[TLS] TLS 1.3 Problem?

Michael D'Errico <mike-list@pobox.com> Sun, 27 September 2020 19:52 UTC

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From: Michael D'Errico <mike-list@pobox.com>
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Date: Sun, 27 Sep 2020 15:51:54 -0400
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Subject: [TLS] TLS 1.3 Problem?
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Hi,

Took a quick look at RFC 8446 and noticed that there is no
definition of ServerKeyExchange or ServerHelloDone which
are part of TLS 1.2 and prior.  A 1.3 client talking to a 1.2
or earlier server is likely going to receive both of these
messages:

RFC 5246                          TLS                        August 2008

       Client                                               Server

       ClientHello                  -------->
                                                       ServerHello
                                                      Certificate*
                                                ServerKeyExchange*
                                               CertificateRequest*
                                    <--------      ServerHelloDone
       Certificate*
       ClientKeyExchange
       CertificateVerify*
       [ChangeCipherSpec]
       Finished                     -------->
                                                [ChangeCipherSpec]
                                    <--------             Finished
       Application Data             <------->     Application Data

              Figure 1.  Message flow for a full handshake

Since RFC 8446 obsoletes RFC 5246, this is a serious problem.

How is this supposed to work?   Sorry but I did not follow the
development of TLS 1.3.  I felt that I was unwelcome in this
group by some of the "angry cryptographers" as I call them.

Mike