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The unilateral exchange is the sequence a claimant and a verifier actually run: a commitment, a challenge and a response, with the verifier accepting only if the response satisfies the verification relation. The clause fixes the order and the checks, and it is where a replay or a skipped check shows up.

What it asks

Record the exchange as implemented, including the number of passes, the source of the challenge and the exact condition the verifier tests. Show a negative test for a replayed response, a wrong challenge and a wrong key, because a positive test alone does not demonstrate the check.

What the revision does

The fourth edition will restate the exchange. The revised sequence is not public, so the three-move description here is the 2009 baseline. The 2009 wording is therefore superseded at publication.

Who it binds

The engineer who implements the verifier and the tester who signs it off.

Evidence an auditor samples

• The exchange sequence as implemented, with the pass count (interface specification)
• The source of the challenge and its freshness rule (design note)
• The negative tests for replay, wrong challenge and wrong key (test suite)

The test you can run

Replay a captured response. If the verifier accepts it, the freshness check is missing whatever the positive tests show.

Common failures

• The challenge is a counter with no freshness window, so a replay succeeds later.
• The verifier checks the response format but not the verification relation.

How to close it

Write the negative tests first. A verifier is defined by what it rejects, and the three tests named here are the minimum an assessor will ask to see. The artefact that opens the work is the exchange sequence as implemented, with the pass count; the row is only closed when that artefact is dated and has a person against it.

Where it sits against the 2009 edition

4.3 is a 2009 clause that the revision replaces. For 4.3, the 2009 obligation is the one to evidence until publication; after it, this row is rebuilt against the published text.

For the file, this row closes when the exchange sequence as implemented, with the pass count exists with a date, a named holder and a pointer, and when the test above has been run and recorded. The engineer who implements the verifier and the tester who signs it off. owns that answer for 4.3.