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2013-02-06
16:17 [Pub][ePrint]

Certificateless public key cryptography (CLPKC), with properties of no key escrow and no certificate, has received a lot of attention since its invention. However, membership revocation in certificateless cryptosystem still remains a non-trivial problem: the existing solutions are not practical for use due to either a costly mediator or enormous computation (secret channel). In this paper, we present a new approach to revocation in CLPKC with a concrete construction of a revocable certificateless signature (RCLS) scheme. In our scheme, a user\'s private key is composed of three parts: an initial partial private key, a time key and a secret value. The transmission of updated-key requires only a public channel, which makes our RCLS scheme more efficient than other methods. We first provide formal definition and security model for a RCLS scheme. The new scheme is proved secure in the random oracle model, based on the Computational Diffie-Hellman problem.

16:17 [Pub][ePrint]

In this paper, we introduce the notion of a joint compartmented threshold access structure (JCTAS). We study the necessary conditions for the existence of an ideal and perfect secret sharing scheme and give a characterization of almost all ideal JCTASes. Then we give an ideal and almost surely perfect construction that realizes such access structures. We prove the asymptotic perfectness of this construction by the Schwartz-Zippel Lemma.

16:17 [Pub][ePrint]

We show that some problems in information security can be solved without using one-way functions. The latter are usually regarded as a central concept of cryptography, but the very existence of one-way functions depends on difficult conjectures in complexity theory, most notably on the notorious \"$P \\ne NP$\" conjecture.

In this paper, we suggest protocols for secure computation of the sum, product, and some other functions, without using any one-way functions. A new input that we offer here is that, in contrast with other proposals, we conceal \"intermediate results\" of a computation. For example, when we compute the sum of $k$ numbers, only the final result is known to the parties; partial sums are not known to anybody. Other applications of our method include voting/rating over insecure channels and a rather elegant and efficient solution of Yao\'s \"millionaires\' problem\".

Then, while it is fairly obvious that a secure (bit) commitment between two parties is impossible without a one-way function, we show that it is possible if the number of parties is at least 3. We also show how our (bit) commitment scheme for 3 parties can be used to arrange an unconditionally secure (bit) commitment between just two parties if they use a \"dummy\" (e.g., a computer) as the third party. We explain how our concept of a \"dummy\" is different from a well-known concept of a \"trusted third party\".

We also suggest a protocol, without using a one-way function, for \"mental poker\", i.e., a fair card dealing (and playing) over distance. We also propose a secret sharing scheme where an advantage over Shamir\'s and other known secret sharing schemes is that nobody, including the dealer, ends up knowing the shares owned by any particular player.

It should be mentioned that computational cost of our protocols is negligible to the point that all of them can be executed without a computer.

16:17 [Pub][ePrint]

Maximum distance separable (MDS) matrices have applications not only in coding theory but also are

of great importance in the design of block ciphers and hash functions. It is highly nontrivial

to find MDS matrices which could be used in lightweight cryptography.

In a crypto 2011 paper, Guo et. al. proposed a new MDS matrix $Serial(1,2,1,4)^4$ over $\\mathbb{F}_{2^8}$.

This representation has a compact hardware implementation of the AES MixColumn operation.

No general study of MDS properties of this newly introduced construction of the form

$Serial(z_0,\\ldots,z_{d-1})^d$ over $\\mathbb{F}_{2^n}$

for arbitrary $d$ and $n$ is available in the literature.

In this paper we study some properties of MDS matrices and provide an insight of

why $Serial(z_0,\\ldots,z_{d-1})^d$ leads to an MDS matrix.

For efficient hardware implementation, we aim to restrict the values of $z_i$\'s in

$\\{1,\\alpha,\\alpha^2,\\alpha+1\\}$, such that $Serial(z_0,\\ldots,z_{d-1})^d$ is MDS for $d = 4 \\mbox{ and } 5$, where

$\\alpha$ is the root of the constructing polynomial of $\\mathbb{F}_{2^n}$.

We also propose more generic constructions of MDS matrices e.g.

we construct lightweight $4 \\times 4$ and $5 \\times 5$ MDS matrices over $\\mathbb{F}_{2^n}$ for all $n \\ge 4$.

An algorithm is presented to check if a given matrix is MDS. The algorithm

directly follows from the basic properties of MDS matrix and is easy to implement.

16:17 [Pub][ePrint]

In 1978, Rivest, Adleman and Dertouzos introduced the basic concept of privacy homomorphism that allows computation on encrypted data without decryption.

It was elegant work that precedes the recent development of fully homomorphic encryption schemes although there were found some security flaws, e.g., ring homomorphic schemes are broken by the known-plaintext attacks.

In this paper, we revisit one of their proposals, in particular the third scheme which is based on the Chinese Remainder Theorem and is ring homomorphic.

The previous result is that only a single pair of known plaintext/ciphertext can break this scheme.

However, by exploiting the standard technique to insert an error to a message before encryption, we can cope with this problem.

We present a secure modification of their proposal by showing that the proposed scheme is

fully homomorphic and secure against the chosen plaintext attacks under the decisional approximate GCD assumption {{and the sparse subset sum assumption}} when the message space is restricted to $\\Z_2^k$.

Interestingly, the proposed scheme can be regarded as a generalization of the DGHV scheme with larger plaintext.

Our scheme has $\\tilde{O}(\\lambda^5)$ overhead while the DGHV has ${\\tilde{O}}(\\lambda^8)$ for the security parameter $\\lambda$.

When restricted to the homomorphic encryption scheme with depth-$O(\\log \\lambda)$, the overhead is reduced to $\\tilde{O}(\\lambda)$.

Our scheme can be used

in applications requiring a large message space $\\Z_Q$ for $\\log Q=O(\\lambda^4)$ or SIMD style operations on $\\Z_Q^k$ for $\\log Q=O(\\lambda), k=O(\\lambda^3)$, with $\\tilde{O}(\\lambda^5)$ ciphertext size as in the DGHV.

2013-02-05
09:46 [Job][New]

Project: In a community, members provide each other with various types of help, usually nonprofessional and nonmaterial, for a particular shared, usually burdensome, characteristic. The help may take the form of providing and evaluating relevant information, relating personal experiences, listening to and accepting advice, providing sympathetic understanding and establishing social networks. Today, an emerging trend is that patients (non-professionals) with similar conditions form communities so that they can reach out to each other for advice and sharing experiences. Healthcare professionals are also involved in the process to improve the services, and to do research on the basis of the data generated by a community.

Goal: design new privacy and security techniques to support self-help communities. The project is a collaboration with several academic and industrial partners.

What we ask and what we offer: You have Master degree or you are about to graduate in computer science or mathematics, and you have a solid background in applied cryptography and/or information security. You like working in a team. You will be appointed for a period of four years, at the end of which you must have completed a PhD thesis. During this period you have the opportunity to broaden your knowledge by joining international exchange programs, to participate in national and international conferences and workshops, and to visit other research institutes and universities worldwide.

The monthly salary of a PhD student ranges from EURO 1956, - gross in the first year to EURO 2502, - gross in the fourth year.

09:46 [Job][New]

The Security Lab at Swedish Institute of Computer Science (SICS) in Stockholm is looking for a talent post doc researcher candidate in the area of system security and especially trusted computing technologies in virtualized software systems. We are looking for a talent researcher with interest in applied research and good knowledge in Trusted Computing Technologies.

Swedish Institute of Computer Science is a non-profit Swedish applied research institute.

The security Lab at SICS was established in 2009. Since then it has grown from 1 to 8 people. The research is directed toward secure systems design in close co-operation with above leading Swedish companies in the IT and telecommunications businesses as well as Swedish universities such as Royal Institute of Technology in Stockholm. The group has developed an own hypervisor providing secure execution in ARM based embedded systems that currently undergo formal verification. Furthermore, the group is performing lots of research on usage of Trusted Computing technologies to secure future cloud infrastructures. The secure systems group at SICS consists for the moment of 4 senior researchers (PhD), 2 PhD students and addition 2 junior researchers with MSc degrees in computer science.

09:07 [Event][New]

From May 13 to May 26
Location: Leiden , The Netherlands

09:07 [Event][New]

Submission: 4 March 2013
From June 24 to June 27
Location: Atlantic City, USA

09:05 [Event][New]

Submission: 10 April 2013
From July 25 to July 26
Location: Seoul, Korea

2013-02-01
12:59 [Job][New]

The research department of the Orange telecommunication company is searching for a suitable candidate for a post-doctoral research position to be involved in our Applied Crypto Group, especially related to cryptographic algorithms for the security of the cloud. Suitable candidates are requested to apply immediately. Candidate selection will continue until the position is filled.