http://www-h.eng.cam.ac.uk/help/jpmg/ssh/ssh-detail.html
http://www.openssh.com/press.html
http://www.ssh.fi/tech/crypto/protocols.html#ssh2
http://www.oreilly.com/catalog/sshtdg/
http://www.oreilly.com/catalog/sshtdg/chapter/index.html
http://www.google.com/search?q=ssh+how+it+works&btnG=Google%B7j%B4M&hl=zh-TW&lr=