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t2z’s two-step signing process (get_sighash + append_signature) is designed for integration with external signers.

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Why External Signing?

  • Hardware wallets — Ledger, Trezor, etc.
  • HSMs — Hardware Security Modules for enterprise
  • Air-gapped systems — Cold storage signing
  • Multi-signature — Different parties contribute signatures

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The Flow

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Implementation

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1. Get Sighash

const sighashHex = t2z.get_sighash(pczt, inputIndex);
const sighashBytes = hexToBytes(sighashHex);

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2. Sign Externally

The sighash is a standard 32-byte hash. Sign it with ECDSA secp256k1: 
// Generic external signing interface
const signature = await externalSigner.signEcdsa({
  message: sighashBytes,
  curve: 'secp256k1',
  // Hardware wallet specific options...
});

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3. Format Signature

The signature must be DER-encoded with sighash type appended: 
// If signer returns DER
const sigWithType = new Uint8Array([...derSignature, 0x01]);

// If signer returns raw (r, s)
import { secp256k1 } from '@noble/curves/secp256k1';
const sig = new secp256k1.Signature(r, s);
const derSig = sig.toDERRawBytes();
const sigWithType = new Uint8Array([...derSig, 0x01]);

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4. Append to PCZT

pczt = t2z.append_signature(
  pczt,
  inputIndex,
  pubkeyHex,
  bytesToHex(sigWithType)
);

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Hardware Wallet Examples

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Ledger

import TransportWebUSB from '@ledgerhq/hw-transport-webusb';
import Btc from '@ledgerhq/hw-app-btc';

const transport = await TransportWebUSB.create();
const btc = new Btc({ transport });

// Get sighash
const sighash = t2z.get_sighash(pczt, 0);

// Sign with Ledger
const result = await btc.signMessage(
  "44'/133'/0'/0/0",  // Zcash BIP44 path
  Buffer.from(sighash, 'hex').toString('hex')
);

// Append signature
pczt = t2z.append_signature(pczt, 0, pubkey, result.signature + '01');

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Trezor

import TrezorConnect from '@trezor/connect';

const sighash = t2z.get_sighash(pczt, 0);

const result = await TrezorConnect.signMessage({
  path: "m/44'/133'/0'/0/0",
  message: sighash,
  hex: true,
});

pczt = t2z.append_signature(pczt, 0, pubkey, result.payload.signature + '01');

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Air-Gapped Signing

For maximum security, the PCZT can be transferred to an offline machine:

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Online Machine

// Create PCZT
const pczt = t2z.propose_transaction(...);

// Export for offline signing
const pcztHex = pczt.to_hex();
// Transfer via QR code, USB drive, etc.

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Offline Machine

// Import PCZT
const pczt = WasmPczt.from_hex(pcztHex);

// Sign all inputs
for (let i = 0; i < inputCount; i++) {
  const sighash = t2z.get_sighash(pczt, i);
  // Sign with cold storage key
  pczt = t2z.append_signature(pczt, i, pubkey, signature);
}

// Export signed PCZT
const signedHex = pczt.to_hex();
// Transfer back to online machine

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Online Machine (continued)

// Import signed PCZT
const signedPczt = WasmPczt.from_hex(signedHex);

// Prove and finalize
const provedPczt = t2z.prove_transaction(signedPczt);
const txHex = t2z.finalize_and_extract_hex(provedPczt);

// Broadcast
await broadcast(txHex);

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Multi-Party Signing

Different parties can sign different inputs: 
// Party A signs input 0
const sighash0 = t2z.get_sighash(pczt, 0);
// Sign and append...
const pcztFromA = pczt.to_hex();

// Transfer to Party B...

// Party B signs input 1
const pczt = WasmPczt.from_hex(pcztFromA);
const sighash1 = t2z.get_sighash(pczt, 1);
// Sign and append...

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Security Considerations

Always verify before signing! If the PCZT came from another party, call verify_before_signing to ensure it hasn’t been modified.
// Verify PCZT matches your intent
t2z.verify_before_signing(pczt, originalPayments, expectedChange);

// Only then sign
const sighash = t2z.get_sighash(pczt, 0);