Biblio

Leakage resilience (LR) and misuse resistance (MR) are two important properties for the deployment of authenticated encryption (AE) schemes. They aim at mitigating the impact of implementation flaws due to side-channel leakages and misused randomness. In this paper, we discuss the interactions and incompatibilities between these two properties. We start from the usual definition of MR for AE schemes from Rogaway and Shrimpton, and argue that it may be overly demanding in the presence of leakages. As a result, we turn back to the basic security requirements for AE: ciphertext integrity (INT-CTXT) and CPA security, and propose to focus on a new notion of CIML security, which is an extension of INT-CTXT in the presence of misuse and leakages. We discuss the extent to which CIML security is offered by previous proposals of MR AE schemes, conclude by the negative, and propose two new efficient CIML-secure AE schemes: the DTE scheme offers security in the standard model, while the DCE scheme offers security in the random oracle model, but comes with some efficiency benefits. On our way, we observe that these constructions are not trivial, and show for instance that the composition of a LR MAC and a LR encryption scheme, while providing a (traditional) MR AE scheme, can surprisingly lose the MR property in the presence of leakages and does not achieve CIML security. Eventually, we show the LR CPA security of DTE and DCE.

Digital signatures are perhaps the most important base for authentication and trust relationships in large scale systems. More specifically, various applications of signatures provide privacy and anonymity preserving mechanisms and protocols, and these, in turn, are becoming critical (due to the recently recognized need to protect individuals according to national rules and regulations). A specific type of signatures called "signatures with efficient protocols", as introduced by Camenisch and Lysyanskaya (CL), efficiently accommodates various basic protocols and extensions like zero-knowledge proofs, signing committed messages, or re-randomizability. These are, in fact, typical operations associated with signatures used in typical anonymity and privacy-preserving scenarios. To date there are no "signatures with efficient protocols" which are based on simple assumptions and truly practical. These two properties assure us a robust primitive: First, simple assumptions are needed for ensuring that this basic primitive is mathematically robust and does not require special ad hoc assumptions that are more risky, imply less efficiency, are more tuned to the protocol itself, and are perhaps less trusted. In the other dimension, efficiency is a must given the anonymity applications of the protocol, since without proper level of efficiency the future adoption of the primitives is always questionable (in spite of their need). In this work, we present a new CL-type signature scheme that is re-randomizable under a simple, well-studied, and by now standard, assumption (SXDH). The signature is efficient (built on the recent QA-NIZK constructions), and is, by design, suitable to work in extended contexts that typify privacy settings (like anonymous credentials, group signature, and offline e-cash). We demonstrate its power by presenting practical protocols based on it.