Private key

A secret number (256 bits on Bitcoin, Ethereum and most major networks) used to produce the digital signatures that authorise spending from the associated addresses. For a standard single-key address, the signature is the only form of authorisation the network recognises: whoever holds the key controls the funds. Exceptions include multisignature addresses and smart-contract wallets, where control may depend on several keys or on rules coded in the contract. Loss of a key results in loss of funds only if no backup exists.