International Association for Cryptologic Research

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2013-03-27
09:09 [Job][New]

The Information Security Centre of Excellence is looking for several stand-out PhD students who want to pursue research in the area of network security and collaborate closely with industry partners. The positions are fully-funded. To apply please send your detailed CV.

2013-03-26
15:17 [Pub][ePrint]

A new implementation of the GHASH function has been recently committed to a Git version of OpenSSL, to speed up AES-GCM. We identified a bug in that implementation, and made sure it was quickly fixed before trickling into an official OpenSSL trunk. Here, we use this (already fixed) bug as a real example that demonstrates the fragility of AES-GCM\'s authentication algorithm (GHASH). One might expect that incorrect MAC tag generation would only cause legitimate message-tag pairs to fail authentication (which is already a serious problem). However, since GHASH is a \"polynomial evaluation\" MAC, the bug can be exploited for actual message forgery.

15:17 [Pub][ePrint]

We demonstrate the high-speed computation of core elliptic curve operations with full protection against timing-type side-channel attacks. We use a state-of-the-art GLV-GLS curve in twisted Edwards form defined over a quadratic extension field of large prime characteristic, which supports a four dimensional decomposition of the scalar. We present highly optimized algorithms and formulas for speeding up the different arithmetic layers, including techniques especially suitable for high-speed, side-channel protected computation on GLV-based implementations. Analysis and performance results are reported for modern x64 and ARM processors. For instance, on an Intel Ivy Bridge processor we compute a variable-base scalar multiplication in 94,000 cycles, a fixed-base scalar multiplication in 53,000 cycles using a table of 6KB, and a double scalar multiplication in 118,000 cycles using a table of 3KB. Similarly, on an ARM Cortex-A15 processor we compute a variable-base scalar multiplication in 244,000 cycles, a fixed-base scalar multiplication in 116,000 cycles (table of 6KB), and a double scalar multiplication in 285,000 cycles (table of 3KB). All these numbers and the proposed techniques represent a significant improvement of the state-of-the-art performance of elliptic curve computations. Most remarkably, our optimizations allow us to reduce the cost of adding protection against timing attacks in the computation of variable-base scalar multiplication to around or below 10%.

15:17 [Pub][ePrint]

In Eurocrypt 2012, Lewko presented a fully secure IBE scheme in the

prime order setting based on the decisional linear assumption. We note that

some random factor involved in the ciphertext can further be used to hide yet another message

, and get a new fully secure IBE scheme with better message-ciphertext rate.

Similar to Lewko\'s

scheme, we use dual pairing vector space in prime order bilinear

groups to simulate the canceling and parameter hiding properties of

composite order settings. The security of our scheme is based on the

subspace assumption, which can be reduced to the decisional linear

assumption. We employ the dual system encryption technique in our

security proof.

15:17 [Pub][ePrint]

How can we encode a communication protocol between two parties to become resilient to adversarial errors on the communication channel? This question dates back to the seminal works of Shannon and Hamming from the 1940\'s, initiating the study of error-correcting codes (ECC). But, even if we encode each message in the communication protocol with a good\'\' ECC, the error rate of the encoded protocol becomes poor (namely $O(1/m)$ where $m$ is the number of communication rounds). Towards addressing this issue, Schulman (FOCS\'92, STOC\'93) introduced the notion of \\emph{interactive coding}.

We argue that whereas the method of separately encoding each message with an ECC ensures that the encoded protocol carries the same amount of information as the original protocol, this may no longer be the case if using interactive coding. In particular, the encoded protocol may completely leak a player\'s private input, even if it would remain secret in the original protocol. Towards addressing this problem, we introduce the notion of \\emph{knowledge-preserving interactive coding}, where the interactive coding protocol is required to preserve the knowledge\'\' transmitted in the original protocol. Our main results are as follows.

\\begin{itemize}

\\item The method of separately applying ECCs to each message is essentially optimal: No knowledge-preserving interactive coding scheme can have an error rate of $1/m$, where $m$ is the number of rounds in the original protocol.

\\item If restricting to computationally-bounded (polynomial-time) adversaries, then assuming the existence of one-way functions (resp. subexponentially-hard one-way functions), for every $\\epsilon>0$, there exists a knowledge-preserving interactive coding schemes with constant error rate and information rate $n^{-\\epsilon}$ (resp. $1/\\polylog(n)$) where $n$ is the security parameter; additionally to achieve an error of even $1/m$ requires the existence of one-way functions.

\\item Finally, even if we restrict to computationally-bounded adversaries, knowledge-preserving interactive coding schemes with constant error rate can have an information rate of at most $o(1/\\log n)$. This results applies even to \\emph{non-constructive} interactive coding schemes.

\\end{itemize}

15:17 [Pub][ePrint]

Since Kilian showed in 1988 that oblivious transfer (OT) is complete in the sense that every secure multi-party computation can be realized from this primitive, cryptographers are working on reduction of OT to various other primitives. A long standing open question in this context is the classification of finite stateless 2-party primitives (so-called \"cryptogates\"), i.e. trusted black boxes that can be jointly queried by two parties, have finite input and output alphabets, and do not change behavior depending on time or input history. Over the decades, completeness criteria have been found for deterministic cryptogates (i.e. primitives without internal randomness), noisy channels, and symmetric (i.e., both parties receive the same output) or asymmetric (i.e., only one party receives any output at all) randomized cryptogates. However, the known criteria for randomized primitives other than noisy channels only hold in presence of passive adversaries (i.e., even corrupted parties still follow the protocol).

We complete this line of research by providing simple but comprehensive combinatorial completeness criteria for ALL finite stateless 2-party primitives. I.e., for the first time there are completeness criteria for randomized primitives that are neither symmetric nor asymmetric (but give different outputs to the querying parties), and we overcome the limitation that previous results for randomized primitives with input from BOTH parties only regarded passive adversaries. A fundamental tool of our approach is a powerful lemma from real algebraic geometry, which allows us to base a cryptographic security proof on a rather \"game-theoretic\" approach.

As a corollary of our work, every non-complete example of a finite stateless 2-party primitive is essentially symmetric. This relationship between non-completeness and symmetric output behavior was previously only known for deterministic cryptogates.

15:17 [Pub][ePrint]

We propose here a non asymptotic complexity analysis of

some variants of information set decoding. In particular, we give this

analysis for the two recent variants { published by May, Meurer and

Thomae in 2011 and by Becker, Joux, May and Meurer in 2012 { for

which only an asymptotic analysis was available. The purpose is to

provide a simple and accurate estimate of the complexity to facilitate

the paramater selection for code-based cryptosystems. We implemented

those estimates and give a comparison at the end of the paper.

15:17 [Pub][ePrint]

Searching on remote encrypted data (commonly known as \\textit{searchable encryption}) is becoming an important technique in secure data outsourcing, since it allows users to outsource encrypted data to the third party and maintains the keyword searching on the data at the same time.

It has been widely accepted in the literature that searchable encryption techniques should leak as little information as possible to the third party. An early classical method called oblivious RAM hides all information at the cost of poly-logarithmic computation and communication overheads, which turns out to be impractical in the real world applications (e.g., cloud computing). A number of efficient searchable encryption schemes have been proposed under weaker security guarantees afterwards, however, such schemes leak statistical information about the user\'s search pattern.

In this paper, we show that the search pattern leakage can result in non-trivial risks. As pioneer work, we present two concrete attack models exploiting user\'s search pattern and some auxiliary background knowledge aiming to disclose the underlying keywords of user\'s queries. To resist these attacks, we develop two new searchable encryption constructions that hide the search pattern. Our constructions are designed to be independent from the underlying searchable encryption scheme. Our experiments, which are based on the real world dataset, demonstrate the effectiveness and efficiency of proposed attack models and new constructions.

15:17 [Pub][ePrint]

In this paper we present the (to the best of our knowledge) first LWE-based encryption scheme that removes the need of Gaussian sampling for the error, i.e. the discrete Gaussian distribution is replaced by the uniform distribution on a (small) set, which at the same time preserves the underlying worst-case hardness. This shows that provable security and efficiency do not necessarily have to mutually exclude each other. We give an asymptotic parameter instantiation for our scheme, as well as some hardness results for LWE which might be of independent interest.

2013-03-25
15:31 [Event][New]

Submission: 8 May 2013