3.4.5.1 Three-Leg DCE-Style Mutual Authentication

An application protocol using the Kerberos protocol must exchange application protocol messages with Kerberos signing or encryption applied in order to verify mutual authentication. DCE, in the authn_dce_secret authentication service (as specified in [C706]) mandated that mutual authentication be verified before any RPC messages were exchanged. To accommodate that requirement, the DCE Kerberos implementation issued an additional AP exchange reply message from the client to the server as part of the AP exchange subprotocol.

Kerberos V5 is not interoperable with the DCE authn_dce_secret security protocol. KILE MUST have compatible extensions for third-party extensions. KILE emulates this behavior as follows:

  • The AP-REQ message MUST NOT have GSS-API wrapping. It is sent as is without encapsulating it in a header ([RFC2743] section 3.1).

  • The signature message and the encryption message MUST NOT include the length of the application data; they are no longer RFC 1964–compliant [RFC1964].

  • The client MUST generate an additional AP exchange reply message exactly as the server would ([RFC4120] section 3.2.4) as the final message to send to the server. The client sets the GSS_C_DCE_STYLE flag ([RFC4757] section 7.1) to TRUE in the authenticator's checksum field ([RFC4121] section 4.1.1). In GSS terms, the client must return success and a message to the server. It is up to the application to deliver the message to the server.

  • The server MUST receive the additional AP exchange reply message and verify that the message is constructed correctly ([RFC4120] section 3.2.5).

    The GSS_Wrap() and GSS_WrapEx() methods are not supported with DCE Style authentication.

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