md5 pack

import md5

def md5_pack(md5_obj, string):
    if len(string) > 0xff:
        return None
    return md5_obj.digest()+chr(len(string))+string

m = md5.new("dakman")
print md5_pack(m, "dakman"),
m = md5.new("jp")
print md5_pack(m, "jp")