Key exchange with the help of a public ledger

Blockchains and other public ledger structures promise a new way to create globally consistent event logs and other records. We make use of this consistency property to detect and prevent man-in-the-middle attacks in a key exchange such as Diffie-Hellman or ECDH. Essentially, the MitM attack creates an inconsistency in the world views of the two honest parties, and they can detect it with the help of the ledger. Thus, there is no need for prior knowledge or trusted third parties apart from the distributed ledger. To prevent impersonation attacks, we require user interaction. It appears that, in some applications, the required user interaction is reduced in comparison to other user-assisted key-exchange protocols

On the Design of Cryptographic Primitives

The main objective of this work is twofold. On the one hand, it gives a brief overview of the area of two-party cryptographic protocols. On the other hand, it proposes new schemes and guidelines for improving the practice of robust protocol design. In order to achieve such a double goal, a tour through the descriptions of the two main cryptographic primitives is carried out. Within this survey, some of the most representative algorithms based on the Theory of Finite Fields are provided and new general schemes and specific algorithms based on Graph Theory are proposed

Analysis of common attacks in LDPCC-based public-key cryptosystems

We analyze the security and reliability of a recently proposed class of public-key cryptosystems against attacks by unauthorized parties who have acquired partial knowledge of one or more of the private key components and/or of the plaintext. Phase diagrams are presented, showing critical partial knowledge levels required for unauthorized decryptionComment: 14 pages, 6 figure

A New View on Worst-Case to Average-Case Reductions for NP Problems

We study the result by Bogdanov and Trevisan (FOCS, 2003), who show that under reasonable assumptions, there is no non-adaptive worst-case to average-case reduction that bases the average-case hardness of an NP-problem on the worst-case complexity of an NP-complete problem. We replace the hiding and the heavy samples protocol in [BT03] by employing the histogram verification protocol of Haitner, Mahmoody and Xiao (CCC, 2010), which proves to be very useful in this context. Once the histogram is verified, our hiding protocol is directly public-coin, whereas the intuition behind the original protocol inherently relies on private coins

Dynamic Group Diffie-Hellman Key Exchange under Standard Assumptions

Authenticated Diffie-Hellman key exchange allows two principals communicating over a public network, and each holding public /private keys, to agree on a shared secret value. In this paper we study the natural extension of this cryptographic problem to a group of principals. We begin from existing formal security models and refine them to incorporate major missing details (e.g., strong-corruption and concurrent sessions). Within this model we define the execution of a protocol for authenticated dynamic group Diffie-Hellman and show that it is provably secure under the decisional Diffie-Hellman assumption. Our security result holds in the standard model and thus provides better security guarantees than previously published results in the random oracle model

Horizon formation and far-from-equilibrium isotropization in supersymmetric Yang-Mills plasma

Using gauge/gravity duality, we study the creation and evolution of anisotropic, homogeneous strongly coupled $\mathcal N=4$ supersymmetric Yang-Mills plasma. In the dual gravitational description, this corresponds to horizon formation in a geometry driven to be anisotropic by a time-dependent change in boundary conditions.Comment: 4 pages, typos corrected, published versio

Chosen-ciphertext security from subset sum

We construct a public-key encryption (PKE) scheme whose security is polynomial-time equivalent to the hardness of the Subset Sum problem. Our scheme achieves the standard notion of indistinguishability against chosen-ciphertext attacks (IND-CCA) and can be used to encrypt messages of arbitrary polynomial length, improving upon a previous construction by Lyubashevsky, Palacio, and Segev (TCC 2010) which achieved only the weaker notion of semantic security (IND-CPA) and whose concrete security decreases with the length of the message being encrypted. At the core of our construction is a trapdoor technique which originates in the work of Micciancio and Peikert (Eurocrypt 2012

Активные фильтры аналогового тракта построителя сейсмических разрезов

Secure, anonymous and unobservable communication is becoming increasingly important due to the gradual erosion of privacy in many aspects of everyday life. This prompts the need for various anonymity- and privacy-enhancing techniques, e.g., group signatures, anonymous e-cash and secret handshakes. In this paper, we investigate an interesting and practical cryptographic construct Oblivious Signature-Based Envelopes (OS-BEs) recently introduced in [15]. OSBEs are very useful in anonymous communication since they allow a sender to communicate information to a receiver such that the receiver s rights (or roles) are unknown to the sender. At the same time, a receiver can obtain the information only if it is authorized to access it. This makes OSBEs a natural fit for anonymity-oriented and privacy-preserving applications, such as Automated Trust Negotiation and Oblivious Subscriptions. Previous results yielded three OSBE constructs: one based on RSA and two based on Identity-Based Encryption (IBE). Our work focuses on the ElGamal signature family: we succeed in constructing practical and secure OSBE schemes for several well-known signature schemes, including: Schnorr, Nyberg-Rueppel, ElGamal and DSA. As experiments with the prototype implementation il-lustrate, our schemes are more efficient than previous techniques. Furthermore, we show that some OSBE schemes, despite offering affiliation privacy for the receiver, introduce no additional cost over schemes that do not offer this feature