Ordering our world: the quest for traces of temporal organization in autobiographical memory

An experiment examined the idea, derived from the Self Memory System model (Conway & Pleydell-Pearce, 2000), that autobiographical events are sometimes tagged in memory with labels reflecting the life era in which an event occurred. The presence of such labels should affect the ease of judgments of the order in which life events occurred. Accordingly, 39 participants judged the order of two autobiographical events. Latency data consistently showed that between-era judgments were faster than within-era judgments, when the eras were defined in terms of either: (a) college versus high school, (b) academic quarter within year, or (c) academic year within school. The accuracy data similarly supported the presence of a between-era judgment effect for the college versus high school dichotomy

New constructions and practical applications for private stream searching (extended abstract

Abstract A system for private stream searching allows a clientto retrieve documents matching some search criteria from a remote server while the server evaluating the re-quest remains provably oblivious to the search criteria. In this extended abstract, we give a high level outlineof a new scheme for this problem and an experimental analysis of its scalability. The new scheme is highlyefficient in practice. We demonstrate the practical applicability of the scheme by considering its performancein the demanding scenario of providing a privacy preserving version of the Google News Alerts service.

Ciphertext-policy attribute-based encryption

In several distributed systems a user should only be able to access data if a user posses a certain set of credentials or attributes. Currently, the only method for enforcing such policies is to employ a trusted server to store the data and mediate access control. However, if any server storing the data is compromised, then the confidentiality of the data will be compromised. In this paper we present a system for realizing complex access control on encrypted data that we call Ciphertext-Policy Attribute-Based Encryption. By using our techniques encrypted data can be kept confidential even if the storage server is untrusted; moreover, our methods are secure against collusion attacks. Previous Attribute-Based Encryption systems used attributes to describe the encrypted data and built policies into user’s keys; while in our system attributes are used to describe a user’s credentials, and a party encrypting data determines a policy for who can decrypt. Thus, our methods are conceptually closer to traditional access control methods such as Role-Based Access Control (RBAC). In addition, we provide an implementation of our system and give performance measurements.

Cryptographic Techniques for Privacy Preserving Identity

Currently, people have a limited range of choices in managing their identity online. They can use their real name or a long-term pseudonym, thereby lending context and credibility to information they publish but retaining no control over their privacy, or they can post anonymously, ensuring strong privacy but lending no additional credibility to their posts. In this work, we aim to develop a new type of online identity that allows users to publish information anonymously and unlinkably while simultaneously backing their posts with the credibility offered by a single, persistent identity. We show how these seemingly contradictory goals can be achieved through a series of new cryptographic techniques.Our consideration of the utility of persistent identities focuses on their ability to develop reputation. In particular, many online forums include systems for recording feedback from a user's prior behavior and using it to filter spam and predict the quality of new content. However, the dependence of this reputation information on a user's history of activities seems to preclude any possibility of anonymity. We demonstrate that useful reputation can, in fact, coexist with strong privacy guarantees by developing a novel cryptographic primitive we call “signatures of reputation” which supports monotonic measures of reputation in a completely anonymous setting. In our system, users can express trust in others by voting for them, collect votes to build up their own reputation, and attach a proof of their reputation to any data they publish, all while maintaining the unlinkability of their actions.Effective use of our scheme for signatures of reputation requires a means of selectively retrieving information while hiding one's search criteria. The sensitivity of search criteria is widely recognized and has previously been addressed through a series of cryptographic schemes for private information retrieval (PIR). Among the more recent of these is a scheme proposed by Ostrovsky and Skeith for a variant of PIR termed “private stream searching.” In this setting, a client encrypts a set of search keywords and sends the resulting query to an untrusted server. The server uses the query on a stream of documents and returns those that match to the client while learning nothing about the keywords in the query. To retrieve documents of total length n, the Ostrovsky-Skeith scheme requires the server to return data of length O(n log n). We present a new private stream searching scheme that improves on this result in several ways. First, we reduce the asymptotic communication to O(n + m log m), where m ≤ n is the number of distinct documents returned. More importantly, our scheme improves the multiplicative constants, resulting in an order of magnitude reduction in communication in typical scenarios. We also provide several extensions to our scheme which increase its flexibility and correspondingly broaden its applicability.With the help of our private stream searching scheme, the proposed signatures of reputation allow users to accumulate positive feedback over time and attach a proof of their current reputation to any information they post online, all while maintaining the unlinkability of their actions. Because of the unlinkability provided, the user is free to use a single identity across all applications, thereby obtaining the most reputation. A detailed analysis of practical performance shows that our proposals, while costly, are within the capabilities of present computing and communications infrastructure.We conclude our investigation into the potential for new forms of online identity with an evaluation of what might be considered the final frontier in attacks on anonymity: the possibility of linking posted information to its author solely through its content. Even if all explicit forms of identity are stripped from information a user posts online, it must remain intelligible to others to be useful. In the case of textual content, we note that the techniques of stylometry might allow an adversary to determine the likely author of an anonymous post by comparing it to material previously posted elsewhere. Through a series of large-scale experiments we show that, in some cases, this is indeed possible, and that individuals who have authored large amounts of content already online are the most vulnerable

Cryptographic methods for storing ballots on a voting machine

A direct recording electronic (DRE) voting machine must satisfy several requirements to ensure voter privacy and the integrity of the election. A recent proposal for a vote storage system due to Molnar et al. provides tamper-evidence properties while maintaining voter privacy by storing ballots on a programmable, read-only memory (PROM). We achieve the same properties and protect against additional threats of memory replacement through cryptographic techniques, without the use of special hardware. Our approach is based on a new cryptographic primitive called History-Hiding Append-Only Signatures.