What is a Hash?
The output of a cryptographic function that turns any input into a fixed-length string of characters. In Bitcoin, hashing is used to secure transactions, create addresses, and validate blocks.
Why It Matters
Hashing is the cryptographic foundation of Bitcoin's security. A hash function takes any input—whether it's a transaction, a block, or a number—and produces a fixed-length output that looks random. Crucially, changing even one character of the input completely changes the hash, making it impossible to tamper with data without detection. Bitcoin uses hashing to create addresses (your public key gets hashed to create your Bitcoin address), to secure transactions (every transaction is hashed), and to validate blocks (each block contains a hash of the previous block, creating the "chain"). This means Bitcoin's entire security model depends on hashing working reliably and being computationally difficult to reverse.
How It Works
A hash function is a mathematical algorithm that converts any input into a fixed-size output string. Bitcoin primarily uses SHA-256 (Secure Hash Algorithm 256-bit), which always produces a 256-bit output regardless of input size. The same input always produces the same hash, but any input change produces a completely different hash (this is called the avalanche effect). Most importantly, it's computationally impractical to reverse a hash—you cannot take a hash and figure out what input created it. Bitcoin uses hashing in multiple places: SHA-256 hashing of transactions creates a transaction ID, double SHA-256 hashing is used for blocks, and hashing is part of the mining process where miners search for a hash output below a certain target.