Since designated confirmer signature schemes were introduced by Chaum and formalized by Okamoto, a number of attempts have been made to design designated confirmer signature schemes which are efficient and at the same time provably secure under standard cryptographic assumptions. Yet, there has been a consistent gap in security claims and analysis between all generic theoretical proposals and any concrete implementation proposal one can envision using in practice. In this paper we propose a modification of Okamoto's definition of security which still captures security against chosen message attack, and yet enables the design of concrete and reasonably efficient designated confirmer signature schemes which can be proved secure without resorting to random oracle assumptions as previously done. In particular, we present simple transformations of the digital signature schemes of Cramer-Shoup, Goldwasser-Micali-Rivest and Gennaro-Halevi-Rabin into secure designated confirmer signature schemes. We prove security of the schemes obtained under the same security assumption made by the digital signature scheme transformed and an encryption scheme we use as a tool.

Monitoring tasks, such as anomaly and DDoS detection, require identifying frequent flow aggregates based on common IP prefixes. These are known as hierarchical heavy hitters (HHH), where the hierarchy is determined based on the type of prefixes of interest in a given application. The per packet complexity of existing HHH algorithms is proportional to the size of the hierarchy, imposing significant overheads.In this paper, we propose a randomized constant time algorithm for HHH. We prove probabilistic precision bounds backed by an empirical evaluation. Using four real Internet packet traces, we demonstrate that our algorithm indeed obtains comparable accuracy and recall as previous works, while running up to 62 times faster. Finally, we extended Open vSwitch (OVS) with our algorithm and showed it is able to handle 13.8 million packets per second. In contrast, incorporating previous works in OVS only obtained 2.5 times lower throughput.

One of the biggest challenges for the Internet of Things (IoT) is to bridge the currently fragmented trust domains. The traditional PKI model relies on a common root of trust and does not fit well with the heterogeneous IoT ecosystem where constrained devices belong to independent administrative domains.In this work we describe a distributed trust model for the IoT that leverages the existing trust domains and bridges them to create endto-end trust between IoT devices without relying on any common root of trust. Furthermore we define a new cryptographic primitive, denoted as obligation chain designed as a credit-based Blockchain with a built-in reputation mechanism. Its innovative design enables a wide range of use cases and business models that are simply not possible with current Blockchain-based solutions while not experiencing traditional blockchain delays. We provide a security analysis for both the obligation chain and the overall architecture and provide experimental tests that show its viability and quality.

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Web search has become an integral part of our lives and we use it daily for business and pleasure. Unfortunately, however, we unwittingly reveal a huge amount of private information about ourselves when we search the web. A look at a user's search terms over a period of a few months paints a frighteningly clear and detailed picture about the user's life. In this paper, we build on previous work by Castellà-Roca et al. (Computer Communications 2009) and show how to achieve privacy in web searches efficiently and practically without resorting to full-blown anonymous routing. In contrast to previous work, our protocol is secure in the presence of malicious adversaries.