Fix IT2Me protocol error connecting to old hosts.

remoting/protocol/negotiating_authenticator_base.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef REMOTING_PROTOCOL_NEGOTIATING_AUTHENTICATOR_BASE_H_
 #define REMOTING_PROTOCOL_NEGOTIATING_AUTHENTICATOR_BASE_H_
 
 #include <memory>
 #include <string>
 #include <vector>
@@ skipping 49 matching lines @@
 //  * The host never sends its own supported methods back to the client, so once
 //      the host picks a method from the client's list, it's final.
 //  * Any change in this class must maintain compatibility between any version
 //      mix of webapp, client plugin and host, for both Me2Me and IT2Me.
 class NegotiatingAuthenticatorBase : public Authenticator {
  public:
   // Method represents an authentication algorithm.
   enum class Method {
     INVALID,
 
+    // SPAKE2 with P224 using access code in plain-text. Used for It2Me.
+    // TODO(sergeyu): Remove and use SHARED_SECRET_SPAKE2_CURVE25519.

[kelvinp, 2016/05/03 00:22:36]

Maybe update the comment saying that this cannot be removed until all hosts or
maybe 95% of our hosts reached M51.

[Jamie, 2016/05/03 00:33:27]

Done.

+    SHARED_SECRET_PLAIN_SPAKE2_P224,
+
     // SPAKE2 PIN or access code hashed with host_id using HMAC-SHA256.
     SHARED_SECRET_SPAKE2_P224,
     SHARED_SECRET_SPAKE2_CURVE25519,
 
     // SPAKE2 using shared pairing secret. Falls back to PIN-based
     // authentication when pairing fails.
     PAIRED_SPAKE2_P224,
     PAIRED_SPAKE2_CURVE25519,
 
     // Authentication using third-party authentication server.
@@ skipping 54 matching lines @@
   RejectionReason rejection_reason_ = INVALID_CREDENTIALS;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(NegotiatingAuthenticatorBase);
 };
 
 }  // namespace protocol
 }  // namespace remoting
 
 #endif  // REMOTING_PROTOCOL_NEGOTIATING_AUTHENTICATOR_BASE_H_