Abstract. This work presents a study of the complexity of the Blum-Kalai-Wasserman (BKW) algorithm when applied to the Learning with Errors (LWE) problem, by providing refined estimates for the data and computational effort requirements for solving concrete instances of the LWE problem. We apply this refined analysis to suggested parameters for various LWE-based cryptographic schemes from the literature and compare with alternative approaches based on lattice reduction. As a result, we provide new upper bounds for the concrete hardness of these LWE-based schemes. Rather surprisingly, it appears that BKW algorithm outperforms known estimates for lattice reduction algorithms starting in dimension n ≈ 250 when LWE is reduced to SIS. However, this assumes access to an unbounded number of LWE samples.

Abstract. In this paper we define small scale variants of the AES. These variants inherit the design features of the AES and provide a suitable framework for comparing different cryptanalytic methods. In particular, we provide some preliminary results and insights when using off-theshelf computational algebra techniques to solve the systems of equations arising from these small scale variants.

In this paper we propose a new cryptanalytic method against block ciphers, which combines both algebraic and statistical techniques. More specifically, we show how to use algebraic relations arising from differential characteristics to speed up and improve key-recovery differential attacks against block ciphers. To illustrate the new technique, we apply algebraic techniques to mount differential attacks against round reduced variants of Present-128.

Gennaro et al. (Crypto 2010) introduced the notion of noninteractive verifiable computation, which allows a computationally weak client to outsource the computation of a function f on a series of inputs x (1) , . . . to a more powerful but untrusted server. Following a preprocessing phase (that is carried out only once), the client sends some representation of its current input x (i) to the server; the server returns an answer that allows the client to recover the correct result f (x (i) ), accompanied by a proof of correctness that ensures the client does not accept an incorrect result. The crucial property is that the work done by the client in preparing its input and verifying the server's proof is less than the time required for the client to compute f on its own.We extend this notion to the multi-client setting, where n computationally weak clients wish to outsource to an untrusted server the computation of a function f over a series of joint inputs (x (1) 1 , . . . , x (1) n ), . . . without interacting with each other. We present a construction for this setting by combining the scheme of Gennaro et al. with a primitive called proxy oblivious transfer.Portions of this work were done while at the University of Maryland.

No abstract

A method for extracting cryptographic key material from DRAM used in modern computers has been recently proposed in [9]; the technique was called Cold Boot attacks. When considering block ciphers, such as the AES and DES, simple algorithms were also proposed in [9] to recover the cryptographic key from the observed set of round subkeys in memory (computed via the cipher's key schedule operation), which were however subject to errors due to memory bits decay. In this work we extend this analysis to consider key recovery for other ciphers used in Full Disk Encryption (FDE) products. Our algorithms are also based on closest code word decoding methods, however apply a novel method for solving a set of non-linear algebraic equations with noise based on Integer Programming. This method should have further applications in cryptology, and is likely to be of independent interest. We demonstrate the viability of the Integer Programming method by applying it against the Serpent block cipher, which has a much more complex key schedule than AES. Furthermore, we also consider the Twofish key schedule, to which we apply a dedicated method of recovery.

No abstract

Abstract. The combination of software-as-a-service and the increasing use of mobile devices gives rise to a considerable difference in computational power between servers and clients. Thus, there is a desire for clients to outsource the evaluation of complex functions to an external server. Servers providing such a service may be rewarded per computation, and as such have an incentive to cheat by returning garbage rather than devoting resources and time to compute a valid result. In this work, we introduce the notion of Revocable Publicly Verifiable Computation (RPVC), where a cheating server is revoked and may not perform future computations (thus incurring a financial penalty). We introduce a Key Distribution Center (KDC) to efficiently handle the generation and distribution of the keys required to support RPVC. The KDC is an authority over entities in the system and enables revocation. We also introduce a notion of blind verification such that results are verifiable (and hence servers can be rewarded or punished) without learning the value. We present a rigorous definitional framework, define a number of new security models and present a construction of such a scheme built upon Key-Policy Attribute-based Encryption.