Encrypt the data in the Cloud Storage bucket by using Customer Managed Encryption Keys. Configure an IAM deny policy for unauthorized groups.

Generate a key in your on-premises environment to encrypt the data before you upload the data to the Cloud Storage bucket. Upload the key to the Cloud Key Management Service (KMS). Activate Key Access Justifications (KAJ) and have the external key system reject unauthorized accesses.

Encrypt the data in the Cloud Storage bucket by using Customer Managed Encryption Keys backed by a Cloud Hardware Security Module (HSM). Enable data access logs.

Generate a key in your on-premises environment and store it in a Hardware Security Module (HSM) that is managed on-premises. Use this key as an external key in the Cloud Key Management Service (KMS). Activate Key Access Justifications (KAJ) and set the external key system to reject unauthorized accesses.