Add SASL PLAIN authentication method

- this method isn't as deprecated as IMAP LOGIN;

 - it allows to keep hashed passwords on the server side;

 - it has the ability to specify that the remote identity
   is different from authenticating username, so it even
   can be useful in some cases (e.g., migrated mailboxes);
   configuration variable "remote_identity" was introduced
   to leverage this functionality.

From: Andreas Mack <andreas.mack@konsec.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

Changelog.rst

@@ -16,6 +16,7 @@ WIP (add new stuff for the next release)
 * Honor the timezone of emails (Tobias Thierer)
 * Allow mbnames output to be sorted by a custom sort key by specifying
   a 'sort_keyfunc' function in the [mbnames] section of the config.
+* Support SASL PLAIN authentication method.  (Andreas Mack)
 
 OfflineIMAP v6.5.5-rc1 (2012-09-05)
 ===================================

offlineimap.conf

@@ -353,6 +353,18 @@ ssl = yes
 # Specify the remote user name.
 remoteuser = username
 
+# Specify the user to be authorized as.  Sometimes we want to
+# authenticate with our login/password, but tell the server that we
+# really want to be treated as some other user; perhaps server will
+# allow us to do that (or, may be, not).  Some IMAP servers migrate
+# account names using this functionality: your credentials remain
+# intact, but remote identity changes.
+#
+# Currently this variable is used only for SASL PLAIN authentication
+# mechanism.
+#
+# remote_identity = authzuser
+
 # There are six ways to specify the password for the IMAP server:
 #
 # 1. No password at all specified in the config file.

offlineimap/imapserver.py

@@ -55,6 +55,7 @@ class IMAPServer:
         self.tunnel = repos.getpreauthtunnel()
         self.usessl = repos.getssl()
         self.username = None if self.tunnel else repos.getuser()
+        self.user_identity = repos.get_remote_identity()
         self.password = None
         self.passworderror = None
         self.goodpassword = None
@@ -133,6 +134,24 @@ class IMAPServer:
         self.ui.debug('imap', 'Attempting IMAP LOGIN authentication')
         imapobj.login(self.username, self.getpassword())
 
+
+    def plainhandler(self, response):
+        """
+        Implements SASL PLAIN authentication, RFC 4616,
+          http://tools.ietf.org/html/rfc4616
+
+        """
+        authc = self.username
+        passwd = self.getpassword()
+        authz = ''
+        if self.user_identity != None:
+            authz = self.user_identity
+        NULL = u'\x00'
+        retval = NULL.join((authz, authc, passwd)).encode('utf-8')
+        self.ui.debug('imap', 'plainhandler: returning %s' % retval)
+        return retval
+
+
     def gssauth(self, response):
         data = base64.b64encode(response)
         try:
@@ -213,6 +232,8 @@ class IMAPServer:
                   "TLS connection: %s" % str(e),
                   OfflineImapError.ERROR.REPO)
 
+	# Hashed authenticators come first: they don't reveal
+	# passwords.
         if 'AUTH=CRAM-MD5' in imapobj.capabilities:
             tried_to_authn = True
             self.ui.debug('imap', 'Attempting '
@@ -224,6 +245,18 @@ class IMAPServer:
                 self.ui.warn('CRAM-MD5 authentication failed: %s' % e)
                 exc_stack.append(('CRAM-MD5', e))
 
+        # Try plaintext authenticators.
+        if 'AUTH=PLAIN' in imapobj.capabilities:
+            tried_to_authn = True
+            self.ui.debug('imap', 'Attempting '
+              'PLAIN authentication')
+            try:
+                imapobj.authenticate('PLAIN', self.plainhandler)
+                return
+            except imapobj.error as e:
+                self.ui.warn('PLAIN authentication failed: %s' % e)
+                exc_stack.append(('PLAIN', e))
+
         # Last resort: use LOGIN command,
         # unless LOGINDISABLED is advertized (RFC 2595)
         if 'LOGINDISABLED' in imapobj.capabilities:

offlineimap/repository/IMAP.py

@@ -115,6 +115,18 @@ class IMAPRepository(BaseRepository):
                                    "'%s' specified." % self,
                                OfflineImapError.ERROR.REPO)
 
+
+    def get_remote_identity(self):
+        """
+        Remote identity is used for certain SASL mechanisms
+        (currently -- PLAIN) to inform server about the ID
+        we want to authorize as instead of our login name.
+
+        """
+
+        return self.getconf('remote_identity', default=None)
+
+
     def getuser(self):
         user = None
         localeval = self.localeval