Ring signatures, first introduced by Rivest, Shamir, and Tauman, enable a user to sign a message so that a ring of possible signers (of which the user is a member) is identified, without revealing exactly which member of that ring actually generated the signature. In contrast to group signatures, ring signatures are completely "ad-hoc" and do not require any central authority or coordination among the various users (indeed, users do not even need to be aware of each other); furthermore, ring signature schemes grant users fine-grained control over the level of anonymity associated with any particular signature. This paper has two main areas of focus. First, we examine previous definitions of security for ring signature schemes and suggest that most of these prior definitions are too weak, in the sense that they do not take into account certain realistic attacks. We propose new definitions of anonymity and unforgeability which address these threats, and then give separation results proving that our new notions are strictly stronger than previous ones. Next, we show two constructions of ring signature schemes in the standard model: one based on generic assumptions which satisfies our strongest definitions of security, and a second, more efficient scheme achieving weaker security guarantees and more limited functionality. These are the first constructions of ring signature schemes that do not rely on random oracles or ideal ciphers.

Ring signatures, first introduced by Rivest, Shamir, and Tauman, enable a user to sign a message so that a ring of possible signers (of which the user is a member) is identified, without revealing exactly which member of that ring actually generated the signature. In contrast to group signatures, ring signatures are completely "ad-hoc" and do not require any central authority or coordination among the various users (indeed, users do not even need to be aware of each other); furthermore, ring signature schemes grant users fine-grained control over the level of anonymity associated with any particular signature. This paper has two main areas of focus. First, we examine previous definitions of security for ring signature schemes and suggest that most of these prior definitions are too weak, in the sense that they do not take into account certain realistic attacks. We propose new definitions of anonymity and unforgeability which address these threats, and then give separation results proving that our new notions are strictly stronger than previous ones. Next, we show two constructions of ring signature schemes in the standard model: one based on generic assumptions which satisfies our strongest definitions of security, and a second, more efficient scheme achieving weaker security guarantees and more limited functionality. These are the first constructions of ring signature schemes that do not rely on random oracles or ideal ciphers.

Mapping the router topology is an important component of Internet measurement. Alias resolution, the process of mapping IP addresses to routers, is critical to accurate Internet mapping. Ally, a popular alias resolution tool, was developed to resolve aliases in individual ISPs, but its probabilistic accuracy and need to send O(n 2 ) probes to infer aliases among n IP addresses make it unappealing for large-scale Internet mapping. In this paper, we present RadarGun, a tool that uses IP identifier velocity modeling to improve the accuracy and scalability of the Ally-based resolution technique. We provide analytical bounds on Ally's accuracy and validate our predicted aliases against Ally. Additionally, we show that velocity modeling requires only O(n) probes and thus scales to Internet-sized mapping efforts.

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Online social networks (OSNs) are immensely popular, with some claiming over 200 million users [10]. Users share private content, such as personal information or photographs, using OSN applications. Users must trust the OSN service to protect personal information even as the OSN provider benefits from examining and sharing that information.We present Persona, an OSN where users dictate who may access their information. Persona hides user data with attribute-based encryption (ABE), allowing users to apply fine-grained policies over who may view their data. Persona provides an effective means of creating applications in which users, not the OSN, define policy over access to private data.We demonstrate new cryptographic mechanisms that enhance the general applicability of ABE. We show how Persona provides the functionality of existing online social networks with additional privacy benefits. We describe an implementation of Persona that replicates Facebook applications and show that Persona provides acceptable performance when browsing privacy-enhanced web pages, even on mobile devices.