Bitcoin Silent Payments Integration

Welcome 👨‍💻

Silent Payments

Special credit to cake wallet.

ℹ️

This is a Dart package. To use an SDK go here.

@shakesco/bitcoin_base

Install

To get started, add this to your pubspec.yaml:

pubspec.yaml

  bitcoin_base:
    git:
      url: https://github.com/shakesco/bitcoin_base
      ref: cake-update-v5

After installing:

index.dart

import 'package:bitcoin_base/bitcoin_base.dart' as bitcoin_base;

Generate Silent Payment address

This will generate the silent payment address. It prepares a receiver to receive silent payments. You can generate a silent payment address in three ways:

Private Keys

If you are not a wallet provider, use this method. More specifically, you can make the user sign a message and then derive b_scan and b_spend from the resulting signature (Use r as b_scan and s as b_spend or vice versa).

⚠️

If you are not using this method, ensure that a cryptographically secure random number generator is being used.

index.dart

void main() {
  final b_scan = "";
  final b_spend = "";
  final paymentOwner = bitcoin_base.SilentPaymentOwner.fromPrivateKeys(
      network: bitcoin_base.BitcoinNetwork.testnet,
      version: 0,
      b_scan: bitcoin_base.ECPrivate.fromHex(b_scan),
      b_spend: bitcoin_base.ECPrivate.fromHex(b_spend));
  print(paymentOwner.toAddress());
}

Mnemonic and HD Key

If you are a wallet provider, use this method.

index.dart

void main() {
  final mnemonic = ""; // 12, 15, 24 word phrase
  final paymentOwner = bitcoin_base.SilentPaymentOwner.fromMnemonic(mnemonic);
  print(paymentOwner.toAddress());

//   final paymentOwner = bitcoin_base.SilentPaymentOwner.fromHd();
//   print(paymentOwner.toAddress());
}

Create a taproot address destination

Here is where you create a destination address for the user to send to a newly generated Taproot address, derived from the receiver’s silent payment address generated above. You will need:

The Unspent Transaction Output(UTXO) of the user, hash and output_index.
The private key of the UTXO in 1 above.
Amount the user wants to send. Should be in satoshis(1 BTC = 100⁶ satoshis)
Finally, the public keys of the 2 secret shares, B_scan and B_spend

index.dart

void main() {
  final tx_hash = "";
  final tx_id_output_index = "";
  final sender_privateKey = "";
  final amount = 1000;
  final B_scan = bitcoin_base.SilentPaymentAddress.fromAddress(paymentOwner.toAddress()).B_scan;
  final B_spend = bitcoin_base.SilentPaymentAddress.fromAddress(paymentOwner.toAddress()).B_spend;

  final payto = bitcoin_base.SilentPaymentBuilder(vinOutpoints: [
    bitcoin_base.Outpoint(
        txid: tx_hash,
        index: tx_id_output_index)
  ]).createOutputs([
    bitcoin_base.ECPrivateInfo(
        bitcoin_base.ECPrivate.fromHex(sender_privateKey),
        false // If the output is from a taproot address
        )
  ], [
    bitcoin_base.SilentPaymentDestination(
        amount: amount,
        network: bitcoin_base.BitcoinNetwork.testnet,
        version: 0,
        scanPubkey: B_scan,
        spendPubkey: B_spend)
  ]);

  print(payto.values.first.first.address
      .toAddress(bitcoin_base.BitcoinNetwork.testnet));
}

Scan for funds

Scanning for funds is a drawback of silent payments. So below is how you can check if a certain transaction belongs to a user. You will need:

The transaction input’s tx_hash and output_index.
Public key outputted.
Script and amount from the outputted taproot address

For more info, go here

index.dart

void main() {
  final tx_hash = "";
  final tx_id_output_index = "";
  final senders_pubKey = "";
  final amount = "";
  final Script = "";

  Map<String, bitcoin_base.SilentPaymentScanningOutput> output =
      bitcoin_base.SilentPaymentBuilder(
    vinOutpoints: [
      bitcoin_base.Outpoint(
          txid: tx_id,
          index: tx_id_output_index)
    ],
    pubkeys: [
      bitcoin_base.ECPublic.fromHex(senders_pubKey),
    ],
  ).scanOutputs(paymentOwner.b_scan, paymentOwner.B_spend, [
    bitcoin_base.BitcoinScriptOutput(
        script: bitcoin_base.Script(script: [Script]),
        value: BigInt.from(amount))
  ]);

  print(payto.values.first.first.address
      .toAddress(bitcoin_base.BitcoinNetwork.testnet));
}

If the address above matches the taproot address from the output in the transaction, it belongs to the user.

Spend funds

If the funds belong to the user, they can spend like so:

First, you will need:

The transaction input’s tx_hash and output_index.
Public key outputted.
Receiver’s spend and scan private keys.

index.dart

void main() {
  final tx_hash = "";
  final tx_id_output_index = "";
  final senders_pubKey = "";

  bitcoin_base.ECPrivate spendprivKey = bitcoin_base.SilentPaymentBuilder(
    vinOutpoints: [
      bitcoin_base.Outpoint(
          txid: tx_hash,
          index: tx_id_output_index),
    ],
    pubkeys: [
      bitcoin_base.ECPublic.fromHex(senders_pubKey)
    ],
  ).spendOutputs(paymentOwner.b_scan, paymentOwner.b_spend);
}

The receiver can use spendprivKey to spend the funds!

Thats it! 🎊🎊🎊

Contribute

If you love what we do to progress privacy, contribute to further development

Silent Payments SDK