curtain is a helper mixin class and accompanying decorator for people who
would like to implement HTTP Digest authentication (RFC 2617) in their tornado
applications (http://tornadoweb.org). 

It is distributed under the same license as tornado itself (namely, the 
Apache License, Version 2.0 http://www.apache.org/licenses/LICENSE-2.0.html)

To use these tools, you can add the DigestAuthHandler mixin class to your
app's handler's inheritance tree, like this: 

from curtain import digest
class MainHandler(digest.DigestAuthMixin, tornado.web.RequestHandler):

From there you *can* call methods from within the DigestAuthMixin class
directly, but you more than likely want to treat the digest auth
implementation as a black box. In that case, the only thing you need is: 

1. A string representing the "realm" 
2. A callback function used to grab user credentials from whatever credential
store you happen to use (LDAP, SQL, NIS, etc)

You'll pass both of these to the digest_auth decorator, and decorate whatever
methods you want protected. Here's a simplistic, contrived example: 

class MainHandler(digest.DigestAuthMixin, tornado.web.RequestHandler):
    def getcreds(uname):
        creds = {'auth_username': 'jonesy', 'auth_password': 'foobar'}
	if uname == creds['auth_username']:
	    return creds

    @digest.digest_auth('Authusers', getcreds)
    def get(self):
	self.write("Hello, World!")

In the above, my realm is "Authusers" and the function that returns the
credentials is called 'getcreds'. The DigestAuthMixin class uses this function
to grab a password for the authenticating user, which is used to determine if
the hashed auth "response" from the user is valid. 


TODO: 
* cleanup
* Implement proper errors and lose the 'print' statements. 
* Return 'stale=True' if nonce is stale, but hashes to a valid response.
* Testing for MD5-Sess and qop=auth-int
* Work through the rest of the RFC to catch other edge cases.