PCZT (Partially Constructed Zcash Transaction)
A PCZT is a standardized format defined in ZIP 374 for building Zcash transactions incrementally across multiple parties or systems.
Why PCZT?
Traditional transaction construction requires all private keys to be available in one place. PCZT enables:
- Hardware wallet integration — Sign on a separate device
- Multi-signature workflows — Multiple parties contribute signatures
- Separation of concerns — Different systems handle different roles
- Auditability — Inspect transaction details before signing
PCZT Roles
The ZIP 374 specification defines several roles that operate on a PCZT:
| Role | Description | t2z Function |
|---|
| Creator | Creates the initial empty PCZT | propose_transaction |
| Constructor | Adds inputs and outputs | propose_transaction |
| IO Finalizer | Finalizes input/output data | propose_transaction |
| Prover | Generates zero-knowledge proofs | prove_transaction |
| Signer | Adds signatures | get_sighash + append_signature |
| Spend Finalizer | Finalizes for extraction | finalize_and_extract |
| Transaction Extractor | Produces final transaction | finalize_and_extract |
In t2z, propose_transaction combines the Creator, Constructor, and IO Finalizer roles for simplicity.
Orchard
Orchard is the latest Zcash shielded protocol, activated with the NU5 network upgrade. It uses the Halo 2 proving system.
Orchard vs Sapling
| Feature | Sapling | Orchard |
|---|
| Proving system | Groth16 | Halo 2 |
| Trusted setup | Required (~50MB download) | None |
| Proving key | Downloaded | Built on demand |
| Address prefix | zs1... | u1... (unified) |
| Privacy | Good | Better (unified addresses) |
Unified Addresses
Orchard receivers are embedded in Unified Addresses (UAs), which can contain multiple receiver types:
u1... → Contains Orchard receiver (and optionally Sapling/transparent)
t1... or tm...).
interface TransparentInput {
pubkey: string; // 33-byte compressed public key (hex)
prevoutTxid: string; // 32-byte transaction ID (little-endian hex)
prevoutIndex: number; // Output index in the previous transaction
value: bigint; // Value in zatoshis
scriptPubkey: string; // P2PKH scriptPubkey (hex)
sequence?: number; // Sequence number (default: 0xffffffff)
}
Transaction ID Byte Order
Block explorers display txids in big-endian format, but Zcash internally uses little-endian. Always reverse the bytes when using txids from explorers.
// From block explorer: 3813f53766c6cc012becc81baa7875e2c5d5e3f452f26e11c568d91316f715ce
// For t2z (reversed): ce15f71613d968c5116ef252f4e3d5c5e27578aa1bc8ec2b01ccc66637f51338
Payments (ZIP 321)
Payments follow the ZIP 321 payment request format:
interface Payment {
address: string; // Unified address or transparent address
amount: bigint; // Amount in zatoshis (1 ZEC = 100,000,000 zatoshis)
memo?: string; // Optional memo for shielded outputs (hex-encoded, max 512 bytes)
label?: string; // Optional display label
}
Memos
Memos are only supported for shielded (Orchard) outputs. They must be hex-encoded:
// Convert text to hex
const memoHex = Buffer.from('Hello Zcash!').toString('hex');
// Use in payment
new Payment('u1...', 100000n, memoHex, null);
Fees (ZIP 317)
t2z automatically calculates fees according to ZIP 317:
fee = marginal_fee × max(grace_actions, logical_actions)
marginal_fee = 5,000 zatoshisgrace_actions = 2logical_actions = max(transparent_inputs, transparent_outputs) + orchard_actions
Typical Fees
| Transaction Type | Typical Fee |
|---|
| 1 transparent input → 1 Orchard output | 10,000 zats (0.0001 ZEC) |
| 1 transparent input → 2 Orchard outputs | 10,000 zats |
| 2 transparent inputs → 1 Orchard output | 10,000 zats |
Network & Expiry
Networks
t2z supports both networks:
mainnet — Production Zcash networktestnet — Test network for development
Expiry Height
Every transaction has an expiry height. If the transaction isn’t mined by that block, it becomes invalid.
// Must be:
// 1. After Nu5 activation (mainnet: 1,687,104 / testnet: 1,842,420)
// 2. At least current_height + 40 to avoid "tx-expiring-soon" errors
const currentHeight = await fetchCurrentBlockHeight();
const expiryHeight = currentHeight + 100; // ~2.5 hour buffer
In production, always fetch the current block height from a lightwalletd server or block explorer API.
Zatoshis
All amounts in t2z are in zatoshis — the smallest unit of ZEC:
1 ZEC = 100,000,000 zatoshis
// Convert ZEC to zatoshis
const zatoshis = BigInt(Math.floor(zec * 100_000_000));
// Convert zatoshis to ZEC for display
const zec = Number(zatoshis) / 100_000_000;
Always use BigInt for amounts to avoid JavaScript floating-point precision issues.
