mineracks-ckbunker-hsm-sign/fixtures

Test PSBTs — how to generate them

The harness needs two pre-crafted PSBTs:

Fixture	Amount	Policy path expected
small.psbt	≤ auto-approve cap (e.g. 9,000 sats if your Rule #2 cap is 10,000)	Signs without TOTP
large.psbt	> auto-approve cap, ≤ user-auth cap (e.g. 100,000 sats)	Rejected without TOTP; signs with TOTP

Both PSBTs must:

• be spendable by the Coldcard bound to your CKBunker (same seed / xpub)
• spend to an address you control (or a burn address — they are test inputs, you never broadcast them)
• use a real UTXO the Coldcard can see (watch-only wallet)

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Method 1 — Sparrow Wallet (recommended for first-time setup)

1. In Sparrow, open or create a watch-only wallet loaded with your Coldcard's xpub. (The Coldcard's HSM-Mode QR or a coldcard.txt export works.)
2. Send yourself a small amount on testnet or signet so you have a UTXO to spend without losing real sats. (For mainnet demos, 10k sats is ~AUD $1.)
3. Build two transactions:
   • Small demo — pay 9,000 sats (or 90% of your Rule #2 per-txn cap) to any receive address in the same wallet. Sparrow → Send → Save PSBT → write to fixtures/small.psbt.
   • Large demo — pay 100,000 sats (or mid-range of your Rule #1 cap) the same way. Save as fixtures/large.psbt.
4. Both PSBTs should show Coldcard as a required signer in Sparrow.

Do NOT broadcast these. The harness signs them, but you verify the signatures in Sparrow and then discard — there's no reason to spend real sats on a validation run.

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Method 2 — bitcoind (CI / automation)

If you're wiring the harness into CI against a regtest or signet deployment, scripting PSBT generation is a one-off:

#!/usr/bin/env bash
# Requires bitcoin-cli on PATH, pointed at a node that sees your wallet.
set -euo pipefail

WALLET="ckbunker-watch"
FEE_RATE=10   # sat/vB

recipient=$(bitcoin-cli -rpcwallet=$WALLET getnewaddress)

small_raw=$(bitcoin-cli -rpcwallet=$WALLET walletcreatefundedpsbt \
  '[]' "[{\"$recipient\":0.00009000}]" 0 \
  "{\"fee_rate\":$FEE_RATE}" | jq -r '.psbt')
echo "$small_raw" | base64 -d > fixtures/small.psbt

large_raw=$(bitcoin-cli -rpcwallet=$WALLET walletcreatefundedpsbt \
  '[]' "[{\"$recipient\":0.00100000}]" 0 \
  "{\"fee_rate\":$FEE_RATE}" | jq -r '.psbt')
echo "$large_raw" | base64 -d > fixtures/large.psbt

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Method 3 — use the same PSBT file over and over

Nothing in the harness requires the PSBT to be spendable right now for the reject-path test (test_04). The Coldcard rejects on amount, not on whether the UTXO is still unspent. So:

• small.psbt can be reused until the UTXO is spent elsewhere.
• large.psbt can be reused indefinitely — every validation run that tests Rule #1 rejection produces a rejection regardless of UTXO state.

If you run the full suite frequently, consider crafting large.psbt deliberately against an already-spent UTXO so the success path (test_05) fails at signature verification (not policy evaluation) — this is arguably safer than running with signable funds live.

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File format

Either binary (psbt\xff... magic bytes) or base64-encoded text is accepted by the harness — it auto-detects via magic bytes. Sparrow exports binary by default; bitcoin-cli returns base64.

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What NOT to do

• Do not commit real PSBTs to git — .gitignore already blocks *.psbt in this directory.
• Do not use a PSBT that spends a UTXO you can't afford to move. The harness does not broadcast, but a leaked signed PSBT can be broadcast by anyone.
• Do not reuse production keys for generating fixtures — prefer testnet or signet.