PoC: malicious KOS receiver recovers the shared delta - #1
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Demonstrates the attack from review of tlsnotary/tlsn#1173: a malicious KOS receiver runs two extensions under the same global `delta` and reuses the same base OT. With no per-instance domain separation the two runs derive identical extension columns, so at any column where the receiver's internal choice bits differ, XOR-ing the sender's (raw, correlated) keys yields `delta` exactly. Test only; no protocol change. Runs against stock KOS.
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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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Reproduces the attack @sinui0 described in review of tlsnotary/tlsn#1173, as a single test against stock KOS (no protocol changes).
A malicious receiver runs two KOS extensions under the same global
deltaand reuses the same base OT. KOS has no per-instance domain separation, so both runs derive identical extension columns; only the receiver's internal choice bits differ. The sender's correlated keys are raw (key_j = t_j ^ choice_j·delta), so at any column where the two runs chose differently,key_a ^ key_b == delta.The test asserts the recovered value equals
delta— the leak is real.Fix (per-instance salt / domain separation): #2.