What is blockchain?

A blockchain is a continuously growing list of records, which are grouped into blocks and maintained in a distributed way. In a blockchain, each block contains a cryptographic hash of the previous block, which prevents changes to existing blocks at a later point in time. In conjunction with protocols for distributed consensus, blockchains are currently used as platforms to support global business transactions.

A blockchain is a public ledger for recording transactions, maintained by many nodes without central authority through a distributed cryptographic protocol. All nodes validate the information to be appended to the blockchain, and a consensus protocol ensures that the nodes agree on a unique order in which entries are appended. Consensus protocols tolerating faults and adversarial attacks—coupled with cryptographic tools—are needed for this.

The recent interest in blockchains has revived research on consensus protocols, ranging from the proof-of-work method in Bitcoin’s “mining” protocol to classical Byzantine agreement. Going far beyond its use in cryptocurrencies, blockchain is regarded today as a promising technology to simplify trusted exchanges of data and goods among companies.

More on Blockchain Solutions at IBM Research – Zurich.

More on Financial Services research at IBM Research.


Hyperledger logoThe Hyperledger project was started in December 2015 by The Linux Foundation to support blockchain-based distributed ledgers. It is a global collaboration including leaders in finance, banking, IoT, supply chain, manufacturing and technology. IBM is a key player in the Hyperledger project.

Hyperledger FabricHyperledger Fabric is an implementation of blockchain technology intended for enterprise applications. With a modular architecture and pluggable implementations, it provides a foundation for blockchain solutions. The blockchain framework aims at confidentiality, scalability and security in business environments.

IBM Research – Zurich is actively involved in the development of the Hyperledger Fabric platform. Important designs and key components of Hyperledger Fabric have been produced by the team. Current research addresses the security, privacy, and consensus protocols in blockchains.

More information about Hyperledger Fabric is available in the online documentation.


A consensus protocol allows a group of distributed and potentially faulty nodes to reach a common decision only by sending messages to each other. Initially, every node may start with a different opinion on the result. During execution of the protocol, the nodes reach consensus and converge on a common output by exchanging messages.

Today, consensus protocols exist in many variations, but all of them need a majority or even a qualified majority (such as 2/3 of the nodes) to be correct, whereas the remaining ones could fail, misbehave, or even act adversarially against finding consensus.

Starting with the celebrated protocols for Byzantine Agreement established in 1982, consensus protocols have found widespread applications for keeping distributed systems healty and making cloud platforms operate continuously.

Consensus protocols are deployed widely today. They represent a core abstraction for building reliable and secure distributed systems, and are covered in textbooks, such as the survey of quorum systems by IBM researcher Marko Vukolic and the introduction to distributed computing co-authored by IBM researcher Christian Cachin.


A blockchain is a digital representation of an immutable ledger. Transactions are executed “on” the blockchain with the help of consensus protocols and may append new records to the ledger. As the first widely used blockchain within the Bitcoin cryptocurrency has demonstrated, blockchains may represent high-valued assets securely, secured only through code running on distributed nodes, and not being backed by any centralized trusted entity.

All security of a blockchain must be gained from cryptographic techniques, such as hashing, public-key digital signatures and encryption schemes. Elaborate cryptographic protocols such as zero-knowledge proofs also play an important role. For example, they allow an entity to prove a statement, such as holding a private digital asset or knowing a secret, without revealing information about the secret.

A textbook co-authored by IBM researcher Elli Androulaki gives an introduction to this fascinating topic.

EU project Priviledge logo

Privacy-Enhancing Cryptography in Distributed Ledgers

The H2020 EU project PRIViLEDGE aims to develop and advance techniques that enhance privacy, anonymity and efficient decentralized consensus for distributed ledger technologies.


Elli Androulaki

Elli Androulaki
Manager, Blockchain security & applications


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Hyperledger Fabric: A distributed operating system for permissioned blockchains,”
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Blockchain Consensus Protocols in the Wild,”
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