Choosing the most effective word-mangling rules to use when performing a dictionary-based password cracking attack can be a difficult task. In this paper we discuss a new method that generates password structures in highest probability order. We first automatically create a probabilistic context-free grammar based upon a training set of previously disclosed passwords. This grammar then allows us to generate word-mangling rules, and from them, password guesses to be used in password cracking. We will also show that this approach seems to provide a more effective way to crack passwords as compared to traditional methods by testing our tools and techniques on real password sets. In one series of experiments, training on a set of disclosed passwords, our approach was able to crack 28% to 129% more passwords than John the Ripper, a publicly available standard password cracking program.
We introduce the notion of sanitizable signatures that offer many attractive security features for certain current and emerging applications. A sanitizable signature allows authorized semi-trusted censors to modify -in a limited and controlled fashion -parts of a signed message without interacting with the original signer. We present constructions for this new primitive, based on standard signature schemes and secure under common cryptographic assumptions. We also provide experimental measurements for the implementation of a sanitizable signature scheme and demonstrate its practicality.
Chameleon signatures were introduced by Krawczyk and Rabin, being non-interactive signature schemes that provide non-transferability. However, that first construction employs a chameleon hash that suffers from a key exposure problem: The non-transferability property requires willingness of the recipient in consequentially exposing a secret key, and therefore invalidating all signatures issued to the same recipient's public key. To address this key-revocation issue, and its attending problems of key redistribution, storage of state information, and greater need for interaction, an identity-based scheme was proposed in [1], while a fully key-exposure free construction, based on the elliptic curves with pairings, appeared later in [7].Herein we provide several constructions of exposure-free chameleon hash functions based on different cryptographic assumptions, such as the RSA and the discrete logarithm assumptions. One of the schemes is a novel construction that relies on a single trapdoor and therefore may potentially be realized over a large set of cryptographic groups (where the discrete logarithm is hard).
Abstract. We investigate an application of RFIDs referred to in the literature as group scanning, in which several tags are "simultaneously" scanned by a reader device. Our goal is to study the group scanning problem in strong adversarial models. We present a security model for this application and give a formal description of the attending security requirements, focusing on the privacy (anonymity) of the grouped tags, and/ or forward-security properties. Our model is based on the Universal Composability framework and supports re-usability (through modularity of security guarantees). We introduce novel protocols that realize the security models, focusing on efficient solutions based on off-the-shelf components, such as highly optimized pseudo-random function designs that require fewer than 2000 Gate-Equivalents.
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