[Twisted-Python] Authentication with Perspective Broker and hashed password file

[Dave Cook]

I'd like to authenticate my PB client against a hashed password file,
but I'm getting an error when I use a hash:

Failure: twisted.cred.error.UnhandledCredentials: No checker for  
twisted.spread.interfaces.IJellyable,
twisted.cred.credentials.IUsernameHashedPassword,
twisted.cred.credentials.ICredentials,
twisted.spread.pb.IUsernameMD5Password

However, if I use plaintext passwords, it works fine.  

I'm working with the example in the book:

http://twistedmatrix.com/documents/current/howto/pb-cred#auto7

with the credential checker set to

import sha
myChecker = checkers.FilePasswordDB("my_hashed_passwd_file",
                                    hash =
                                    lambda u, p, h: sha.new(p).hexdigest())

I've tested this checker and hash, and they work fine for logging in
to my nevow site.

The example works, though if I use plaintext passwords and don't pass in a hash:

myPlaintextChecker = checkers.FilePasswordDB("my_plaintext_passwd_file")

Thanks for any hints,
Dave Cook