*09:17* [Pub][ePrint]
On Black-Box Complexity of Universally Composable Security in the CRS model, by Carmit Hazay and Muthuramakrishnan Venkitasubramaniam
In this work, we study the intrinsic complexity of black-box Universally Composable (UC) secure computation based on general assumptions. We present a thorough study in various corruption modelings while focusing on achieving security in the common reference string (CRS) model. Our results involve the following:1. Static UC secure computation. Designing the first static UC secure oblivious transfer protocol based on public-key encryption and stand-alone semi-honest oblivious transfer. As a corollary we obtain the first black-box constructions of UC secure computation assuming only two-round semi-honest oblivious transfer.

2. One-sided UC secure computation. Designing adaptive UC secure two-party computation with single corruptions assuming public-key encryption with oblivious ciphertext generation.

3. Adaptive UC secure computation. Designing adaptively secure UC commitment scheme assuming only public-key encryption with oblivious ciphertext generation. As a corollary we obtain the first black-box constructions of adaptive UC secure computation assuming only (trapdoor) simulatable public-key encryption (as well as a variety of concrete assumptions). We remark that such a result was not known even under non-black-box constructions.

*21:17* [Pub][ePrint]
Improved security proofs in lattice-based cryptography: using the R\\\'enyi divergence rather than the statistical distance, by Shi Bai and Adeline Langlois and Tancr{\\`e}de Lepoint and Damien Stehl\
The R\\\'enyi divergence is a measure of closeness of two probability distributions. We show that it can often be used as an alternative

to the statistical distance in security proofs for lattice-based

cryptography. Using the R\\\'enyi divergence is particularly suited

for security proofs of primitives in which the attacker is required

to solve a search problem (e.g., forging a signature). We show that

it may also be used in the case of distinguishing problems (e.g.,

semantic security of encryption schemes), when they enjoy a public

sampleability property. The techniques lead to security proofs for

schemes with smaller parameters, and sometimes to simpler security

proofs than the existing ones.

*21:17* [Pub][ePrint]
Turning Online Ciphers Off, by Elena Andreeva and Guy Barwell and Dan Page and Martijn Stam
CAESAR has caused a heated discussion regarding the merits of one-pass encryption and online ciphers. The latter is a keyed, length preserving function which outputs ciphertext blocks as soon as the respective plaintext block is received. The immediacy of an online cipher gives a clear performance advantage, yet it comes at a price. Since ciphertext blocks cannot depend on later plaintext blocks, diffusion and hence security is limited. We show how one can attain the best of both worlds by providing provably secure constructions,achieving full cipher security, based on applying an online cipher and reordering blocks.

Explicitly, we show that with just two calls to the online cipher, security up to the birthday bound is both attainable and maximal. Moreover, we demonstrate that three calls to the online cipher suffice to obtain beyond birthday bound security, and (for suitably long messages) arbitrarily strong security. As part of our investigation, we extend an observation by Rogaway and Zhang, highlighting the close relationship between online ciphers and tweakable blockciphers with variable-length tweaks.

*21:17* [Pub][ePrint]
How to detect unauthorised usage of a key, by Jiangshan Yu and Mark Ryan and Cas Cremers
Encryption is useful only if the decryption key has not been exposed to adversaries; in particular, it requires that the device performing the crypto operations is free of malware. We explore ways in which some security guarantees can be achieved even if an attacker has succeeded in obtaining access to all the keys in a device, e.g. by exploiting software vulnerabilities.We develop a new protocol concept that allows the device owner to detect if another party is using the device\'s long-term key. We achieve this by making it necessary for uses of the key to be inserted in an append-only log, which the device owner can interrogate. We propose a multi-device messaging protocol that exploits our concept to allow users to detect unauthorised usage of their device keys. We prove the main properties of our protocol using the Tamarin prover.

The methods we introduce are not intended to replace existing methods used to keep keys safe (such as hardware devices or careful procedures). Rather, our methods provide a useful and effective additional layer of security.

*15:17* [Job][New]
Lecturer, *Royal Holloway, University of London*
Applications are invited for the post of Lecturer in the Information Security Group at Royal Holloway, University of LondonApplications are invited from researchers whose interests are related to, or complement, current strengths of the ISG. We are particularly interested in applicants who will be able to interact with our research groups in cryptography and systems security. However, applications from strong candidates working in other cyber security fields will also be given serious consideration.

Applicants should have a Ph.D. in a relevant subject or equivalent, be a self-motivated researcher, and have a strong publication record. Applicants should be able to demonstrate an enthusiasm for teaching and communicating with diverse audiences, as well as show an awareness of contemporary issues relating to cyber security.

This is a full time and permanent post, available from 1st September, 2015, or as soon as possible thereafter. This post is based in Egham, Surrey, where the College is situated in a beautiful, leafy campus near to Windsor Great Park and within commuting distance from London.

*09:17* [Pub][ePrint]
Time-release Protocol from Bitcoin and Witness Encryption for SAT, by Jia Liu and Flavio Garcia and Mark Ryan
We propose a new time-release protocol based on the bitcoin protocol and witness encryption. We derive a ``public key\'\' from the bitcoin block chain for encryption. The decryption key are the unpredictable information in the future blocks (e.g., transactions, nonces)that will be computed by the bitcoin network. We build this protocol by witness encryption and encrypt with the bitcoin proof-of-work constraints. The novelty of our protocol is that the decryption key will be automatically and publicly available in the bitcoin block chain when the time is due.

Witness encryption was originally proposed by Garg, Gentry, Sahai and Waters. It provides a means to encrypt to an instance, $x$, of an NP language and to decrypt by a witness $w$ that $x$ is in the language.

Encoding CNF-SAT in the existing witness encryption schemes generate poly(n*k) group elements in the ciphertext where n is the number of variables and k is the number of clauses of the CNF formula.

We design a new witness encryption for CNF-SAT which achieves ciphertext size of 2n+2k group elements. Our witness encryption is based on an intuitive reduction from SAT to Subset-Sum problem. Our scheme uses the framework of multilinear maps, but it is independent of the implementation details of multilinear maps.

*21:17* [Pub][ePrint]
A Provably Secure Group Signature Scheme from Code-Based Assumptions, by Martianus Frederic Ezerman and Hyung Tae Lee and San Ling and Khoa Nguyen and Huaxiong Wang
We solve an open question in code-based cryptography by introducing the first provably secure group signature scheme from code-based assumptions. Specifically, the scheme satisfies the CPA-anonymity and traceability requirements in the random oracle model, assuming the hardness of the McEliece problem, the Learning Parity with Noise problem, and a variant of the Syndrome Decoding problem. Our construction produces smaller key and signature sizes than all of the existing post-quantum group signature schemes from lattices, as long as the cardinality of the underlying group does not exceed the population of the Netherlands ($\\approx 2^{24}$ users).The feasibility of the scheme is supported by implementation results.

Additionally, the techniques introduced in this work might be of independent interest: a new verifiable encryption protocol for the randomized McEliece encryption and a new approach to design formal security reductions from the Syndrome Decoding problem.