant to follow, and may actually degrade user privacy. Notably, the hub can link interactions of the same user across different service providers and has visibility over private identifiable information of citizens. In case of malicious compromise it is also able to undetectably impersonate users. Within the structural design constraints placed on these nation-scale brokered identification systems, we propose feasible technical solutions to the privacy and security issues we identified. We conclude with a strong recommendation that FCCX and GOV.UK Verify be subject to a more in-depth technical and public review, based on a defined and comprehensive threat model, and adopt adequate structural adjustments.

Abstract. A secure two-party computation (S2PC) protocol allows two parties to compute over their combined private inputs, as if intermediated by a trusted third party. In the malicious model, this can be achieved with a cut-and-choose of garbled circuits (C&C-GCs), where some GCs are verified for correctness and the remaining are evaluated to determine the circuit output. This paper presents a new C&C-GCs-based S2PC protocol, with significant advantages in efficiency and applicability. First, in contrast with prior protocols that require a majority of evaluated GCs to be correct, the new protocol only requires that at least one evaluated GC is correct. In practice this reduces the total number of GCs to approximately one third, for the same statistical security goal. This is accomplished by augmenting the C&C with a new forge-and-lose technique based on bit commitments with trapdoor. Second, the output of the new protocol includes reusable XOR-homomorphic bit commitments of all circuit input and output bits, thereby enabling efficient linkage of several S2PCs in a reactive manner. The protocol has additional interesting characteristics (which may allow new comparison tradeoffs), such as needing a low number of exponentiations, using a 2-out-of-1 type of oblivious transfer, and using the C&C structure to statistically verify the consistency of input wire keys.

A special metric of interest about Boolean functions is multiplicative complexity (MC): the minimum number of AND gates sufficient to implement a function with a Boolean circuit over the basis {XOR, AND, NOT}. In this paper we study the MC of symmetric Boolean functions, whose output is invariant upon reordering of the input variables. Based on the Hamming weight method from Muller and Preparata (J. ACM 22(2), [195][196][197][198][199][200][201] 1975), we introduce new techniques that yield circuits with fewer AND gates than upper bounded by Boyar et al.

This paper considers the estimation of reliability and availability of intrusion-tolerant systems subject to non-detectable intrusions caused by stealth attacks. We observe that typical intrusion tolerance techniques may in certain circumstances worsen the dependability properties they were meant to improve. We model intrusions as a probabilistic effect of adversarial efforts and analyze different strategies of attack and rejuvenation. We compare several configurations of intrusion-tolerant replication and proactive rejuvenation, and varying mission times and expected times to node-intrusion. In doing so, we identify thresholds that distinguish between improvement and degradation of dependability, with a focus on security. We highlight the complementarity of replication and rejuvenation, showing improvements of resilience not attainable with any of the techniques alone, but possible when they are combined. We advocate the need for thorougher system models, by showing vulnerabilities arising from incomplete specifications.