Abstract. The fair evaluation and comparison of side-channel attacks and countermeasures has been a long standing open question, limiting further developments in the field. Motivated by this challenge, this work makes a step in this direction and proposes a framework for the analysis of cryptographic implementations that includes a theoretical model and an application methodology. The model is based on commonly accepted hypotheses about side-channels that computations give rise to. It allows quantifying the effect of practically relevant leakage functions with a combination of information theoretic and security metrics, measuring the quality of an implementation and the strength of an adversary, respectively. From a theoretical point of view, we demonstrate formal connections between these metrics and discuss their intuitive meaning. From a practical point of view, the model implies a unified methodology for the analysis of side-channel key recovery attacks. The proposed solution allows getting rid of most of the subjective parameters that were limiting previous specialized and often ad hoc approaches in the evaluation of physically observable devices. It typically determines the extent to which basic (but practically essential) questions such as "How to compare two implementations? " or "How to compare two side-channel adversaries? " can be answered in a sound fashion.

We show that for low-density paritycheck (LDPC) codes with sufficient expansion, the Linear Programming (LP) Decoder corrects a constant fraction of errors.

We consider re-keying protocols for secure multicasting in a dynamic multicast group with a center. There is a variety of different scenarios using multicast, presenting a wide range of efficiency requirements with respect to several parameters. We give an upper bound on the tradeoff between storage and communication parameters. In particular, we suggest an improvement of the schemes by Wallner et al. and Wong et al. [13,14] with sub-linear center storage, without a significant loss in other parameters. Correctly selecting the parameters of our scheme we can efficiently accommodate a wide range of scenarios. This is demonstrated by Applying the protocol to some known benchmark scenarios. We also show lower bounds on the tradeoff between communication and user storage, and show that our scheme is almost optimal with respect to these lower bounds.

Abstract. Traditionally, secure cryptographic algorithms provide security against an adversary who has only black-box access to the secret information of honest parties. However, such models are not always adequate. In particular, the security of these algorithms may completely break under (feasible) attacks that tamper with the secret key. In this paper we propose a theoretical framework to investigate the algorithmic aspects related to tamper-proof security. In particular, we define a model of security against an adversary who is allowed to apply arbitrary feasible functions f to the secret key sk, and obtain the result of the cryptographic algorithms using the new secret key f (sk). We prove that in the most general setting it is impossible to achieve this strong notion of security. We then show minimal additions to the model, which are needed in order to obtain provable security. We prove that these additions are necessary and also sufficient for most common cryptographic primitives, such as encryption and signature schemes. We discuss the applications to portable devices protected by PINs and show how to integrate PIN security into the generic security design. Finally we investigate restrictions of the model in which the tampering powers of the adversary are limited. These restrictions model realistic attacks (like differential fault analysis) that have been demonstrated in practice. In these settings we show security solutions that work even without the additions mentioned above.

Abstract.We construct the first efficient forward-secure digital signature scheme where the total number of time periods for which the public key is used does not have to be fixed in advance. The number of time periods for which our scheme can be used is bounded only by an exponential function of the security parameter (given this much time, any scheme can be broken by exhaustive search), and its performance depends (minimally) only on the time elapsed so far. Our scheme achieves excellent performance overall, is very competitive with previous schemes with respect to all parameters, and outperforms each of the previous schemes in at least one parameter. Moreover, the scheme can be based on any underlying digital signature scheme, and does not rely on specific assumptions. Its forward security is proven in the standard model, without using a random oracle. As an intermediate step in designing our scheme, we propose and study two general composition operations that can be used to combine any existing signature schemes (whether standard or forward-secure) into new forward-secure signature schemes.