# Annex C Parameter guidance, and how to use a comparison

Annex C is the informative guidance on parameter choice and on comparing the mechanisms. It is where a design team looks for a modulus size, a challenge length or a reason to prefer one family, and it is the annex most often quoted without its informative status.

## What it asks

Record the guidance you relied on, the parameter you chose and the reason, and mark the citation as informative. Where the guidance gives a range, record the point in the range and the constraint that fixed it, rather than quoting the range as if it were a requirement.

## What the revision does

The fourth edition will update the guidance for the revised mechanism set. The public record stops before the new text.

## Who it binds

The architect who chose the parameters and the reviewer who accepted them.

## Evidence an auditor samples

- The parameter choice record, with the guidance citation and its status (design note)
- The constraint that fixed the point in the range (capacity plan)
- The review that accepted the choice (risk register)

## The test you can run

Take a parameter and ask which constraint fixed it. If the answer is a range with no point, the choice has not been made.

## Common failures

- A range from the guidance is quoted as the approved value.
- The guidance is cited without saying it is informative.

## How to close it

Write the constraint before the number. A parameter chosen against a named constraint survives a review; a parameter copied from a table does not.

## Source

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