Abstract. In this paper, we investigate the recent paradigm for group signatures proposed by Rivest et al . at Asiacrypt '01. We first improve on their ring signature paradigm by showing that it holds under a strictly weaker assumption, namely the random oracle model rather than the ideal cipher. Then we provide extensions to make ring signatures suitable in practical situations, such as threshold schemes or ad-hoc groups. Finally we propose an efficient scheme for threshold scenarios based on a combinatorial method and provably secure in the random oracle model.
Abstract. In this paper, we describe a three-stage attack against Revised NSS, an NTRU-based signature scheme proposed at the Eurocrypt 2001 conference as an enhancement of the (broken) proceedings version of the scheme. The first stage, which typically uses a transcript of only 4 signatures, effectively cuts the key length in half while completely avoiding the intended hard lattice problem. After an empirically fast second stage, the third stage of the attack combines lattice-based and congruence-based methods in a novel way to recover the private key in polynomial time. This cryptanalysis shows that a passive adversary observing only a few valid signatures can recover the signer's entire private key. We also briefly address the security of NTRUSign, another NTRUbased signature scheme that was recently proposed at the rump session of Asiacrypt 2001. As we explain, some of our attacks on Revised NSS may be extended to NTRUSign, but a much longer transcript is necessary. We also indicate how the security of NTRUSign is based on the hardness of several problems, not solely on the hardness of the usual NTRU lattice problem.
Abstract. We present a technique for Merkle tree traversal which requires only logarithmic space and time. For a tree with N leaves, our algorithm computes sequential tree leaves and authentication path data in time 2 log 2 (N ) and space less than 3 log 2 (N ), where the units of computation are hash function evaluations or leaf value computations, and the units of space are the number of node values stored. This result is an asymptotic improvement over all other previous results (for example, measuring cost = space * time). We also prove that the complexity of our algorithm is optimal: There can exist no Merkle tree traversal algorithm which consumes both less than O(log 2 (N )) space and less than O(log 2 (N )) time. Our algorithm is especially of practical interest when space efficiency is required.
User authentication in computing systems traditionally depends on three factors: something you have (e.g., a hardware token), something you are (e.g., a fingerprint), and something you know (e.g., a password). In this paper, we explore a fourth factor, the social network of the user, that is, somebody you know.Human authentication through mutual acquaintance is an age-old practice. In the arena of computer security, it plays roles in privilege delegation, peer-level certification, helpdesk assistance, and reputation networks. As a direct means of logical authentication, though, the reliance of human being on another has little supporting scientific literature or practice.In this paper, we explore the notion of vouching, that is, peer-level, human-intermediated authentication for access control. We explore its use in emergency authentication, when primary authenticators like passwords or hardware tokens become unavailable. We describe a practical, prototype vouching system based on SecurID, a popular hardware authentication token. We address traditional, cryptographic security requirements, but also consider questions of social engineering and user behavior.
Abstract. Naor, Pinkas, and Sumner introduced and implemented a sealed-bid, two-server auction system that is perhaps the most efficient and practical to date. Based on a cryptographic primitive known as oblivious treinsfer, their system aims to ensure priveicy and correctness provided that at least one auction server behaves honestly. As observed in [19], however, the NFS system suffers from a security flaw in which one of the two servers can cheat so as to modify bids almost arbitrarily and without detection. We propose a means of repairing this flaw while preserving the attractive practical elements of the NFS protocol, including minimal round complexity for servers and minimal computation by players providing private inputs. Our proposal requires a slightly greater amount of computation and communication on the part of the two auction servers, but actually involves much less computation on the part of bidders. This latter feature makes our proposal particularly attractive for use with low-power devices. While the original proposal of NFS involved several dozen exponentiations for a typical auction, ours by contrast involves only several dozen modular multiplications. The key idea in our proposal is a form of oblivious transfer that we refer to as verifiable proxy oblimous transfer (VPOT).
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