Extended Public Key (Xpub)

An extended public key, or xpub, is the public counterpart of a BIP 32 extended private key, containing a public key plus a 32-byte chain code from which unlimited receive addresses can be derived without exposing any private key. Wallets and services use it to generate a fresh address for every payment.

Why it matters

The xpub separates receiving from spending. A merchant's web server can mint new addresses for every customer while holding nothing a hacker could steal, and a watch-only wallet built from an xpub monitors cold storage in real time with the keys safely offline. Multisignature coordination also runs on xpubs, with each participant sharing public material so any cosigner can construct the wallet's addresses.

The cost is privacy: an xpub links every address it derives, so whoever obtains it can read the wallet's full transaction history. There is also a sharper technical edge case, since an xpub combined with any leaked non-hardened child private key mathematically reveals the parent extended private key.

How it works

Non-hardened child public keys are derived with HMAC-SHA512 over the parent public key, chain code, and index, mirroring private derivation without any secret material. The structure serializes as a Base58Check string beginning with xpub, and variant prefixes such as ypub and zpub signal which address type the keys are meant to produce.