A developer team requests integration of their legacy application with Vault to encrypt and decrypt...

Comprehensive and Detailed In-Depth Explanation:

The Vault Agent with Auto-Auth is ideal for legacy apps unable to modify for authentication. The Vault documentation states:

"Legacy applications often suffer from the ability to integrate with modern platforms such as Vault. To assist with this, you can use the Vault Agent to authenticate and manage a Vault token automatically. The token is written to a sink (local file) that the application can pick up and use. The Vault Agent Auto Auth feature will manage the lifecycle of the token to ensure there is always a valid token that the application can use."

—Vault Agent Auto Auth

D: Correct. The Agent handles tokens for Transit encryption:

"Running the Vault Agent on the application server(s) and utilizing the Auto Auth feature is the best way to integrate Vault with the legacy application."

—Vault Agent Auto Auth

A: Transit doesn’t send data directly.

B: Requires app modification, not feasible.

C: Kubernetes auth requires app changes and Kubernetes context.