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Signing is a two-step process per ZIP 374:
  1. Get the sighash — Compute the signature hash for an input
  2. Append the signature — Add the signed result to the PCZT
This separation enables external signing with hardware wallets, HSMs, or air-gapped systems.

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Two Approaches

External Signing

Use get_sighash + append_signature when:
  • Using a hardware wallet
  • Key is in an HSM
  • Signing on a separate system
Recommended for production

Convenience Method

Use sign_transparent_input when:
  • Key is available in memory
  • Testing/development
  • Simple use cases
Combines both steps internally

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External Signing Flow

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Step 1: Get Sighash

// Get the 32-byte sighash for input 0
const sighashHex = t2z.get_sighash(pczt, 0);
// Returns: "64a1ef0a32a709943685a39db55a5abf..."

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Step 2: Sign Externally

Sign the sighash with ECDSA secp256k1. The signature must be DER-encoded with the sighash type byte appended. 
import { secp256k1 } from '@noble/curves/secp256k1';

// Sign the sighash
const sighashBytes = hexToBytes(sighashHex);
const signature = secp256k1.sign(sighashBytes, privateKeyBytes);

// Get DER-encoded signature
const derSignature = signature.toDERRawBytes();

// Append SIGHASH_ALL type byte (0x01)
const signatureWithType = new Uint8Array([...derSignature, 0x01]);
const signatureHex = bytesToHex(signatureWithType);

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Step 3: Append Signature

pczt = t2z.append_signature(
  pczt,
  0,                    // Input index
  pubkeyHex,            // 33-byte compressed pubkey
  signatureHex          // DER signature + sighash type
);

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Convenience Method

For simple cases where the private key is available: 
// Sign input 0 with private key
pczt = t2z.sign_transparent_input(
  pczt,
  0,                    // Input index
  privateKeyHex         // 32-byte private key
);

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Signing Multiple Inputs

Each input must be signed individually: 
// Sign all inputs
for (let i = 0; i < inputs.length; i++) {
  const sighash = t2z.get_sighash(pczt, i);
  const signature = await sign(sighash, keys[i]);
  pczt = t2z.append_signature(pczt, i, pubkeys[i], signature);
}

// Verify all signed
const info = t2z.inspect_pczt(pczt.to_hex());
console.log('All signed:', info.all_inputs_signed);

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Signature Format

The signature appended to the PCZT must be: 
[DER-encoded ECDSA signature] + [sighash type byte]
ComponentSizeDescription
DER signature70-72 bytesStandard DER-encoded ECDSA
Sighash type1 byte0x01 for SIGHASH_ALL

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Example DER Signature

30440220                  # DER sequence header
359fd725c1bd0d5506c6...   # r value (32 bytes)
0220                      # Integer header
11574b391407ba5e04ea...   # s value (32 bytes)
01                        # SIGHASH_ALL

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Hardware Wallet Integration

The external signing flow is designed for hardware wallets: 
// 1. Get sighash on the host
const sighash = t2z.get_sighash(pczt, inputIndex);

// 2. Send to hardware wallet for signing
const signature = await hardwareWallet.signEcdsa({
  message: hexToBytes(sighash),
  keyPath: "m/44'/133'/0'/0/0",  // Zcash transparent path
});

// 3. Append signature on the host
pczt = t2z.append_signature(pczt, inputIndex, pubkey, signature);
The sighash is a standard 32-byte hash. Any ECDSA secp256k1 signer that supports raw message signing will work.

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Verifying Signatures

After signing, use inspect_pczt to verify: 
const info = t2z.inspect_pczt(pczt.to_hex());

// Check each input
info.transparent_inputs.forEach((input, i) => {
  console.log(`Input ${i}: ${input.is_signed ? '✓ Signed' : '○ Not signed'}`);
});

// Check all signed
if (info.all_inputs_signed) {
  console.log('All inputs signed, ready for proving');
}

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Common Errors

The signature doesn’t validate for the sighash. Check:
  • Correct private key for the input
  • Proper DER encoding
  • Sighash type byte (0x01) appended
The public key doesn’t match the input’s scriptPubkey. Ensure you’re using the correct key for each input.
The input index is out of bounds. Check inputs.length from inspect_pczt.

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Next Step

After all inputs are signed, proceed to generating proofs for Orchard outputs.