PSBT Format¶

Technical details about Partially Signed Bitcoin Transactions (PSBTs) used in Theft Shield.

What is a PSBT?¶

A PSBT (BIP-174) is a standardized format for creating, signing, and finalizing Bitcoin transactions across multiple parties or devices.

Structure¶

Global Fields¶

Key	Description
`PSBT_GLOBAL_UNSIGNED_TX`	The unsigned transaction
`PSBT_GLOBAL_XPUB`	Extended public keys
`PSBT_GLOBAL_VERSION`	PSBT version number

Input Fields¶

Key	Description
`PSBT_IN_NON_WITNESS_UTXO`	Full previous transaction (legacy)
`PSBT_IN_WITNESS_UTXO`	UTXO being spent (SegWit)
`PSBT_IN_PARTIAL_SIG`	Partial signatures
`PSBT_IN_SIGHASH_TYPE`	Signature hash type
`PSBT_IN_REDEEM_SCRIPT`	Redeem script (P2SH)
`PSBT_IN_WITNESS_SCRIPT`	Witness script (P2WSH)
`PSBT_IN_BIP32_DERIVATION`	Key derivation paths

Output Fields¶

Key	Description
`PSBT_OUT_REDEEM_SCRIPT`	Redeem script for output
`PSBT_OUT_WITNESS_SCRIPT`	Witness script for output
`PSBT_OUT_BIP32_DERIVATION`	Derivation for change outputs

Theft Shield PSBTs¶

Structure¶

Vigil's Theft Shield PSBTs include:

PSBT {
  Global:
    - Unsigned transaction spending all monitored UTXOs
    - Version: 2

  Inputs (one per UTXO):
    - Previous output reference
    - Witness UTXO data
    - BIP32 derivation path
    - (Empty partial signatures - to be filled)

  Outputs:
    - Single output to safe address
    - Amount: Total - fee
}

Signing Process¶

Creator (Vigil) constructs PSBT
Signer (Your hardware wallet) adds signatures
Finalizer (Vigil) verifies completeness
Extractor (Vigil) creates broadcastable transaction

Encoding Formats¶

Base64¶

Human-readable encoding:

cHNidP8BAHECAAAAAZvDiCVxT73PiMpxKCEHULgJHBkKR5B3/yCaW2VU+...

Binary¶

Raw PSBT format:

70 73 62 74 ff 01 00 71 02 00 00 00 01 9b c3 88 ...

Hex¶

Hexadecimal encoding:

70736274ff010071020000...

Validation¶

Required Checks¶

Vigil validates PSBTs for:

Signature Completeness
All inputs have required signatures
Signature types are correct (SIGHASH_ALL)
Output Verification
Output address matches safe address
Amount calculation is correct
Fee Verification
Fee matches expected round budget
Fee rate is reasonable
UTXO Coverage
All monitored UTXOs are included
No unexpected inputs

Common PSBT Operations¶

Combining PSBTs¶

Merge multiple partially signed PSBTs:

bitcoin-cli combinepsbt '["psbt1...", "psbt2..."]'

Analyzing PSBT¶

Inspect PSBT contents:

bitcoin-cli decodepsbt "cHNidP8BAHECAAAAAZvDiCVxT..."

Finalizing PSBT¶

Convert signed PSBT to transaction:

bitcoin-cli finalizepsbt "cHNidP8BAHECAAAAAZvDiCVxT..."

Security Considerations¶

What PSBTs Reveal¶

PSBTs contain: - ✅ Transaction structure (inputs, outputs) - ✅ Addresses involved - ✅ Amounts - ✅ Derivation paths (if included)

PSBTs do NOT contain: - ❌ Private keys - ❌ Seed phrases

PSBT Storage¶

Vigil stores signed PSBTs securely:

Encrypted at rest
Access-controlled
Regular integrity checks

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