You are migrating an application into the cloud. The application will need to read data from a Cloud Storage bucket. Due to local regulatory requirements, you need to hold the key material used for…

Customer Managed Encryption Keys (CMEK) are managed by Cloud KMS, where Google has control over the key lifecycle, which does not meet the requirement for the customer to have *full* control over the key material and a rationale for accessing it outside of KMS logs.

Generating a key on-premises and encrypting data before upload using Customer-Supplied Encryption Keys (CSEK) provides customer control over the key, but the question's emphasis on a 'valid rationale for accessing the key material' suggests a more integrated, auditable solution like EKM rather than just supplying a key for encryption at rest.

This option is identical to A and is incorrect for the same reason: CMEK through Cloud KMS does not grant the customer 'full control' over the key material in the sense of managing its physical storage and having a specific rationale for access to the material itself.

Generating an encryption key in your on-premises environment and storing it in a Hardware Security Module (HSM) ensures full customer control over the key material. Integrating this with Google Cloud External Key Manager (Cloud EKM) allows the key to be used for Cloud Storage encryption while ensuring that all access requests to the key material generate an auditable rationale and are subject to your on-premises security policies, meeting the specific regulatory requirements.