Key Insulation and Intrusion Resilience Over a Public Channel
Key insulation (KI) and Intrusion resilience (IR) are methods to protect a user's key against exposure by utilizing periodic communications with an auxiliary helper. But existing work assumes a secure channel between user and helper. If we want to realize KI or IR in practice we must realize this secure channel. This paper looks at the question of how to do this when the communication is over what we are more likely to have in practice, namely a public channel such as the Internet or a wireless network. We explain why this problem is not trivial, introduce models and definitions that capture the desired security in a public channel setting, and provide a complete (and surprising) answer to the question of when KI and IR are possible over a public channel. The information we provide is important to guide practitioners with regard to the usage of KI and IR and also to guide future research in this area.
The Knowledge-of-Exponent Assumptions and 3-Round Zero-Knowledge Protocols
Hada and Tanaka showed the existence of 3-round, negligible-error zero-knowledge arguments for NP based on a pair of non-standard assumptions, here called KEA1 and KEA2. In this paper we show that KEA2 is false. This renders vacuous the results of Hada and Tanaka. We recover these results, however, under a suitably modified new assumption called KEA3. What we believe is most interesting is that we show that it is possible to ``falsify'' assumptions like KEA2 that, due to their nature and quantifier-structure, do not lend themselves easily to ``efficient falsification'' (Naor).
Towards Plaintext-Aware Public-Key Encryption without Random Oracles
We consider the problem of defining and achieving plaintext-aware encryption without random oracles in the classical public-key model. We provide definitions for a hierarchy of notions of increasing strength: PA0, PA1 and PA2, chosen so that PA1+IND-CPA => IND-CCA1 and PA2+IND-CPA => IND-CCA2. Towards achieving the new notions of plaintext awareness, we show that a scheme due to Damgard, denoted DEG, and the ``lite'' version of the Cramer-Shoup scheme, denoted CSL, are both PA0 under the KEA0 assumption of Damgard, and PA1 under an extension of this assumption called KEA1. As a result, DEG is the most efficient proven IND-CCA1 scheme known.
An Uninstantiable Random-Oracle-Model Scheme for a Hybrid Encryption Problem
We present a simple, natural random-oracle (RO) model scheme, for a practical goal, that is uninstantiable, meaning is proven in the RO model to meet its goal yet admits NO standard-model instantiation that meets this goal. The goal in question is IND-CCA-preserving asymmetric encryption which formally captures security of the most common practical usage of asymmetric encryption, namely to transport a symmetric key in such a way that symmetric encryption under the latter remains secure. The scheme is an ElGamal variant, called Hash ElGamal, that resembles numerous existing RO-model schemes, and on the surface shows no evidence of its anomalous properties. More generally, we show that a certain goal, that we call key-verifiable, ciphertext-verifiable IND-CCA-preserving asymmetric encryption, is achievable in the RO model (by Hash ElGamal in particular) but unachievable in the standard model. This helps us better understand the source of the anomalies in Hash ElGamal and also lifts our uninstantiability result from being about a specific scheme to being about a primitive or goal. These results extend our understanding of the gap between the standard and RO models, and bring concerns raised by previous work closer to practice by indicating that the problem of RO-model schemes admitting no secure instantiation can arise in domains where RO schemes are commonly designed.
Secure Proxy Signature Schemes for Delegation of Signing Rights
A proxy signature scheme permits an entity to delegate its signing rights to another entity. These schemes have been suggested for use in numerous applications, particularly in distributed computing. But to date, no proxy signature schemes with guaranteed security have been proposed; no precise definitions or proofs of security have been provided for such schemes. In this paper, we formalize a notion of security for proxy signature schemes and present provably-secure schemes. We analyze the security of the well-known delegation-by-certificate scheme and show that after some slight but important modifications, the resulting scheme is secure, assuming the underlying standard signature scheme is secure. We then show that employment of the recently introduced aggregate signature schemes permits bandwidth and computational savings. Finally, we analyze the proxy signature scheme of Kim, Park and Won, which offers important performance benefits. We propose modifications to this scheme that preserve its efficiency, and yield a proxy signature scheme that is provably secure in the random-oracle model, under the discrete-logarithm assumption.
Building Secure Cryptographic Transforms, or How to Encrypt and MAC
We describe several notions of ``cryptographic transforms,'' symmetric schemes designed to meet a variety of privacy and authenticity goals. We consider goals, such as replay-avoidance and in-order packet delivery, that have not been fully addressed in previous works in this area. We then provide an analysis of possible ways to combine standard encryption and message authentication schemes in order to provably meet these goals. Our results further narrow the gap between the provable-security results from the theoretical community and the needs of developers who implement real systems.
Protecting against Key Exposure: Strongly Key-Insulated Encryption with Optimal Threshold
A new framework for protection against key exposure was recently suggested by Dodis et. al.. We take its realization further towards practice by presenting simple new schemes that provide benefits over previous ones in terms of scalability, performance and security. Our first contribution is a simple, practical, scalable scheme called SKIE-OT that achieves the best possible security in their framework. SKIE-OT is based on the Boneh-Franklin identity-based encryption (IBE) scheme and exploits algebraic properties of the latter. We also show that the role of identity-based encryption is not coincidental by proving that IBE is equivalent to (not strongly) key-insulated encryption with optimal threshold and allowing random-access key updates.