Which is the correct simplified TLS handshake sequence needed to authenticate clients using a mutual TLS exchange?

In a simplified mutual TLS (mTLS) handshake, the client initiates a connection. The server responds with its own certificate and a request for the client's certificate. The client, after verifying the server's certificate, sends its own certificate and a "Certificate Verify" message to prove possession of the private key. Once both parties have validated each other's certificates and established a shared secret key, the secure connection is established, and the server considers the client authenticated based on its trusted certificate.

A. This sequence is incorrect because the server sends its certificate before it can validate the client's certificate, not after.

B. This sequence is incorrect. The server presents its certificate and requests the client's before the client sends its own certificate.

C. This sequence is incorrect. The client does not send its certificate in the initial request; it waits for the server's "Certificate Request" message.

1. Dierks, T., & Rescorla, E. (2008). The Transport Layer Security (TLS) Protocol Version 1.2. RFC 5246. Internet Engineering Task Force. Section 7.3, "Handshake Protocol Overview". This RFC details the full handshake, where the Server sends its Certificate and a CertificateRequest message before the Client sends its Certificate.

2. MIT OpenCourseWare. (2014). 6.857 Computer and Network Security, Fall 2014. Lecture 10: "Transport Layer Security (TLS)". The lecture notes describe the handshake where the server sends its certificate first, followed by the client sending its certificate in response to a server request for mutual authentication.