email: Added initial support for cancelling authentication

Should a client application fail to decode an authentication message
received from a server, or not support any of the parameters given by
the server in the message, then the authentication phrase should be
cancelled gracefully by the client rather than simply terminating the
connection.

The authentication phrase should be cancelled by simply sending a '*'
to the server, in response to erroneous data being received, as per
RFC-3501, RFC-4954 and RFC-5034.

This patch adds the necessary state machine constants and appropriate
response handlers in order to add this functionality for the CRAM-MD5,
DIGEST-MD5 and NTLM authentication mechanisms.

1 files changed, 22 insertions, 2 deletions

diff --git a/lib/imap.c b/lib/imap.c
index f856dcd3d..b17b7e533 100644
--- a/lib/imap.c
+++ b/lib/imap.c
@@ -426,6 +426,7 @@ static void state(struct connectdata *conn, imapstate newstate)
     "AUTHENTICATE_NTLM",
     "AUTHENTICATE_NTLM_TYPE2MSG",
     "AUTHENTICATE_XOAUTH2",
+    "AUTHENTICATE_CANCEL",
     "AUTHENTICATE_FINAL",
     "LOGIN",
     "LIST",
@@ -1287,7 +1288,7 @@ static CURLcode imap_state_auth_ntlm_type2msg_resp(struct connectdata *conn,
 }
 #endif
 
-/* For AUTH XOAUTH2 (without initial response) responses */
+/* For AUTHENTICATE XOAUTH2 (without initial response) responses */
 static CURLcode imap_state_auth_xoauth2_resp(struct connectdata *conn,
                                              int imapcode,
                                              imapstate instate)
@@ -1325,7 +1326,22 @@ static CURLcode imap_state_auth_xoauth2_resp(struct connectdata *conn,
   return result;
 }
 
-/* For final responses to the AUTHENTICATE sequence */
+/* For AUTHENTICATE cancellation responses */
+static CURLcode imap_state_auth_cancel_resp(struct connectdata *conn,
+                                            int imapcode,
+                                            imapstate instate)
+{
+  struct SessionHandle *data = conn->data;
+
+  (void)imapcode;
+  (void)instate; /* no use for this yet */
+
+  failf(data, "Authentication cancelled");
+
+  return CURLE_LOGIN_DENIED;
+}
+
+/* For final responses in the AUTHENTICATE sequence */
 static CURLcode imap_state_auth_final_resp(struct connectdata *conn,
                                            int imapcode,
                                            imapstate instate)
@@ -1678,6 +1694,10 @@ static CURLcode imap_statemach_act(struct connectdata *conn)
       result = imap_state_auth_xoauth2_resp(conn, imapcode, imapc->state);
       break;
 
+    case IMAP_AUTHENTICATE_CANCEL:
+      result = imap_state_auth_cancel_resp(conn, imapcode, imapc->state);
+      break;
+
     case IMAP_AUTHENTICATE_FINAL:
       result = imap_state_auth_final_resp(conn, imapcode, imapc->state);
       break;