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What is a private key?
A private key is a secret number that lets its holder sign transactions from a crypto account. It is one half of a key pair: the matching public key and address can be shared, but a private key cannot be worked out from them.
What it means for you
Anyone who learns your private key can send your funds anywhere, and the transfer cannot be undone. If it is lost with no backup, the funds cannot be moved by anyone. No real support desk or wallet ever needs you to type it into a website or message.
A common mistake: “If I lose my private key, the wallet company can reset it.”
In fact: There is no reset. SEC staff note that a private key cannot be changed or replaced once created, and losing it means permanently losing access, unless a seed phrase or other backup can regenerate it.
How it works
On Bitcoin and Ethereum a private key is a 256-bit number within a range set by the secp256k1 elliptic curve, written on Ethereum as 64 hex characters. The public key is computed from it on the curve, a step that is easy one way and computationally infeasible to reverse. Signing a transaction with the private key produces an ECDSA signature that anyone can check against the public key without learning the private key. Most wallets derive many private keys from one seed, so the seed phrase stands in for all of them.
Sources: ethereum.org: Ethereum accounts, Bitcoin developer guide: Wallets, SEC Office of Investor Education: Crypto asset custody basics for retail investors · checked 4 October 2026
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Educational content, not financial advice. Written by hand and checked against the source named above. Something wrong? Tell us and we reply within two business days.