# 7.2 Key production and the two modes

Key production here depends on the mode. One mode fixes a private key size and derives the public value from the base, and the other uses a different construction. Mixing the two produces keys that look valid and fail only at verification, which is why the record has to name the mode.

## What it asks

Record the mode, the private key size and the derivation for each key, and show that the key was generated under the same mode the verifier expects. Keep a negative test for a key from the other mode, and review the key size against the guidance rather than the library default.

## What the revision does

The fourth edition will restate key production. The revised rules are not public.

## Who it binds

The key management owner and the verifier operator.

## Evidence an auditor samples

- The key record with mode, size and derivation (key management)
- The key size review against the guidance (design note)
- The negative test with a key from the other mode (test suite)

## The test you can run

Generate a key in the other mode and present it. If the verifier accepts it, the mode check is absent.

## Common failures

- The key size is inherited from a library default with no review.
- A key from the other mode is accepted because the verifier does not check the mode.

## How to close it

Put the mode in the key identifier and check it on every verification. The identifier is the cheapest place to stop a cross-mode key, and it makes the record self describing.

## Source

ISO/IEC 9798-5:2009(E), 7.2, 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.