[TLS] [Technical Errata Reported] RFC8446 (7250)

[RFC Errata System <[e-mail]>]

The following errata report has been submitted for RFC8446,
"The Transport Layer Security (TLS) Protocol Version 1.3".

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You may review the report below and at:
https://www.rfc-editor.org/errata/eid7250

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Type: Technical
Reported by: Alan DeKok <[e-mail]>

Section: 4.6.1

Original Text
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   The client MAY use this PSK for future handshakes by including the
   ticket value in the "pre_shared_key" extension in its ClientHello
   (Section 4.2.11).

Corrected Text
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(to add)

  Where the client does not support session tickets, this extension MUST be ignored.

Notes
-----
I've seen a TLS implementation which doesn't implement session tickets.  That's fine, but the implementation doesn't *ignore* session tickets it receives.  Instead, it treats reception of the ticket as un recoverable error, and drops the TLS connection.

It's also worth adding a note to section 4.2 at the bottom of page 38.  To note that in general, f an extension isn't supported AND doesn't materially affect the TLS exchange, THEN it should be ignored.

i.e. there's nothing in the spec which mentions Postel's law "be conservative in what you send, be liberal in what you accept".  So implementors reading this document are free to do all kinds of odd things.

In addition, the text in Section 4.2 at the bottom of page 38 says:

"
      Designers
      and implementors should be aware of the fact that until the
      handshake has been authenticated, active attackers can modify
      messages and insert, remove, or replace extensions.
"

The implicit conclusion here is that an implementation receiving extensions must sanity check them.  e.g. an attacker adding an undefined / unknown extension should not cause the entire session to be torn down.

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RFC8446 (draft-ietf-tls-tls13-28)
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Title               : The Transport Layer Security (TLS) Protocol Version 1.3
Publication Date    : August 2018
Author(s)           : E. Rescorla
Category            : PROPOSED STANDARD
Source              : Transport Layer Security
Area                : Security
Stream              : IETF
Verifying Party     : IESG