Hash Chains & Block Linking

A hash chain links data blocks so that each block contains the hash of the previous block. Modifying any block changes its hash, which invalidates all subsequent blocks. This creates a tamper-evident, append-only ledger.

Each block contains: (1) data payload (transactions), (2) previous block hash (pointer to prior block), (3) timestamp, (4) nonce (for PoW), (5) Merkle root of transactions. The first block (genesis) has previous_hash = "0"*64.

import hashlib, json class Block: def __init__(self, data, prev_hash): self.data = data self.prev_hash = prev_hash self.hash = hashlib.sha256( json.dumps({'data':data,'prev':prev_hash}).encode() ).hexdigest()

If an attacker modifies block 3's data, block 3's hash changes. Block 4 references block 3's old hash — the chain breaks at block 4. The attacker must recompute hashes for blocks 3, 4, 5, ... all the way to the tip. With PoW, this is computationally infeasible.

Hash chains provide tamper evidence, not immutability by themselves. Without additional mechanisms (PoW, digital signatures, distribution), an attacker who controls the chain can rewrite it. Distributed consensus is what makes blockchain tamper-resistant in practice.

Bitcoin's longest chain rule: the valid chain with the most accumulated proof-of-work is canonical. If an attacker forks the chain, honest miners extend the honest chain faster (51% assumption). The attacker's fork is shorter and discarded.

Bitcoin's SPV (Simple Payment Verification) clients download only 80-byte block headers (containing previous hash, Merkle root, nonce, timestamp, bits). Full nodes store all transactions. SPV clients verify a transaction by Merkle proof against the header Merkle root.

Bitcoin's genesis block (block 0, Jan 3 2009) has prev_hash = 0x000...0. Satoshi embedded the headline "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" in the coinbase transaction as proof of timestamp.

Hash chains predate blockchain. Uses: certificate revocation lists (linked by hash), log integrity (hash of previous log entry), DNSSEC chain of trust. The concept generalizes: any append-only structure where each entry commits to all previous entries.

To speed up initial sync, Bitcoin Core hardcodes checkpoints: known-valid block hashes at certain heights. A client starting sync can skip PoW verification for blocks before the checkpoint, trusting the hardcoded hash. Trade-off: reduces trust-minimization.

Git uses the same principle: each commit is identified by SHA-1/SHA-256 of its content + parent commit hash. Checking out a specific commit hash gives exactly that state. Git's entire history is a hash-linked tree of objects.

Why must an attacker recompute all subsequent block hashes after modifying one block?

Hash chains link blocks via cryptographic pointers. Modification cascades to all subsequent blocks. Combined with PoW and distributed consensus, this makes blockchain immutable in practice. Next: Merkle trees for efficient transaction integrity.