Our research topics are cryptography, data security, and distributed algorithms, with emphasis on cryptography and secure protocols for distributed systems and blockchain networks.
During his 20-year career in industrial research at IBM Research - Zurich Christian Cachin has become an expert in distributed, cryptographic protocols. He contributed to solutions for cloud security, developed consensus protocols, and helped to create the Hyperledger Fabric blockchain platform (more info).
Blockchains and consensus protocols
Blockchains are trustworthy distributed networks, maintained by many nodes using distributed a consensus process and relying on cryptographic techniques. Some systems regulate which nodes may participate in the consensus protocol, but others leave this completely open and operate without any central authority. Cryptocurrencies rely on blockchains and hold substantial value.
The participating nodes validate transactions executed by the network and append information to the blockchain, which also takes the form of a ledger. A distributed consensus protocol tolerating faults and adversarial attacks ensures that the nodes agree on a unique order in which entries are appended. Advanced cryptographic algorithms play an important role for achieving consistency and privacy. Guaranteeing security and safety for blockchain networks requires mathematical models and sophisticated arguments, drawn from theoretical computer science, cryptography, and the theory of distributed computing.
Based on earlier work on Byzantine-fault tolerant (BFT) consensus for distributing trust on the Internet, we are exploring consensus protocols and security mechanisms, and apply them to blockchain systems.
We are currently exploring models for subjective or asymmetric trust. In traditional consensus protocols, all involved nodes adhere to a global, symmetric trust assumption, typically only expressed through bounds on the number of faulty processes. Some practical blockchain systems have recently suggested to let each participating node express its own subjective trust assumption. In this model of asymmetric trust, every node is free to choose which others it trusts and which ones might collude against it.
Papers and background
Christian Cachin and Luca Zanolini. From Symmetric to Asymmetric Asynchronous Byzantine Consensus. e-print, arXiv:2005.08795v2 [cs.DC].
Ignacio Amores-Sesar, Christian Cachin, and Jovana Mićić. Security Analysis of Ripple Consensus. Proceedings of OPODIS 2020.
Christian Cachin and Björn Tackmann. Asymmetric Distributed Trust. Proceedings of OPODIS 2019.
Elli Androulaki, Artem Barger, Vita Bortnikov, Christian Cachin, Konstantinos Christidis, Angelo De Caro and others. Hyperledger Fabric: A distributed operating system for permissioned blockchains. Proceedings of EuroSys 2018.
Christian Cachin and Marko Vukolic. Blockchain consensus protocols in the wild. Proceedings of DISC 2017.
Christian Cachin, Rachid Guerraoui, and Luís Rodrigues. Introduction to Reliable and Secure Distributed Programming. Springer, 2011.
Distributed programs running without trusted coordinator, such as the smart contracts executed by a blockchain network, cannot perform cryptographic operations today because no single node can hold a secret key. As one faulty node alone may leak any secrets it knows, keys cannot simply be distributed among the participating nodes. Distributed cryptography, also known as threshold cryptography, provides well-known methods to secure cryptosystems in the model of Byzantine fault tolerance.
Our research in the realm of distributed cryptosystems aims to provide such cryptographic operations for smart contracts. We have explored consensus protocols with generalized quorums, which encapsulate flexible trust structures motivated by practice. For realizing them, methods to specify the trust models are needed. We have developed efficient implementations for trust assumptions expressed by a monotone Boolean formula or by a monotone span program.
Ongoing work extends these trust models to distributed cryptosystems.
- Orestis Alpos and Christian Cachin Consensus Beyond Thresholds: Generalized Byzantine Quorums Made Live. Proceedings of SRDS 2020.