# Clause 8 Asymmetric encryption systems, and the only mutual exchange

This family builds authentication on an asymmetric encryption system rather than on a bare one way function. It is the only family in the part with a mutual exchange, so it is the one to read when both parties have to be assured of each other rather than one of the other.

## What it asks

Record whether each deployment is unilateral or mutual, and show that the mutual exchange really gives both parties assurance rather than running two unilateral exchanges back to back. Keep the encryption system parameters with the mechanism record, because the two are chosen together.

## What the revision does

The fourth edition keeps the family and will restate it. The revised text is not public.

## Who it binds

The architect who chose the mechanism and the operator of the mutual service.

## Evidence an auditor samples

- The deployment record, naming unilateral or mutual (architecture)
- The exchange diagram with both assurance points marked (design note)
- The encryption system parameters and their review (cryptography standard)

## The test you can run

In a mutual deployment, ask what the first party knows at the end that it did not know at the start. If the answer is nothing, the exchange is not mutual.

## Common failures

- Two unilateral exchanges are presented as mutual authentication.
- The encryption system is chosen by the vendor and never recorded.

## How to close it

Draw the assurance, not the messages. A mutual exchange is defined by what each party knows at the end, and a diagram with two arrows can hide the absence of it.

## Source

ISO/IEC 9798-5:2009(E), Clause 8, 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.