# 9.3 The exchange with no coupon strategy

This subclause is the fourth group in the introduction, the one with no coupon strategy. Both parties do the work each time, which makes it simpler to operate and more sensitive to the cost of the arithmetic on a constrained device.

## What it asks

Record the exchange and the device cost, and show that the verifier checks point membership and the response relation on every run. Test a point outside the subgroup and a replayed challenge, and record the measured device cost rather than an estimate.

## What the revision does

The fourth edition will restate the exchange. No public copy of the new sequence exists yet.

## Who it binds

The implementer and the capacity owner for the device estate.

## Evidence an auditor samples

- The exchange sequence and the device cost measurement (test report)
- The point validation and response checks, with code pointers (source repository)
- The negative tests and their outputs (test report)

## The test you can run

Measure the exchange on the slowest supported device. If the measurement is an estimate, the capacity case is unproven.

## Common failures

- The device cost is estimated from a desktop benchmark.
- Subgroup validation is skipped to save time on the constrained device.

## How to close it

Measure on the device you support, not the one on your desk. The absence of a coupon is a design choice, and the cost that comes with it has to be evidenced rather than assumed.

## Source

ISO/IEC 9798-5:2009(E), 9.3, 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.