Hi @Guy_Harrison,
At the moment, MongoDB drivers support these KMS providers:
- Amazon Web Services KMS
- Azure Key Vault
- Google Cloud Platform Key Management
- Local KMS provider
So I guess it’s all about which cloud provider your client is using.
Doc: https://docs.mongodb.com/drivers/security/client-side-field-level-encryption-local-key-to-kms/
Cheers,
Maxime.