MD5 PasswordPacket

Where can I read up on the encoding of the String in the PasswordPacket 
required when I get an AuthenticationMD5Password message from the backend?

I've looked at some code that implements the encoding, but reading the 
specification (the intent) would be most useful.

One question in particular springs to mind: can usernames and passwords 
be made up of multi-byte characters, and how does this affect the MD5 
encoding?

Thanks.

P.S. Many thanks for the help with my character string encoding 
questions.  I have that all working nicely, now. :-)