International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Bogdan Warinschi

Affiliation: University of Bristol, UK

Publications

Year
Venue
Title
2017
TCC
2016
CRYPTO
2016
PKC
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
PKC
2015
ASIACRYPT
2014
CRYPTO
2014
EPRINT
2012
PKC
2012
ASIACRYPT
2011
EUROCRYPT
2010
EPRINT
The Fiat--Shamir Transform for Group and Ring Signature Schemes
M.-F. Lee Nigel P. Smart B. Warinschi
The Fiat-Shamir (FS) transform is a popular tool to produce particularly efficient digital signature schemes out of identification protocols. It is known that the resulting signature scheme is secure (in the random oracle model) if and only if the identification protocol is secure against passive impersonators. A similar results holds for constructing ID-based signature schemes out of ID-based identification protocols. The transformation had also been applied to identification protocols with additional privacy properties. So, via the FS transform, ad-hoc group identification schemes yield ring signatures and identity escrow schemes yield group signature schemes. Unfortunately, results akin to those above are not known to hold for these latter settings and the security of the resulting schemes needs to be proved from scratch, or worse, it is often simply assumed. Therefore, the security of the schemes obtained this way does not clearly follow from that of the base identification protocol and needs to be proved from scratch. Even worse, some papers seem to simply assume that the transformation works without proof. In this paper we provide the missing foundations for the use of the FS transform in these more complex settings.We start with defining a formal security model for identity escrow schemes (a concept proposed earlier but never rigorously formalized). Our main result constists of necessary and sufficient conditions for an identity escrow scheme to yield (via the FS transform) a secure group signature schemes. In addition, we discuss several variants of this result that account for the constructions of group signatures that fulfill weaker notions of security. In addition, using the similarity between group and ring signature schemes we give analogous results for the latter primitive.
2010
PKC
2010
JOFC
2009
EPRINT
Foundations of Non-Malleable Hash and One-Way Functions
Non-malleability is an interesting and useful property which ensures that a cryptographic protocol preserves the independence of the underlying values: given for example an encryption Enc(m) of some unknown message m, it should be hard to transform this ciphertext into some encryption Enc(m*) of a related message m*. This notion has been studied extensively for primitives like encryption, commitments and zero-knowledge. Non-malleability of one-way functions and hash functions has surfaced as a crucial property in several recent results, but it has not undergone a comprehensive treatment so far. In this paper we initiate the study of such non-malleable functions. We start with the design of an appropriate security definition. We then show that non-malleability for hash and one-way functions can be achieved, via a theoretical construction that uses perfectly one-way hash functions and simulation-sound non-interactive zero-knowledge proofs of knowledge (NIZKPoK). We also discuss the complexity of non-malleable hash and one-way functions. Specifically, we give a black-box based separation of non-malleable functions from one-way permutations (which our construction bypasses due to the 'non-black-box' NIZKPoK). We exemplify the usefulness of our definition in cryptographic applications by showing that non-malleability is necessary and sufficient to securely replace one of the two random oracles in the IND-CCA encryption scheme by Bellare and Rogaway, and to improve the security of client-server puzzles.
2009
ASIACRYPT
2009
ASIACRYPT
2008
EPRINT
A Modular Security Analysis of the TLS Handshake Protocol
We study the security of the widely deployed Secure Session Layer/Transport Layer Security (TLS) key agreement protocol. Our analysis identifies, justifies, and exploits the modularity present in the design of the protocol: the {\em application keys} offered to higher level applications are obtained from a {\em master key}, which in turn is derived, through interaction, from a {\em pre-master key}. Our first contribution consists of formal models that clarify the security level enjoyed by each of these types of keys. The models that we provide fall under well established paradigms in defining execution, and security notions. We capture the realistic setting where only one of the two parties involved in the execution of the protocol (namely the server) has a certified public key, and where the same master key is used to generate multiple application keys. The main contribution of the paper is a modular and generic proof of security for the application keys established through the TLS protocol. We show that the transformation used by TLS to derive master keys essentially transforms an {\em arbitrary} secure pre-master key agreement protocol into a secure master-key agreement protocol. Similarly, the transformation used to derive application keys works when applied to an arbitrary secure master-key agreement protocol. These results are in the random oracle model. The security of the overall protocol then follows from proofs of security for the basic pre-master key generation protocols employed by TLS.
2008
ASIACRYPT
2007
CRYPTO
2007
PKC
2007
EPRINT
A Cryptographic Model for Branching Time Security Properties -- the Case of Contract Signing Protocols
Some cryptographic tasks, such as contract signing and other related tasks, need to ensure complex, branching time security properties. When defining such properties one needs to deal with subtle problems regarding the scheduling of non-deterministic decisions, the delivery of messages sent on resilient (non-adversarially controlled) channels, fair executions (executions where no party, both honest and dishonest, is unreasonably precluded to perform its actions), and defining strategies of adversaries against all possible non-deterministic choices of parties and arbitrary delivery of messages via resilient channels. These problems are typically not addressed in cryptographic models and these models therefore do not suffice to formalize branching time properties, such as those required of contract signing protocols. In this paper, we develop a cryptographic model that deals with all of the above problems. One central feature of our model is a general definition of fair scheduling which not only formalizes fair scheduling of resilient channels but also fair scheduling of actions of honest and dishonest principals. Based on this model and the notion of fair scheduling, we provide a definition of a prominent branching time property of contract signing protocols, namely balance, and give the first \emph{cryptographic} proof that the Asokan-Shoup-Waidner two-party contract signing protocol is balanced.
2006
EPRINT
Key Exchange Protocols: Security Definition, Proof Method and Applications
We develop a compositional method for proving cryptographically sound security properties of key exchange protocols, based on a symbolic logic that is interpreted over conventional runs of a protocol against a probabilistic polynomial-time attacker. Since key indistinguishability and other previous specifications of secure key exchange suffer from specific compositionality problems, we develop a suitable specification of acceptable key generation. This definition is based on a simple game played by an adversary against a key exchange protocol and a conventional challenger characterizing secure encryption (or other primitives of interest). The method is illustrated using a sample protocol.
2006
EPRINT
Computationally Sound Symbolic Secrecy in the Presence of Hash Functions
The standard symbolic, deducibility-based notions of secrecy are in general insufficient from a cryptographic point of view, especially in presence of hash functions. In this paper we devise and motivate a more appropriate secrecy criterion which exactly captures a standard cryptographic notion of secrecy for protocols involving public-key enryption and hash functions: protocols that satisfy it are computationally secure while any violation of our criterion directly leads to an attack. Furthermore, we prove that our criterion is decidable via an NP decision procedure. Our results hold for standard security notions for encryption and hash functions modeled as random oracles.
2004
TCC
2003
EUROCRYPT
2003
EPRINT
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.

Program Committees

Crypto 2019
PKC 2018
Eurocrypt 2015
Crypto 2014
Eurocrypt 2014
Crypto 2011
PKC 2010
TCC 2010
TCC 2007