How Blocks Connect: Hashes and Cryptographic Linking

What you'll learn

• The Chain of Trust
• Understanding Hash Functions
• How Blocks Link Together
• Merkle Trees: Compressing Many Transactions
• For Deeper Reading

Key terms

Hash function

A mathematical function that converts any input into a fixed-size string of characters. Same input always produces same output (deterministic), tiny changes produce wildly different outputs (avalanche effect), and reversing the function is computationally infeasible (one-way).

SHA-256

Secure Hash Algorithm 256-bit, designed by the NSA and standardized by NIST in 2001. Produces a 256-bit (64 hex character) output. Used by Bitcoin for mining and transaction hashing.

Keccak-256

A variant of SHA-3 used by Ethereum. Produces 256-bit output like SHA-256 but with a different internal algorithm (sponge construction). Slightly different from official NIST SHA-3 due to historical timing.

Avalanche effect

A property of cryptographic hash functions where any change to the input, no matter how small, produces a completely different and unpredictable output. Critical for security.

Collision resistance

The property that it is computationally infeasible to find two different inputs producing the same hash output. SHA-256 has no known collisions.

Genesis block

The very first block in a blockchain. Bitcoin's genesis block was mined on January 3, 2009 and contains the message "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks."

Merkle tree

A binary tree of hashes invented by Ralph Merkle in 1979. Used in Bitcoin to compress all transactions in a block into a single Merkle root, enabling efficient inclusion proofs (Simplified Payment Verification).

Merkle root

The single hash at the top of a Merkle tree, included in the block header. Summarizes all transactions in the block in a way that allows efficient verification.

Simplified Payment Verification (SPV)

A way for lightweight clients (like mobile wallets) to verify transactions without downloading the entire blockchain, using Merkle proofs from full nodes.

Block header

A small (80-byte for Bitcoin) summary of a block containing the previous block hash, Merkle root, timestamp, difficulty target, and nonce. Used for chain verification without full block data.

Immutability

The property that historical blockchain data cannot practically be changed. Not absolute — alteration requires redoing all proof-of-work since the change point, which is economically infeasible for major chains.

51% attack

An attack where a single entity controls more than half of a blockchain's computational power (or stake), potentially allowing them to rewrite recent history. Has happened to small chains; never successfully to Bitcoin or Ethereum.