A blockchain is a distributed ledger in which participants use agreed rules to record and validate transactions. Its security depends on the network design, consensus mechanism, software and operation. It is not automatically a private or universally trustworthy database.
Hashes, signatures and records
Blocks are linked using cryptographic hashes; this is different from encrypting their contents. Digital signatures can help establish that a transaction was authorised by a key holder. Neither a hash nor a signature proves that an external real-world claim entered into the system is true.
What “immutable” means
Under normal operation, the ledger is designed to make past changes evident and difficult. The practical guarantees depend on consensus, control of participants and the possibility of reorganisations or governance changes. “Immutable” should not be read as an unconditional promise that no record can ever change.
Privacy and appropriate uses
Public ledgers can expose transaction information. Permissioned networks restrict participation but still need access controls, key management and a data-protection assessment. Do not place sensitive personal data on a ledger simply because it uses cryptography.
Before choosing blockchain for traceability, shared records or smart contracts, identify the participants, trust assumptions and need for distributed agreement. Compare it with a conventional database and assess recovery, governance and the quality of external inputs.
Reference: NIST IR 8202 — Blockchain Technology Overview. Editorial review: 30 October 2026.
