Hash

A fixed-length string generated by a cryptographic function to uniquely represent data in the Bitcoin blockchain.

What is Hash?

In the context of Bitcoin, a hash is the output of a cryptographic hash function, typically SHA-256, which transforms input data of any size into a fixed-length string of 256 bits. This process is deterministic, meaning the same input always produces the same hash, but it is computationally infeasible to reverse-engineer the original data or find two different inputs with the same hash (collision resistance).

In Bitcoin, hashes are critical for securing transactions, blocks, and the blockchain’s integrity. For example, a transaction’s hash (TxID) uniquely identifies it, like “1a2b3c4d…” for a transfer of 0.5 BTC, and is used in Merkle trees to summarize multiple transactions in a block.

Hashes underpin Bitcoin’s proof-of-work (PoW) mechanism, where miners compete to find a block hash meeting a difficulty target (e.g., a hash with 20 leading zeros as of 2025, requiring ~10^21 computations per block).

The block hash, computed from the block header (including the previous block’s hash, Merkle root, timestamp, nonce, and difficulty), links blocks chronologically, ensuring immutability; altering any transaction would change all subsequent block hashes, requiring enormous computational power to rewrite. However, quantum computing advancements, like Google’s 2025 Willow chip with 105 qubits, raise theoretical concerns about SHA-256’s long-term collision resistance.