PLAINAuthenticator in twisted.mail.imap4 does not follow RFCs

New submission from khorn <None>:

PLAINAuthenticator.challengeResponse() uses the following statement to send auth credentials to the server

        return '%s\0%s\0' % (self.user, secret)

which would give auth credentials of the form:

        authid<NUL>password<NUL>

        (where <NUL> is the NUL character)

However, both RFC2595 and RFC4616 (both define the PLAIN SASL mechanism), say that credentials should be passed this way:

        [authzid]<NUL>authnid<NUL>password

        (where <NUL> is the NUL character and [authzid] is optional)

Now even if one was to leave the authzid out of the equation, you would end up with something like this:

        <NUL>authnid<NUL>password

and the version Twisted's IMAP code uses appears to be invalid.

NOTE: PLAINCredentials may also need to be modified to follow RFCs

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Type     : defect
Component: mail
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/3939