Problem

P2PKH (Pay-to-Public-Key-Hash) outputs initially hide the public key behind a hash, providing a layer of quantum resistance. However, once an address has been used to send bitcoin, the public key is revealed in the scriptSig of the spending transaction.

Any remaining unspent UTXOs at that same address are now quantum-vulnerable: a quantum attacker can compute the private key from the exposed public key and sweep the remaining balance.

Detection

When processing inputs, extract the public key from:

1. scriptSig.asm for P2PKH (last element is the pubkey)
2. txinwitness[1] for P2WPKH (second witness element is the pubkey)

Once extracted, tag all remaining UTXOs at that address:

// Mark remaining UTXOs at the address
UPDATE utxos
SET pubkey_exposed = TRUE, pubkey_hex = $2,
    loss_rules = array_append(loss_rules, '016')
WHERE address = $1 AND pubkey_exposed = FALSE

Classification

Loss bucket: 4 (Quantum only) for UTXOs that have no other loss classification.

Note: UTXOs already classified as provably or probably lost are not re-tagged with '016' — the quantum tag is only meaningful for UTXOs that are otherwise spendable.

Relationship to Proposal 015

• Proposal 015: Public key is always visible (in P2PK script), exposure is static
• Proposal 016: Public key becomes visible at first spend, exposure is dynamic

Combined, Proposals 015 + 016 define the full quantum exposure set tracked in the quantum_all_exposed snapshot.