[TLS] [Editorial Errata Reported] RFC8446 (6204)

[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/eid6204

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Type: Editorial
Reported by: Chris Wood <[e-mail]>

Section: E.1

Original Text
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Implementations MUST NOT combine external PSKs with certificate-based authentication of either the client or the server unless negotiated by some extension.

Corrected Text
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Implementations MUST NOT combine external PSKs with certificate-based authentication of either client or the server. Future specifications MAY provide an extension to permit this. 

Notes
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The existing text can be misread as permitting this combination upon negotiation of the "post_handshake_auth" extension, which would be incorrect. [1] describes an attack that can occur based on this misinterpretation. The proposed text aims to make clear that a *new* extension is required for this combination. 

[1] https://link.springer.com/article/10.1007%2Fs11416-020-00352-0

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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