# 4.3 The unilateral exchange and its three moves

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.

## 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.

## Source

ISO/IEC 9798-5:2009(E), 4.3, publisher preview; replaced by ISO/IEC DIS 9798-5, stage 40.20. Read on 21 October 2026. Our wording, not the instrument's.
https://www.iso.org/standard/89516.html

Not legal advice; does not certify.