International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Max Tuengerthal

Publications

Year
Venue
Title
2020
JOFC
Joint State Composition Theorems for Public-Key Encryption and Digital Signature Functionalities with Local Computation
Ralf Küsters Max Tuengerthal Daniel Rausch
In frameworks for universal composability, complex protocols can be built from sub-protocols in a modular way using composition theorems. However, as first pointed out and studied by Canetti and Rabin, this modular approach often leads to impractical implementations. For example, when using a functionality for digital signatures within a more complex protocol, parties have to generate new verification and signing keys for every session of the protocol. This motivates to generalize composition theorems to so-called joint state (composition) theorems, where different copies of a functionality may share some state, e.g., the same verification and signing keys. In this paper, we present a joint state theorem which is more general than the original theorem of Canetti and Rabin, for which several problems and limitations are pointed out. We apply our theorem to obtain joint state realizations for three functionalities: public-key encryption, replayable public-key encryption, and digital signatures. Unlike most other formulations, our functionalities model that ciphertexts and signatures are computed locally, rather than being provided by the adversary. To obtain the joint state realizations, the functionalities have to be designed carefully. Other formulations proposed in the literature are shown to be unsuitable. Our work is based on the IITM model. Our definitions and results demonstrate the expressivity and simplicity of this model. For example, unlike Canetti’s UC model, in the IITM model no explicit joint state operator needs to be defined and the joint state theorem follows immediately from the composition theorem in the IITM model.
2020
JOFC
The IITM Model: A Simple and Expressive Model for Universal Composability
Ralf Küsters Max Tuengerthal Daniel Rausch
The universal composability paradigm allows for the modular design and analysis of cryptographic protocols. It has been widely and successfully used in cryptography. However, devising a coherent yet simple and expressive model for universal composability is, as the history of such models shows, highly non-trivial. For example, several partly severe problems have been pointed out in the literature for the UC model. In this work, we propose a coherent model for universal composability, called the IITM model (“Inexhaustible Interactive Turing Machine”). A main feature of the model is that it is stated without a priori fixing irrelevant details, such as a specific way of addressing of machines by session and party identifiers, a specific modeling of corruption, or a specific protocol hierarchy. In addition, we employ a very general notion of runtime. All reasonable protocols and ideal functionalities should be expressible based on this notion in a direct and natural way, and without tweaks, such as (artificial) padding of messages or (artificially) adding extra messages. Not least because of these features, the model is simple and expressive. Also the general results that we prove, such as composition theorems, hold independently of how such details are fixed for concrete applications. Being inspired by other models for universal composability, in particular the UC model and because of the flexibility and expressivity of the IITM model, conceptually, results formulated in these models directly carry over to the IITM model.
2010
EPRINT
Ideal Key Derivation and Encryption in Simulation-based Security
Ralf Küsters Max Tuengerthal
Many real-world protocols, such as SSL/TLS, SSH, IPsec, IEEE 802.11i, DNSSEC, and Kerberos, derive new keys from other keys. To be able to analyze such protocols in a composable way, in this paper we extend an ideal functionality for symmetric and public-key encryption proposed in previous work by a mechanism for key derivation. We also equip this functionality with message authentication codes (MACs) and ideal nonce generation. We show that the resulting ideal functionality can be realized based on standard cryptographic assumptions and constructions, hence, providing a solid foundation for faithful, composable cryptographic analysis of real-world security protocols. Based on this new functionality, we identify sufficient criteria for protocols to provide universally composable key exchange and secure channels. Since these criteria are based on the new ideal functionality, checking the criteria requires merely information-theoretic or even only syntactical arguments, rather than involved reduction arguments. As a case study, we use our method to analyze two central protocols of the IEEE 802.11i standard, namely the 4-Way Handshake Protocol and the CCM Protocol, proving composable security properties. As to the best of our knowledge, this constitutes the first rigorous cryptographic analysis of these protocols.
2009
EPRINT
Universally Composable Symmetric Encryption
Ralf Küsters Max Tuengerthal
For most basic cryptographic tasks, such as public key encryption, digital signatures, authentication, key exchange, and many other more sophisticated tasks, ideal functionalities have been formulated in the simulation-based security approach, along with their realizations. Surprisingly, however, no such functionality exists for symmetric encryption, except for a more abstract Dolev-Yao style functionality. In this paper, we fill this gap. We propose two functionalities for symmetric encryption, an unauthenticated and an authenticated version, and show that they can be implemented based on standard cryptographic assumptions for symmetric encryption schemes, namely IND-CCA security and authenticated encryption, respectively. We also illustrate the usefulness of our functionalities in applications, both in simulation-based and game-based security settings.
2008
EPRINT
Joint State Theorems for Public-Key Encryption and Digital Signature Functionalities with Local Computation
Ralf Küsters Max Tuengerthal
Composition theorems in simulation-based approaches allow to build complex protocols from sub-protocols in a modular way. However, as first pointed out and studied by Canetti and Rabin, this modular approach often leads to impractical implementations. For example, when using a functionality for digital signatures within a more complex protocol, parties have to generate new verification and signing keys for every session of the protocol. This motivates to generalize composition theorems to so-called joint state theorems, where different copies of a functionality may share some state, e.g., the same verification and signing keys. In this paper, we present a joint state theorem which is more general than the original theorem of Canetti and Rabin, for which several problems and limitations are pointed out. We apply our theorem to obtain joint state realizations for three functionalities: public-key encryption, replayable public-key encryption, and digital signatures. Unlike most other formulations, our functionalities model that ciphertexts and signatures are computed locally, rather than being provided by the adversary. To obtain the joint state realizations, the functionalities have to be designed carefully. Other formulations are shown to be unsuitable. Our work is based on a recently proposed, rigorous model for simulation-based security by K{\"u}sters, called the IITM model. Our definitions and results demonstrate the expressivity and simplicity of this model. For example, unlike Canetti's UC model, in the IITM model no explicit joint state operator needs to be defined and the joint state theorem follows immediately from the composition theorem in the IITM model.

Coauthors

Ralf Küsters (5)
Daniel Rausch (2)