*00:17* [Pub][JoC]
An Efficient State Recovery Attack on the X-FCSR Family of Stream Ciphers
Abstract We describe a state recovery attack on the X-FCSR family of stream ciphers. In this attack we analyse each block of output keystream and try to solve for the state. The solver will succeed when a number of state conditions are satisfied. For X-FCSR-256, our best attack has a computational complexity of only 24.7 table lookups per block of keystream, with an expected 244.3 such blocks before the attack is successful. The precomputational storage requirement is 233. For X-FCSR-128, the computational complexity of our best attack is 216.3 table lookups per block of keystream, where we expect 255.2 output blocks before the attack comes through. The precomputational storage requirement for X-FCSR-128 is 267.

- Content Type Journal Article
- Pages 1-22
- DOI 10.1007/s00145-012-9130-9
- Authors

- Paul Stankovski, Dept. of Electrical and Information Technology, Lund University, P.O. Box 118, 221 00 Lund, Sweden
- Martin Hell, Dept. of Electrical and Information Technology, Lund University, P.O. Box 118, 221 00 Lund, Sweden
- Thomas Johansson, Dept. of Electrical and Information Technology, Lund University, P.O. Box 118, 221 00 Lund, Sweden

- Journal Journal of Cryptology
- Online ISSN 1432-1378
- Print ISSN 0933-2790

From: Fri, 07 Sep 2012 16:46:28 GMT
*15:17* [Pub][ePrint]
Dynamic Searchable Symmetric Encryption, by Seny Kamara and Charalampos Papamanthou and Tom Roeder
Searchable symmetric encryption (SSE) allows a client to encrypt its data in such a way that this data can still be searched. The most immediate application of SSE is to cloud storage, where it enables a client to securely outsource its data to an untrusted cloud provider without sacrificing the ability to search over it. SSE has been the focus of active research and a multitude of schemes that achieve various levels of security and efficiency have been proposed. Any practical SSE scheme, however, should (at a minimum) satisfy the following properties: sublinear search time, security against adaptive chosen-keyword attacks, compact indexes and the ability to add and delete files efficiently. Unfortunately, none of the previously-known SSE constructions achieve all these properties at the same time. This severely limits the practical value of SSE and decreases its chance of deployment in real-world cloud storage systems.

To address this, we propose the first SSE scheme to satisfy all the properties outlined above. Our construction extends the inverted index approach (Curtmola et al., CCS 2006) in several non-trivial ways and introduces new techniques for the design of SSE. In addition, we implement our scheme and conduct a performance evaluation, showing that our approach is highly efficient and ready for deployment.

*15:17* [Pub][ePrint]
Generic Construction of Trace and Revoke Schemes, by Murat Ak, Aggelos Kiayias, Serdar Pehlivanoglu, Ali AydÄ±n Selcuk
Broadcast encryption (BE) is a cryptographic primitive that allows a broadcaster to encrypt a content to a specific group of users called privileged users and prevent revoked users from decrypting the content. In BE schemes, a group of users, called traitor s may leak their keys and allow illegal reception of the content. Such malicious users can be detected through traitor tracing (TT) schemes. The ultimate goal in a content distribution system would be combining traitor tracing and broadcast encryption (trace and revoke mechanisms) so that any receiver key found to be compromised in a tracing process would be revoked in the future transmissions.In this paper, we propose a generic method to transform a broadcast encryption scheme into a trace and revoke scheme. This transformation involves imposing a fingerprinting code over the underlying BE transmissions. In conventional usage of fingerprinting codes, this will inflate the public key size with an additional data linear in the length of the code. To restrain from such increase in public key size, we introduce a new property, called public samplability, of a fingerprinting code. This property enables us to simulate the code independently from the actual code generated for tracing purposes. We have proved this property for the open fingerprinting code of [10].

We have instantiated our generic transformation with the BE schemes of [4, 12, 19]: we introduce (i) trace and revoke schemes with constant private key size and short ciphertext size, (ii) the first ID-based trace and revoke scheme, (iii) the first publicly traceable scheme with constant private key size and (iv) the first trace and revoke scheme against pirate rebroadcasting attack in the public key setting.

*21:17* [Pub][ePrint]
Invertible Polynomial Representation for Private Set Operations, by Hyung Tae Lee and Hyunsook Hong and Jung Hee Cheon
In many private set operations, a set is represented by a polynomial over a ring $\\Z_\\sigma$ for a composite integer $\\sigma$, where $\\Z_\\sigma$ is the message space of some additive homomorphic encryption. While it is useful for implementing set operations with polynomial additions and multiplications, a polynomial representation has a limitation due to the hardness of polynomial factorizations over $\\Z_\\sigma$. That is, it is hard to recover a corresponding set from a resulting polynomial over $\\Z_\\sigma$ if $\\sigma$ is not a prime. In this paper, we propose a new representation of a set by a polynomial over $\\Z_\\sigma$, in which $\\sigma$ is a composite integer with {\\em known factorization} but a corresponding set can be efficiently recovered from a polynomial except negligible probability. Note that $\\Z_\\sigma[x]$ is not a unique factorization domain, so a polynomial may be written as a product of linear factors in several ways. To exclude irrelevant linear factors, we introduce a special encoding function which supports early abort strategy. As a result, our representation can be efficiently inverted by computing all the linear factors of a polynomial in $\\Z_\\sigma[x]$ whose root locates in the image of encoding function.

When we consider group decryption as in most private set operation protocols, inverting polynomial representations should be done without a single party possessing a factorization of $\\sigma$. This is very hard for Paillier\'s encryption whose message space is $\\Z_N$ with unknown factorization of $N$. Instead, we detour this problem by using Naccache-Stern encryption with message space $\\Z_\\sigma$ where $\\sigma$ is a smooth integer with public factorization. As an application of our representation, we obtain a constant round privacy preserving set union protocol. Our construction improves the complexity than the previous without honest majority assumption. It can be also used for constant round multi-set union protocol and private set intersection protocol even when decryptors do not possess a superset of the resulting set.

*21:17* [Pub][ePrint]
Cryptanalysis of a recent two factor authentication scheme , by Michael Scott
Very recently a scheme has been proposed by Wang and Ma for a robust smart-card based password authentication scheme, which claimsto be secure against a Smart Card security breach. In this short note we attempt an initial cryptanalysis of this scheme.