Overview
An extended public key (commonly called an xpub) is a component of HD wallets defined in BIP32. It contains both a public key and a chain code, which together allow the deterministic derivation of all child public keys in a specific branch of the key hierarchy. This enables watch-only wallets, accounting systems, and payment processors to generate fresh receiving addresses without ever having access to the corresponding private keys.
Structure
An extended public key consists of several fields serialized into a Base58Check-encoded string:
xpub661MyMwAqRbcF... (typical xpub string, 111 characters)
Internal structure:
┌──────────────────────────────────────────────────┐
│ Version bytes (4) → Identifies key type │
│ Depth (1) → Level in the HD tree │
│ Parent fingerprint (4) → First 4 bytes of parent │
│ Child index (4) → Which child this is │
│ Chain code (32) → Used for key derivation │
│ Public key (33) → Compressed public key │
└──────────────────────────────────────────────────┘
Total: 78 bytes → Base58Check encoded
Version Prefixes
Different version bytes produce different prefixes that indicate the address type:
┌────────┬──────────────┬────────────────────────┐
│ Prefix │ Script Type │ Derivation Path │
├────────┼──────────────┼────────────────────────┤
│ xpub │ P2PKH │ m/44'/0'/account' │
│ ypub │ P2SH-P2WPKH │ m/49'/0'/account' │
│ zpub │ P2WPKH │ m/84'/0'/account' │
└────────┴──────────────┴────────────────────────┘
Use Cases
- Watch-only wallets: Import an xpub to monitor balances and generate invoices without spending capability
- Payment processors: Generate unique addresses for each customer or order without storing private keys on the server
- Accounting: Track all wallet activity without risking fund security
- Multi-device setups: Share the xpub with a phone wallet for monitoring while keeping private keys on a hardware wallet
Privacy Implications
Sharing an xpub is a significant privacy decision. Anyone with access to an xpub can:
- Derive all past and future addresses in that derivation path
- See the complete transaction history and balance
- Correlate all addresses as belonging to the same wallet
For this reason, xpubs should be treated as sensitive information and shared only with trusted parties. Leaking an xpub to a third party gives them full visibility into your financial activity.
Common Misconceptions
- An xpub cannot be used to spend funds. It only enables address generation and balance monitoring.
- However, combining an xpub with any single child private key from a non-hardened derivation path can expose the master private key. This is why hardened derivation is used at critical levels of the key hierarchy.
- An xpub is specific to one account in the HD tree. A wallet with multiple accounts has multiple xpubs.