Question on DH key exchange

Question

The server is configured to support the cipher suite TLS_DHE_RSA_WITH_AES_128_CBC_SHA and validate the client's certificate.

My understanding is that the server uses DH for key exchange which does not require encryption or decryption but a math formula (g MOD p) to derive a secret key for both the server and client.

So in this case, if the client initiates the handshake (client hello), the TLS communication can be established just with public certificates of server and client. Is the client's private key is not required? If so, is this not an insecure configuration?

The concern is, if client's private key is not being used, any one who has client's public key should be able to connect to server over tls..? this should not be the case in IOT which uses PKI for device authentication.

Answer 1

Is the client's private key is not required?

The TLS key exchange has nothing to do with authentication using client certificates, i.e. it is fully independent from it. This means both RSA key exchange and DH key exchange can be used with or without authentication by client certificates.

Client certificates are requested by the server but only if the server is configured accordingly (usually not the case). Only then the server will issue a CertificateRequest and the client will provide its own certificate and only in this case it needs the matching private key.

In more detail: the certificate is send in clear inside the ClientCertificate message. Additionally the client uses the private key of its certificate to sign the previously exchanged messages and sends this signature inside the CertificateVerify message. This way the client proves that it owns the private key matching the certificate. See TLS 1.2 section 7.4.8 for details.