Adding custom cypher

My goal is to add support of custom cypher so it could be used in TLS/SSL along with RSA and AES. As far as I understand NPAPI has gone and WebExtensions is the only way. I am new at WebExtension development and most of information I find is about changing interface etc. Could you give me a clue, some starting point?

There is no way to look at or even interfere with the TLS handshake.