Private key
Also · Public key · Key pair · Signing key
A private key is the secret number that lets you sign Bitcoin transactions and spend the coins locked to its public key. Whoever holds it controls the funds.
A Bitcoin private key is a 256-bit number. From it, elliptic-curve mathematics on the secp256k1 curve produces a public key, and from the public key an address. The relationship runs 1 way. The public key can be shared and used to verify signatures. The private key must never leave the device that signs.
Most wallets today do not store individual keys. They derive an unlimited tree of them from a single seed, which the recovery phrase encodes. Losing 1 copy of a key is not fatal if a backup exists. Funds become unreachable only when nothing can satisfy the spending conditions anymore, and in a multisig setup that depends on how many of the required keys survive.
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