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The mutual exchange is the clause that makes this family distinct. It gives both entities assurance of the other identity, and it does so with a sequence that has to be read as a whole. Splitting it into two unilateral runs is the classic implementation error.

What it asks

Record the mutual sequence and mark the point at which each party is assured, and test that a party which stops early does not leave the other assured. Keep the evidence for both directions, because a mutual claim with one direction tested is a unilateral claim.

What the revision does

The fourth edition will restate the mutual exchange. The revised sequence is not in the public record. The 2009 wording is therefore superseded at publication.

Who it binds

The operator of the mutual service and the tester who signs it off.

Evidence an auditor samples

• The mutual sequence with both assurance points marked (design note)
• The early stop test and its output (test report)
• The evidence for each direction, kept separately (test report)

The test you can run

Stop the exchange after the first direction. If the second party still reports success, the mutual claim is false.

Common failures

• The mutual exchange is implemented as two independent unilateral exchanges.
• Only one direction is tested and the other is assumed symmetric.

How to close it

Test each direction separately and test the early stop. Mutual authentication is an ordering property, and an ordering property is only shown by breaking the order. Put the mutual sequence with both assurance points marked in the register first, then work down the evidence list until every line has a date.

Where it sits against the 2009 edition

The clause exists today as 8.3 in the 2009 edition. The fourth edition replaces it, and the public record stops before the new wording, so the obligation below is the one in force.

For the file, this row closes when the mutual sequence with both assurance points marked exists with a date, a named holder and a pointer, and when the test above has been run and recorded. The operator of the mutual service and the tester who signs it off. owns that answer for 8.3.