Mine blocks with proof-of-work, watch hash chain integrity, tamper & re-mine, explore consensus attacks
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Each block's header contains a cryptographic hash of the previous block. Any modification cascades forward, making tampering immediately detectable without a central authority.
Miners must find a nonce such that SHA-256(data + nonce) starts with N leading zeros. Difficulty adjusts to maintain target block time as hash rate changes across the network.
Transactions are summarised by a binary hash tree whose root appears in the block header. This allows O(log n) proof that a transaction is included without downloading the full block.
Nakamoto consensus: the valid chain is the one with the most cumulative proof-of-work. A 51% attacker can re-write history only by outpacing the honest network — economically prohibitive on large networks.
| Network | Consensus | Block Time | Throughput | Energy |
|---|---|---|---|---|
| Bitcoin (BTC) | Proof-of-Work (SHA-256) | ~10 min | ~7 TPS | ~130 TWh/yr |
| Ethereum (ETH) | Proof-of-Stake (Casper) | ~12 sec | ~15 TPS | ~0.01 TWh/yr |
| Solana (SOL) | Proof-of-History + PoS | ~0.4 sec | ~65,000 TPS | Very low |
| Binance Smart Chain | Proof-of-Staked Authority | ~3 sec | ~300 TPS | Low |
| Litecoin (LTC) | Proof-of-Work (Scrypt) | ~2.5 min | ~56 TPS | Moderate |
| Ripple (XRP) | Federated Byzantine Agreement | ~4 sec | ~1,500 TPS | Minimal |
| Cardano (ADA) | Ouroboros PoS | ~20 sec | ~250 TPS | Very low |
| Hyperledger Fabric | Pluggable (PBFT/Raft) | <1 sec | ~3,500 TPS | Minimal |
Mining requires finding a nonce N such that SHA-256(prevHash + data + N) has ≥D leading zero bits. Since hash outputs are pseudorandom, this requires on average 2^D attempts. Bitcoin adjusts D every 2016 blocks (~2 weeks) to maintain a 10-minute block time regardless of total network hash rate.
Each block hash depends on its content AND the previous block's hash. Changing any data in block k changes its hash, which is embedded in block k+1, changing k+1's hash, and so on. An attacker must re-mine every block from k to the chain tip faster than honest miners add new blocks — requiring >50% of total hash power.
A binary hash tree where leaf nodes are transaction hashes and each parent is hash(left child + right child). The Merkle root compactly proves all transactions are included. Verifying a single transaction requires only O(log₂ n) hashes — a "Merkle proof" — rather than downloading the full block.
PoW requires expending computational energy to mine blocks; PoS selects validators proportional to staked coins. Ethereum switched to PoS in September 2022 ("The Merge"), reducing its energy consumption by ~99.95%. PoS requires >33% stake to disrupt; PoW requires >50% hash rate.
Deep dive into distributed ledger technology, cryptographic hash functions, consensus mechanisms, smart contracts, and the environmental debate.
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