Secure proxy

If one (A) connects through a secure proxy (B), beit webbased or not, and you connect through that proxy to another secure website (C), is it possible for B to listen to and decipher the networktraffic?

If you're doing it right man in the middle proxy B can't decrypt the traffic.

it doesn't sound like you have enough knowledge to do it right and that all your data will be raeped by MITM attacks.

don't forget that proxies are normally about anonymity and encryption is normally about privacy.  Those are not the same, and it's sometimes tricky to combine the two.