Blockchain: Concept of distributed ledger
1. Definition
A blockchain is a distributed, append-only digital ledger in which records (transactions) are grouped into blocks, each block being cryptographically linked to the previous one, and the whole ledger is replicated across many independent nodes in a peer-to-peer network. There is no central authority: every node keeps a copy, every node can verify the chain, and a new block is added only when the network reaches consensus. Once written, a record cannot practically be altered without redoing the work of every later block and convincing the majority of the network — which is what gives blockchain its immutability.
The concept came from Satoshi Nakamoto's 2008 Bitcoin paper, which solved the "double spending" problem for digital money; the underlying technology later generalised into Distributed Ledger Technology (DLT), with Ethereum (2015) adding smart contracts.
2. Why a Distributed Ledger?
A traditional ledger (a bank's database) is centralised: one authority records the truth, and we trust it. A distributed ledger replaces that trust in an institution with trust in cryptography, replication and consensus:
- No single point of failure — thousands of copies exist.
- Tamper-evidence — changing history requires changing every copy.
- Transparency/auditability — participants can verify the ledger themselves.
- Disintermediation — transactions can settle without a central clearing party.
3. Structure of the Chain
GENESIS BLOCK 1 BLOCK 2 BLOCK 3
┌──────────┐ ┌──────────────┐ ┌──────────────┐ ┌──────────────┐
│ prev: -- │ │ prev: H0 │ │ prev: H1 │ │ prev: H2 │
│ txs │ ───► │ txs │ ─────► │ txs │ ────► │ txs │
│ nonce │ │ nonce/root │ │ nonce/root │ │ nonce/root │
│ hash H0 │ │ hash H1 │ │ hash H2 │ │ hash H3 │
└──────────┘ └──────────────┘ └──────────────┘ └──────────────┘
blocks are linked BACKWARDS: block N stores the hash of block N-1
tamper with Block 1 → H1 changes → Block 2's "prev" no longer matches
→ every later block is invalidated → network rejects the chain
Each block typically contains: the hash of the previous block, a timestamp, a nonce (or equivalent proof-of-work field), the Merkle root of the transactions, and the transactions themselves. Because the linking is done through hashes, the ledger is a hash chain — cheap to verify, extremely expensive to forge.
4. How a Transaction Gets Recorded (six steps)
- A user creates a transaction and signs it with their private key (the public key verifies authorship — this is a digital signature, not encryption).
- The transaction is broadcast to the peer-to-peer network of nodes.
- Every node validates it: signature correct, inputs exist, sufficient balance, no double spending, no rule violations.
- Valid transactions wait in a mempool.
- Through the consensus mechanism, a node (or a set of validators) is chosen to propose the next block; other nodes verify the block's correctness and its link to the previous hash.
- The block is appended; all nodes update their copy. Later confirmations deepen the certainty — six confirmations is the classical Bitcoin rule of thumb for finality.
5. Consensus Mechanisms
| Mechanism | How it decides | Energy | Examples |
|---|---|---|---|
| Proof of Work (PoW) | Miners race to find a nonce whose hash meets a difficulty target (51% attack requires majority hash power) | Very high | Bitcoin, Litecoin, early Ethereum |
| Proof of Stake (PoS) | Validators stake coins; the right to propose is proportional to stake, with slashing for cheating | Very low | Ethereum (post-2022), Cardano, Solana |
| Delegated PoS / Proof of Authority | A small elected or trusted set of validators | Very low | EOS, Hyperledger-based and private chains |
| PBFT / Raft-style | Voting rounds among known validators (Byzantine fault tolerant) | Negligible | Hyperledger Fabric, permissioned consortiums |
| Proof of History / others | Timestamping to speed ordering | Low | Solana |
6. Types of Blockchain and Key Properties
| Type | Access | Examples | Trade-off |
|---|---|---|---|
| Public / permissionless | Anyone can read, write, validate | Bitcoin, Ethereum | Maximum decentralisation; lower throughput, visible data |
| Private / permissioned | One organisation controls membership | Hyperledger Fabric, Quorum | Fast, private; trust is partly back to the operator |
| Consortium | Several organisations share control | Trade finance, banking consortia (R3 Corda) | Balance of privacy and shared trust |
Core properties: decentralisation · immutability/tamper-evidence · transparency and auditability · pseudonymity (addresses, not names) · high availability and fault tolerance (no single point of failure) · traceability of provenance · programmability through smart contracts · finality of settlement.
Limitations: throughput and latency (public chains handle far fewer transactions per second than centralised payment networks) · storage growth (every node keeps everything) · energy use of PoW · privacy on public chains (the ledger is visible to all, and analysis can de-anonymise) · key management — lose the private key and the assets are gone · smart-contract bugs and hacks · governance and forks · regulatory uncertainty · quantum computing as a long-term threat to current signatures.
7. Applications
Cryptocurrency and payments (Bitcoin, Ether, USDT; cross-border remittances cheaper and faster than correspondent banking) · CBDC (India's RBI e-rupee pilot) · supply-chain traceability (farm-to-fork provenance, anti-counterfeit medicines, conflict minerals) · land and property records (Indian state pilots in Andhra Pradesh, Telangana, Maharashtra) · certificates and diplomas (tamper-proof verification) · voting (auditable, remote) · healthcare records (patient-controlled access, audit trail) · insurance and trade finance (automatic claims via smart contracts) · digital identity (DID) and KYC · NFTs and digital ownership · DeFi (lending, exchanges without intermediaries) · carbon credits and renewable-energy trading · e-governance and public procurement with tamper-evident audit trails.
8. Exam Points
- Define blockchain and explain the concept of a distributed ledger — 5-7 marks, almost certain.
- Explain the structure of a block and how blocks are linked by hashes; why is tampering impractical? — 5-7 marks, very frequent.
- Explain how a transaction is added to the blockchain (step by step) — 5 marks, frequent.
- Explain consensus mechanisms (PoW, PoS, PBFT) and compare their energy use — 5 marks, very frequent.
- Differentiate public, private and consortium blockchains — 5 marks.
- List the advantages (immutability, transparency, decentralisation) and limitations (scalability, energy, privacy) — 5-7 marks.
- Explain any four applications of blockchain with a benefit each (or write about the Indian e-rupee/land records) — 5 marks.
- What are smart contracts? — 3 marks.
- Differentiate a blockchain from a centralised database — 5 marks.