feat(kos): domain-separate KOS instances by instance_id - #445
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Add an `instance_id` salt to `kos::Sender`/`Receiver`, mixed into the base-OT-derived setup PRG seeds via a tweakable correlation-robust hash (`FIXED_KEY_AES.tccr(instance_id, seed)`). Two KOS instances that share one global `delta` and the same base OT then produce independent extension transcripts, so per-instance consistency-check leakage can no longer be composed to sample or recover `delta` (demonstrated in #1). The mix is non-invertible in `seed` on purpose: a linear XOR mix (`seed ^ instance_id`) would let a malicious receiver, who controls the base-OT seeds, pre-compensate them to cancel the salt and collapse two instances onto one PRG stream. API-breaking: `Sender::new`/`Receiver::new` gain an `instance_id: Block`. Paired sender and receiver must use the same id; distinct instances reusing one `delta` must use distinct ids.
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Add an
instance_iddomain separator tokos::Sender/Receiverand mix it into the base-OT-derived PRG seeds (Prg::from_seed(seed ^ instance_id)).This makes two KOS instances that share the same global
deltaand reuse the same base OT produce independent extension transcripts, preventing a malicious receiver from composing per-instance consistency-check leakages to sample or recoverdelta.API-breaking:
Sender::new/Receiver::newgain aninstance_id: Block.