Reading and manipulating binary data

[Dave]

Hi,

I am really confused as to how to use binary data with python.  I am
currently developing a lightweight RADIUS server, which gets UDP datagrams
from a client formatted like so:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Request Authenticator                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

I can read() the data easily enough from the socket.  However, when I have
read it, it is very difficult to manipulate.  I can't convert it to an
integer or a string which is driving me crazy...  I have seen a lot of posts
recommending struct for dealing with binary data, but I can't get it to like
mine.  I get all sorts of errors, like lengths don't match format etc.

Also part of the authentication system involves MD5 hashes.  For this I need
to be able to take a string, find its MD5 digest, and then xor it with
another string.  But since the MD5 digest is binary, and I don't know how to
change it, I can't xor it...  Please, I am sure I am missing something
really obvious but I am stumped as to what it is.

The basic RADIUS rfc is 2865.  Please say if I have seriously misinterpreted
it (as you can probably tell, this is my first time trying something more
than "Hello World" with sockets).

Thanks in advance

Dave