What does a key check value tell you?

A key check value, or KCV, is a short value calculated from a cryptographic key. It helps people compare keys without displaying the complete key in every step of a test workflow.

Agree on the method first

The KCV Calculator supports several methods. A common TDES method encrypts eight zero bytes and retains the first three output bytes. Other methods use a different block, algorithm or truncation rule, so two outputs are only comparable when their methods agree.

Try this published test key with the DES/TDES zero-block method:

0123456789ABCDEFFEDCBA9876543210

The first three ciphertext bytes are 08D7B4. Preserve leading zeroes when recording that result.

A match has limits

A three-byte KCV has far fewer possible values than the key itself. Different keys can share the same KCV. A match is a useful consistency check, not proof of ownership, authorization or a key’s security.

DES parity bits also need separate treatment: changing only those bits does not change the effective DES key. A matching DES KCV therefore does not establish that the supplied bytes have your required parity.

Follow the workflow

Use Key Validation to inspect the key’s properties and Parity Enforcement when the receiving system requires odd or even parity. If you then protect the key under a KEK, check the agreed transport format with the receiving party. A KCV is not an authenticated key block.