PCI compliance - use of ANSI X9.17 for export keys

Question

we have a concern about a key export. We completed the migration to Key Block LMK in our environment (with HSM Thales 10K). Now, we have to exchange keys with third-parties that still use Keys in variant form. We generated the keys and formed the ZMK using the Key Block standard. To resolve the issue we have to export keys using "X" Key Scheme.

About compliance, we are still exporting keys in the correct key block format? or using the "X" we are converting them for the export?

It may seem like an excess of attention, but the fact that exports with scheme X must be authorized has raised safety concerns. it would be interesting to have a technical clarification on the use of scheme X

Answer 1

I'm not an expert with the Thales 10K, but the impression I have from what I have built with it is that anything that involves reducing the security of a key requires explicit enablement. Since converting from a key block to a TDES key reduces its security, you need to explicitly flip the switch to allow it.

Specifically, exporting a key as X means that you're removing the key restrictions in the key block header, even if you don't get access to the key itself. For example, if you export a key that has mode of use E (encrypt only) and then re-import it with a usage of D or N, you're now able to use it to decrypt things that aren't supposed to be decryptable. This principle is also why there's no way to go directly from a keyblock LMK to a variant LMK (which is explicitly not supported, with no config to enable it).

If the third party has no way to support key blocks, then you're just going to have to decide between not working with them, re-loading that key under a variant LMK in the first place, or enabling the export option and setting up appropriate controls/procedures around it.