I am looking for a free open source solution that is able to capture and decrypt SSH traffic given the fact that I can provide the server's RSA private key (something like SSH CryptoAuditor).
Please advise.
mitmssh is the only tool I'm aware of capable of MITM'ing SSHv2 sessions. If you're looking for after-the-fact capture decode, that's not possible, as SSH universally uses ephemeral key exchange mechanisms.