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Crypto & TokensGlossary

Private key

A private key is the secret code that authorizes transfers of crypto from your wallet. Plain-English definition, why lost keys mean lost crypto, and an example.

Also called: secret key, signing key

By The Token Thesis Desk2 min read3 cited sources

Antique iron skeleton keys laid on a grey surface
“Four Skeleton Keys on a blue background (51865175725)” by Cindy Shebley — CC BY 2.0 (edited: cropped, recolored)

Quick answer

A private key is the secret half of a cryptographic key pair. In crypto, it is the code that authorizes transactions from your wallet; whoever holds it controls the assets, and if it is lost, access is permanently lost.

What does private key mean?#

In cryptography, NIST defines a private key as the secret part of an asymmetric key pair that is typically used to digitally sign or decrypt data, and notes it is uniquely associated with its owner and not made public[1]. Its partner, the public key, can be shared freely.

In crypto, the SEC's investor education office describes a private key as a randomly generated passcode that allows you to authorize transactions for a crypto asset, while the public key lets others send crypto to your wallet[2]. A seed phrase is a sequence of words that can restore a wallet if you lose it[2].

Why can't someone just guess a private key?#

Worked example

How large a 64-character key space is

Suppose a key is written as 64 hexadecimal characters (0–9 and a–f). We compare it with a 12-character code, assuming a hypothetical attacker who tries one trillion guesses per second and needs, on average, to search half the possibilities.

Key written asPossible keysAverage time to guess
12 hexadecimal characters281,474,976,710,656About 141 seconds
64 hexadecimal charactersAbout 1.16 × 10⁷⁷About 1.8 × 10⁵⁷ years

Figures computed in code from the stated inputs; rounded to the nearest cent or tenth.

What happens if a private key is lost or stolen?#

The same size that stops guessing also stops recovery. NIST explains that if a user loses a private key, any digital asset associated with it is lost, because it is computationally infeasible to regenerate the same key[3]. The SEC puts it plainly: if you lose your private key, you permanently lose access to the crypto assets in your wallet[2]. Anyone who copies your key or seed phrase can move the assets.

That is why custody choices matter. Read the risks of crypto investing for wallet and platform risks, how blockchains work for where signatures fit, and bitcoin basics for how keys move bitcoin. The ledger the key controls is explained under blockchain.

Sources

Numbers in brackets in the text point here. Grade A = primary source (regulator, statistics agency, law or official document).

  1. 1
    Private key (CSRC Glossary)National Institute of Standards and Technology — Computer Security Resource Center (n.d.) · Grade A
  2. 2
    Crypto Asset Custody Basics for Retail Investors – Investor BulletinU.S. SEC — Office of Investor Education and Assistance (Investor.gov) (2025) · Grade A
  3. 3
    NISTIR 8202: Blockchain Technology OverviewNational Institute of Standards and Technology (Yaga, Mell, Roby, Scarfone) (2018) · Grade A

How we checked this note

Every number, date and rule above links to its source. This note cites 3 sources, 3 of them primary (Grade A). Worked examples were calculated in code, and a second editor compared each figure with its source before publishing. Spotted an error? Tell us — corrections are listed on the note. Read our editorial policy.