P2PK outputs where the pushed key has an invalid length or an invalid prefix byte are permanently unspendable because OP_CHECKSIG immediately returns false for any key it cannot parse.

Abstract

A P2PK scriptPubKey has the form [push-byte] [key-data] OP_CHECKSIG. Bitcoin's OP_CHECKSIG implementation validates the key before attempting signature verification:

• An uncompressed public key must be exactly 65 bytes with prefix 04.
• A compressed public key must be exactly 33 bytes with prefix 02 or 03.

Any other encoding causes OP_CHECKSIG to return false unconditionally, making the output permanently unspendable.

Relationship to Proposal 005

Proposal 005 (Off-Curve Public Key) already handles:

• 65-byte push (0x41) with 04 prefix + off-curve point
• 33-byte push (0x21) with 02/03 prefix + off-curve x-coordinate

This proposal covers the remaining gaps:

• 33-byte push with prefix ≠ 02/03 (e.g. 05, 00): rejected before any curve check
• Any push length other than 33 or 65: wrong-size key, rejected immediately

Script Format

[push-byte] [key-bytes] ac

• push-byte is a direct push opcode: 1–0x4b (1–75 bytes of key data)
• ac = OP_CHECKSIG
• Total hex length = (push-byte + 2) × 2 characters

Detection

// Proposal 010 - P2PK with invalid key encoding
if (hex.endsWith('ac')) {
    const pb = parseInt(hex.slice(0, 2), 16);
    if (pb >= 1 && pb <= 0x4b && hex.length === (pb + 2) * 2) {
        if (pb !== 33 && pb !== 65) {
            return true; // wrong-length key — can never be valid
        } else if (pb === 33 &&
                   hex.slice(2, 4) !== '02' && hex.slice(2, 4) !== '03') {
            return true; // 33-byte with invalid prefix
        }
        // pb === 65 with invalid prefix: already caught by Proposal 005
    }
}

Scope Notes

• Only direct push opcodes (≤ 0x4b) are checked. An OP_PUSHDATA variant would produce a non-standard script that is also unspendable for other reasons.
• The 65-byte / invalid-prefix case (41 + non-04 prefix) is already caught by Proposal 005 (isOnSecp256k1Curve returns false for unrecognised prefixes), so this proposal does not double-count it.
• This detector runs after Proposals 007–009 inside the outer else block.