# 8.2 The unilateral exchange built on encryption

The unilateral exchange uses the encryption system to carry or protect the values the verifier checks. The claimant proves knowledge of the private operation, and the verifier must be sure that what it decrypted or checked came from this exchange and not from an earlier one.

## What it asks

Record the exchange and the binding between the encrypted material and the exchange context, and test a replayed message and a message from another session. Show that the verifier checks the context rather than only the plaintext.

## What the revision does

The fourth edition will restate the exchange. The public record stops before the revised sequence.

## Who it binds

The implementer and the tester.

## Evidence an auditor samples

- The exchange sequence and the context binding (interface specification)
- The replay test and its output (test report)
- The cross session test and its output (test report)

## The test you can run

Replay a captured message in a new session. If it is accepted, the context is not bound.

## Common failures

- The encrypted payload carries no session identifier.
- The verifier checks the plaintext but not the context.

## How to close it

Bind the exchange context into the protected material and test the binding by replay. The test is short, and it is the one that distinguishes a mechanism from a message format.

## Source

ISO/IEC 9798-5:2009(E), 8.2, 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.