Verify Before Signing

The verify_before_signing function checks that a PCZT matches your original request. This is a security check to detect malleation if the PCZT was handled by a third party.

Per ZIP 374: “If the entity that invoked propose_transaction is the same as the entity that is adding the signatures, and no third party may have malleated the PCZT before signing, this step may be skipped.”

When to Verify

Scenario	Should Verify?
You created and will sign the PCZT	Optional
PCZT came from another system	Required
Hardware wallet workflow	Required
Multi-party transaction	Required

Function Signature

TypeScript
Go
Kotlin

function verify_before_signing(
  pczt: WasmPczt,
  payments: WasmPayment[],
  expected_change: WasmExpectedTxOut[]
): void  // Throws on failure

func VerifyBeforeSigning(
  pczt *Pczt,
  payments []Payment,
  expectedChange []ExpectedTxOut,
) error

fun verifyBeforeSigning(
  pczt: Pczt,
  payments: List<Payment>,
  expectedChange: List<ExpectedTxOut>
)  // Throws on failure

Parameters

pczt

Pczt

required

The PCZT to verify

payments

Payment[]

required

The original payments you requested (same as passed to propose_transaction)

expected_change

ExpectedTxOut[]

required

Expected change outputs. Use inspect_pczt to get the actual change amount:

interface ExpectedTxOut {
  address: string;   // Change address
  amount: bigint;    // Change amount in zatoshis
}

Example

TypeScript
Go

import * as t2z from '@d4mr/t2z-wasm';

// Original payments (same as used in propose_transaction)
const payments = [
  new t2z.WasmPayment('u1recipient...', 500_000n, null, null)
];

// Get the actual change from the PCZT
const info = t2z.inspect_pczt(pczt.to_hex());

// Calculate change: total_orchard_output - payment_amount
const paymentTotal = payments.reduce((sum, p) => sum + p.amount, 0n);
const changeAmount = BigInt(info.total_orchard_output) - paymentTotal;

// Build expected change
const expectedChange = changeAmount > 0n 
  ? [new t2z.WasmExpectedTxOut(changeAddress, changeAmount)]
  : [];

// Verify
try {
  t2z.verify_before_signing(pczt, payments, expectedChange);
  console.log('✓ Verification passed');
} catch (err) {
  console.error('✗ Verification failed:', err.message);
  // DO NOT SIGN - PCZT may have been tampered with!
}

// Original payments
payments := []t2z.Payment{{
    Address: "u1recipient...",
    Amount:  500_000,
}}

// Get actual change from PCZT
info, _ := t2z.InspectPczt(pczt)
changeAmount := info.TotalOrchardOutput - 500_000

// Build expected change
var expectedChange []t2z.ExpectedTxOut
if changeAmount > 0 {
    expectedChange = []t2z.ExpectedTxOut{{
        Address: changeAddress,
        Amount:  changeAmount,
    }}
}

// Verify
err := t2z.VerifyBeforeSigning(pczt, payments, expectedChange)
if err != nil {
    log.Fatal("Verification failed - DO NOT SIGN:", err)
}

What Gets Verified

Payment Matching

Every payment in your original request must be present in the PCZT with the exact amount and address.

Change Verification

If you expect change, it must go to the expected address with the expected amount.

No Extra Outputs

The PCZT must not contain any unexpected outputs (malleation check).

Verification Errors

Payment not found in PCZT

One of your original payments is missing from the PCZT. The PCZT may have been modified.

Unexpected output

The PCZT contains an output that wasn’t in your original request. Possible malleation.

Expected change not found

The change output doesn’t match expectations (wrong address or amount).

Security Considerations

Never sign a PCZT that fails verification. A malicious party could have:

Changed the recipient address
Modified the amounts
Added extra outputs to steal funds

If verification fails:

Do not sign the PCZT
Discard it and create a new one
Investigate how the PCZT was modified

Skipping Verification

You can skip verification when:

You created the PCZT yourself
The PCZT never left your control
You’re in a development/testing environment

// Skip verification (only when safe to do so)
console.log('Skipping verification (same entity created and will sign)');

Next Step

After verification passes (or is skipped), proceed to signing the transparent inputs.

Getting Started

Transaction Flow

Platform Guides

Advanced

Verify Before Signing

When to Verify

Function Signature

Parameters

Example

What Gets Verified

Verification Errors

Security Considerations

Skipping Verification

Next Step

​When to Verify

​Function Signature

​Parameters

​Example

​What Gets Verified

​Verification Errors

​Security Considerations

​Skipping Verification

​Next Step

When to Verify

Function Signature

Parameters

Example

What Gets Verified

Verification Errors

Security Considerations

Skipping Verification

Next Step