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.