To implement double encryption for data at rest in Azure Synapse Analytics, the solution requires two key components:

B: An RSA key - This is essential because RSA keys (specifically RSA 2048 or 3072 byte-sized keys) are used for customer-managed keys (CMK) in Azure Synapse Analytics. When you enable double encryption, the first layer is platform-managed encryption, and the second layer uses customer-managed RSA keys to encrypt the data encryption keys. This provides the additional encryption layer required for double encryption.

E: An Azure key vault that has purge protection enabled - This is critical because Azure Key Vault serves as the secure storage location for the RSA keys. Purge protection ensures that deleted keys cannot be permanently removed during the retention period, providing additional security and compliance. The key vault must be properly configured to store and manage the encryption keys used for the second layer of encryption.

Why other options are not suitable:

• A: An X.509 certificate - While X.509 certificates are used for authentication and securing communications, they are not the primary mechanism for implementing double encryption of data at rest in Azure Synapse Analytics.
• C: An Azure virtual network with NSG - This provides network security and access control but does not directly contribute to data encryption at rest.
• D: An Azure Policy initiative - Azure Policy helps enforce organizational standards and compliance but does not directly implement the encryption mechanisms required for double encryption.

The combination of an RSA key stored in a properly configured Azure Key Vault with purge protection enables the customer-managed key encryption layer that works alongside Azure's platform-managed encryption to achieve true double encryption.