How can the CISO be assured that AWS KMS and Amazon S3 are addressing the concerns?

Explanations:

There is indeed no API operation that retrieves an S3 object in its encrypted form. Instead, when a user requests an object, it is decrypted in transit before being delivered, ensuring that the object is never exposed in its encrypted state outside of AWS. This helps mitigate the risk of key exposure, as the encryption keys are managed and secured by AWS KMS.

The encryption of S3 objects is not performed within the secure boundary of the KMS service. Instead, S3 uses KMS to handle the management of the encryption keys, but the actual encryption and decryption of objects happen in the S3 service itself. Therefore, this statement does not accurately address the CISO’s concerns.

Amazon S3 uses AWS KMS to generate a unique data key for each individual object. This practice minimizes the blast radius in case a key is compromised because it isolates the encrypted data associated with that specific key, thus reducing the potential impact of a single key compromise on the overall data security.

Using a single master key to encrypt all data increases the risk associated with key compromise, as it creates a single point of failure. While it may provide a centralized approach for auditing and validation, the blast radius would be extensive if the master key were compromised, which heightens the CISO’s concerns.

The KMS encryption envelope does not digitally sign the master key during encryption to prevent cryptographic wear-out. Instead, KMS employs a layered encryption model where a unique data key is used for data encryption, and the master key itself is used to encrypt this data key. The concept of wear-out pertains more to key rotation and management rather than a signing process.