bitcoin-hardware-psbt-flows
PSBT signing flow patterns across HW vendors: USB direct, microSD airgap, QR airgap, NFC, multi-device coordination. USE WHEN: designing HW signing UX, debugging cross-device PSBT flow, supporting multi-vendor multisig.
What this skill does
# PSBT Signing Flow Patterns
A summary of how PSBTs flow through different hardware wallet
transports.
## Flow types
### USB direct (online signer)
```
Host wallet ──── PSBT ────► HW (via USB)
◄── signed PSBT
```
Wallets: Trezor, Ledger, BitBox02, Coldcard via USB, Jade USB,
Keystone (charging only).
Pro: fastest. Con: USB connectivity = potentially compromised host.
### microSD airgap (true offline)
```
Host wallet → save PSBT to SD → eject → insert into HW → sign on
HW → eject → insert into Host
```
Wallets: Coldcard, Passport, Jade (with SD), Keystone.
Pro: no USB connection; truly airgapped. Con: SD card handling
overhead.
### QR airgap (animated QR / BCUR)
```
Host wallet displays animated QR → HW reads via camera
→ HW signs → HW displays animated QR → Host reads
```
Wallets: Passport, Coldcard Q, Jade, Keystone, SeedSigner, Krux.
Pro: no physical connection. Con: scan time for big PSBTs.
BCUR (Blockchain Commons UR Format) is the standard encoding for
multi-frame QR. Compatible wallets must understand BCUR.
### NFC tap
```
Host (phone) → NFC tap to HW → HW signs → NFC tap back
```
Wallets: Coldcard Mk4 / Q, Passport (limited), some others.
Pro: convenient with Android. Con: iOS NFC limited; PSBT size
limits.
## Coordinator role
For multisig, a **coordinator** wallet handles:
1. Build PSBT.
2. Send to signer 1, get partial sig.
3. Send to signer 2, get partial sig.
4. Combine + finalize.
5. Broadcast.
Coordinators:
- **Sparrow Desktop** — best for cross-vendor multisig.
- **Specter Desktop** — multisig + multiple HW.
- **Caravan** — web-based multisig.
- **Bitcoin Core** with HWI external signer.
## PSBT versions
- v0 (BIP174): widely supported.
- v2 (BIP370): newer; spec-compliant signing only on modern devices.
For multi-device multisig, **all** signers must agree on PSBT version
or coordinator transcodes between them.
## Cross-vendor multisig
Best practice: use **descriptors with key origins**:
```
wsh(sortedmulti(2,
[fp1/48'/0'/0'/2']xpub1.../<0;1>/*,
[fp2/48'/0'/0'/2']xpub2.../<0;1>/*,
[fp3/48'/0'/0'/2']xpub3.../<0;1>/*
))#chk
```
Each signer's device must:
- Have its xpub at the declared path.
- Match key origin fingerprint.
- Recognize the wallet policy (Ledger requires explicit registration
with HMAC).
## Common pitfalls
- **PSBT missing data**: device rejects if `non_witness_utxo` or
`tap_internal_key` missing for the input type.
- **Path mismatch**: device can't find the key for declared
derivation.
- **Multi-vendor wallet policy registration**: Ledger requires
explicit wallet-policy + HMAC registration before multisig signing.
- **Stale PSBT**: signer signed an old version; coordinator must
reconcile.
- **Air-gap delays**: long PSBTs via QR/SD take minutes; users
abandon.
## See also
- [hwi/SKILL.md](../hwi/SKILL.md)
- [multi-vendor-multisig/SKILL.md](../multi-vendor-multisig/SKILL.md)
- [../wallets/hd/SKILL.md](../../wallets/hd/SKILL.md)
- [../../protocol/psbt/SKILL.md](../../protocol/psbt/SKILL.md)
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