> ## Documentation Index
> Fetch the complete documentation index at: https://docs.unwallet.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Stealth Address Technical Guide (x402)

> Technical deep dive into stealth addresses and the x402 payment protocol

<Info>
  **Who is this for?**
  Developers, integrators, and advanced users who want to understand or build on stealth address technology and the x402 standard.
</Info>

This document explains the cryptography, address derivation, and integration details behind stealth addresses in the x402 payment protocol.

## What are Stealth Addresses?

Stealth addresses are a privacy-enhancing technology for blockchain payments. They allow a recipient to publish a single address, while each payment is sent to a unique, unlinkable address on-chain.

<Info>
  Stealth addresses prevent observers from linking multiple payments to the same recipient, even if the stealth address is reused.
</Info>

## How Stealth Addresses Work

1. **Key pairs**: The recipient generates a stealth address, which is derived from two key pairs:
   * A **scan key** (public/private)
   * A **spend key** (public/private)
2. **Stealth address**: The recipient shares the stealth address (usually a combination of the scan and spend public keys).
3. **Sender derives one-time address**:
   * The sender generates an ephemeral key pair.
   * Using the recipient's scan public key and the sender's ephemeral private key, the sender computes a shared secret.
   * The sender uses this secret and the recipient's spend public key to derive a unique one-time public key (address).
4. **Payment**: The sender sends funds to the one-time address. The ephemeral public key is included in the transaction.
5. **Detection**: The recipient's wallet scans the blockchain, using their scan private key and the ephemeral public key from each transaction to compute the shared secret and check if the one-time address belongs to them.
6. **Spending**: If a match is found, the recipient can use their spend private key and the shared secret to spend the funds.

<CodeGroup>
  <CodeBlock language="typescript" title="Address Derivation (simplified)">
    // Sender side
    const sharedSecret = ecdh(ephemeralPrivKey, recipientScanPubKey);
    const oneTimePubKey = add(recipientSpendPubKey, hash(sharedSecret));
    // Recipient side
    const sharedSecret = ecdh(scanPrivKey, ephemeralPubKey);
    const oneTimePrivKey = add(spendPrivKey, hash(sharedSecret));
  </CodeBlock>
</CodeGroup>

## Security Properties

* **Unlinkability**: Each payment uses a new address, unlinkable to the stealth address or other payments.
* **Forward secrecy**: Compromising a single one-time address does not reveal others.
* **Receiver privacy**: Only the recipient can detect and spend the funds.

<Warning>
  If the scan or spend private keys are lost, funds cannot be recovered. Always back up your keys securely.
</Warning>

## Integration Steps

1. **Wallets**: Implement stealth address generation (scan/spend key pairs) and blockchain scanning for one-time addresses.
2. **Senders**: Add support for ephemeral key generation and one-time address derivation.
3. **Transactions**: Include the ephemeral public key in each payment transaction.
4. **User experience**: Hide complexity—users only see their stealth address and balance.

## References

* [BIP47: Reusable Payment Codes](https://github.com/bitcoin/bips/blob/master/bip-0047.mediawiki)
* [Ethereum Stealth Addresses (Vitalik)](https://vitalik.ca/general/2023/01/20/stealth.html)
* [x402 Payment Standard](https://github.com/x402-protocol/spec)

***

For a user-focused guide, see the [Unwallet User Guide](./x402-user-guide.mdx).

<Info>
  See also: [Fluid Key Implementation](./fluid-key.mdx) for a product-specific example and UX.
</Info>