[Emu] Commitment Message in draft-ietf-emu-eap-tls13

[Hannes Tschofenig <[e-mail]>]

Hi all,

This has probably been discussed extensively in the EMU group. I am sorry to bring it up again but I believe this is a bad design decision. I raised it in my short review just sent to the list but I believe it is worthwhile to point it out separately.

draft-ietf-emu-eap-tls13 introduces a new message to EAP-TLS, namely the Commitment Message. This requires extra code in an implementation because the normal behavior would be to run a TLS stack and then send encrypted data.
EAP-TLS does, however, not send application data*. This message changes this. Not only does it not send encrypted application data it requires an implementation to transmit a plaintext application data record after the application traffic secret has been created and before that application traffic secret is used to protect post handshake messages. This will make it difficult to re-use an off-the-shelf TLS 1.3 stack.


There is very little motivation about this message other than



"

   When an EAP server has sent its last handshake message (Finished or a

   Post-Handshake), it commits to not sending any more handshake

   messages by sending a Commitment Message.

"

I might miss something important here but why cannot the EAP-Success or EAP-Failure serve that purpose?

Here are two examples to explain what I mean:


  1.  Failed exchange



    EAP Peer                                              EAP Server



                                                         EAP-Request/

                                 <--------                  Identity

    EAP-Response/

    Identity (Privacy-Friendly)  -------->



                                                         EAP-Request/

                                                    EAP-Type=EAP-TLS

                                 <--------                (TLS Start)

    EAP-Response/

    EAP-Type=EAP-TLS

   (TLS ClientHello)             -------->

                                                         EAP-Request/

                                                    EAP-Type=EAP-TLS

                                                    (TLS ServerHello,

                                             TLS EncryptedExtensions,

                                              TLS CertificateRequest,

                                                     TLS Certificate,

                                               TLS CertificateVerify,

                                                        TLS Finished,

                                 <--------        Commitment Message)

    EAP-Response/

    EAP-Type=EAP-TLS

   (TLS Certificate,

    TLS CertificateVerify,

    TLS Finished)                -------->

                                                         EAP-Request/

                                                    EAP-Type=EAP-TLS

                                 <--------          (TLS Fatal Alert)

    EAP-Response/

    EAP-Type=EAP-TLS             -------->

                                 <--------               EAP-Failure



  1.  Successful Exchange with Post-Handshake NewSession Ticket


    EAP Peer                                              EAP Server



                                                         EAP-Request/

                                 <--------                  Identity

    EAP-Response/

    Identity (Privacy-Friendly)  -------->

                                                         EAP-Request/

                                                    EAP-Type=EAP-TLS

                                 <--------                (TLS Start)

    EAP-Response/

    EAP-Type=EAP-TLS

   (TLS ClientHello)             -------->

                                                         EAP-Request/

                                                    EAP-Type=EAP-TLS

                                                    (TLS ServerHello,

                                             TLS EncryptedExtensions,

                                              TLS CertificateRequest,

                                                     TLS Certificate,

                                               TLS CertificateVerify,

                                 <--------              TLS Finished)

    EAP-Response/

    EAP-Type=EAP-TLS

   (TLS Certificate,

    TLS CertificateVerify,

    TLS Finished)                -------->

                                                         EAP-Request/

                                                    EAP-Type=EAP-TLS

                                               (TLS NewSessionTicket,

                                 <--------        Commitment Message)

    EAP-Response/

    EAP-Type=EAP-TLS             -------->

                                 <--------               EAP-Success


Ciao
Hannes

(*): FWIW Post handshake messages are protected with the application traffic secrets.

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