Construction of Response-Authenticator

Hello, I use FreeRADIUS 2.1.8 on Ubuntu 10.4. For my course at university I have to write a program in which I need to re-calculate the response-authenticator (MD5(Code+ID+Length+Request-Auth.+Attributes+Secret)). Does anybody know how FreeRADIUS calculates it? Does it take the binary code of the message fields or an other encoding? When I try to re-calculate it, I never get the same Response-Authenticator as in the Access-Accept-Message. I'm programming with Python and using the libraries of scapy and hashlib. Perhaps anyone of you can help me. Thank you very much!