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Re: [TLS] supported_versions question

Xiaoyin Liu <xiaoyin.l@outlook.com> Mon, 31 October 2016 19:33 UTC

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From: Xiaoyin Liu <xiaoyin.l@outlook.com>
To: Matt Caswell <frodo@baggins.org>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] supported_versions question
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/bNWITrOk6dIG2BpeqNaQHB00uh8>
Subject: Re: [TLS] supported_versions question
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I think for question 1, it should ignore legacy_version, and select a version from supported_versions, because if a client only supports TLS 1.1 and TLS 1.3, in order to connect to pre-TLS1.3 server, it has to set legacy_version to TLS 1.1.



I have no idea about questions 2 or 3.



Best,

Xiaoyin



From: Matt Caswell<mailto:frodo@baggins.org>
Sent: Monday, October 31, 2016 2:44 PM
To: tls@ietf.org<mailto:tls@ietf.org>
Subject: [TLS] supported_versions question



A few supported_versions questions:

1) What should a server do if supported_versions is received but
ClientHello.legacy_version != TLS1.2? Fail the handshake, or just
ignore legacy_version?

2) What should a server do if supported_versions is received,
ClientHello.legacy_version == TLS1.2, but supported_versions does not
contain TLS1.3 or TLS1.2 (e.g. it contains TLS1.1 or below)? Fail the
handshake, use the legacy_version, or use use the versions in
supported_versions?

3) If the answer to (2) above is ignore the legacy_version, and just
use the versions in supported_versions, which client_version should be
used in the RSA pre-master secret calculation? The one in
legacy_version, or the highest one in supported_versions? Presumably
it has to be the one in legacy_version, otherwise thing will fail when
the client talks to a server that doesn't understand
supported_versions?

Matt

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