[TLS] [Editorial Errata Reported] RFC8446 (6205)

RFC Errata System <rfc-editor@rfc-editor.org> Thu, 04 June 2020 02:04 UTC

To: ekr@rtfm.com, rdd@cert.org, kaduk@mit.edu, caw@heapingbits.net, joe@salowey.net, sean+ietf@sn3rd.com
From: RFC Errata System <rfc-editor@rfc-editor.org>
Cc: mt@lowentropy.net, tls@ietf.org, rfc-editor@rfc-editor.org
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Message-Id: <20200604020416.E2F39F40757@rfc-editor.org>
Date: Wed, 03 Jun 2020 19:04:16 -0700
Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/jOM1lITHqkFfuPCgdN9snLY5nDs>
Subject: [TLS] [Editorial Errata Reported] RFC8446 (6205)
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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/eid6205

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Type: Editorial
Reported by: Martin Thomson <mt@lowentropy.net>

Section: 4.3.2

Original Text
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Servers which are authenticating with a PSK MUST NOT send the
CertificateRequest message in the main handshake, though they MAY
send it in post-handshake authentication (see Section 4.6.2) provided
that the client has sent the "post_handshake_auth" extension (see
Section 4.2.6).

Corrected Text
--------------
Servers which are authenticating with a resumption PSK MUST NOT send the
CertificateRequest message in the main handshake, though they MAY
send it in post-handshake authentication (see Section 4.6.2) provided
that the client has sent the "post_handshake_auth" extension (see
Section 4.2.6). Servers which are authenticating with an external PSK
MUST NOT send the CertificateRequest message either in the main handshake
or request post-handshake authentication. Future specifications MAY
provide an extension to permit this.

Notes
-----
The lack of qualification on "authenticating with a PSK" implies that the statement applies equally to both external and resumption PSKs. However, there are two conditions being governed: whether a certificate can be requested during the handshake, and whether a certificate can be requested post-handshake. The latter of these requires different rules depending on the type of PSK.

We know from the analysis of resumption (see https://mailarchive.ietf.org/arch/msg/tls/TugB5ddJu3nYg7chcyeIyUqWSbA/) that combining a PSK handshake of either type with a client certificate is not safe. Thus, the prohibition on CertificateRequest during the handshake applies equally to both resumption and external PSKs.

For post-handshake, Appendix E.1 already discusses the risks of combining PSKs with certificates, citing the same analysis as above.

[...] It is unsafe to use certificate-based client
authentication when the client might potentially share the same
PSK/key-id pair with two different endpoints.

For this reason an external PSK is not safe to use with post-handshake authentication. A resumption PSK does not have this property, so the same prohibition doesn't apply.

Splitting the requirements as proposed makes this split clearer.

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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

[TLS] [Editorial Errata Reported] RFC8446 (6205) RFC Errata System