tree c9b72524f94ca6c058ed9f39c72188f02ff08804
parent 4e5a26183ecb4f9a0f85c8f8dbe7982885435436
author Roland Shoemaker <bracewell@google.com> 1700510778 -0800
committer Roland Shoemaker <roland@golang.org> 1702917188 +0000

ssh: implement strict KEX protocol changes

Implement the "strict KEX" protocol changes, as described in section
1.9 of the OpenSSH PROTOCOL file (as of OpenSSH version 9.6/9.6p1).

Namely this makes the following changes:
  * Both the server and the client add an additional algorithm to the
    initial KEXINIT message, indicating support for the strict KEX mode.
  * When one side of the connection sees the strict KEX extension
    algorithm, the strict KEX mode is enabled for messages originating
    from the other side of the connection. If the sequence number for
    the side which requested the extension is not 1 (indicating that it
    has already received non-KEXINIT packets), the connection is
    terminated.
  * When strict kex mode is enabled, unexpected messages during the
    handshake are considered fatal. Additionally when a key change
    occurs (on the receipt of the NEWKEYS message) the message sequence
    numbers are reset.

Thanks to Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk from Ruhr
University Bochum for reporting this issue.

Fixes CVE-2023-48795
Fixes golang/go#64784

Change-Id: I96b53afd2bd2fb94d2b6f2a46a5dacf325357604
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/550715
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>