Is the information present in client certificate used while computing the pre-master secret of the session in TLS? In other words,for a given session will the pre-master secret be different depending on whether or not client certificate is used in the session?

1 Answer 1


The short answer is no. The way the premaster secret is computed depends on the ciphersuite. With the deprecated RSA-based ciphersuites with no forward secrecy (no key exchange involved), the premaster secret is chosen by the client and is supposed to be random. For ciphersuites based on a Diffie-Hellman key exchange, the premaster secret is the result of the DH key exchange and is thus normally effectively randomly co-generated by the client and the server. The only case where the client certificate is used to set the premaster secret is if the client certificate contains a (non-ephemeral) DH key, and that feature is very rarely used.

Where the client certificate matters (in addition to whatever validation the server performs) is that the Finished message contains data to verify the whole key exchange, including the ClientCertificate message.

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