[openssh-unix-announce] Announce: OpenSSH 8.4 released

 ---------- Forwarded message ---------

From: Damien Miller <djm@openbsd.org>

Date: Sep 27, 2020 6:59PM

Future deprecation notice

=========================

It is now possible[1] to perform chosen-prefix attacks against the

SHA-1 algorithm for less than USD$50K. For this reason, we will be

disabling the "ssh-rsa" public key signature algorithm by default in a

near-future release.

This algorithm is unfortunately still used widely despite the

existence of better alternatives, being the only remaining public key

signature algorithm specified by the original SSH RFCs.

The better alternatives include:

 * The RFC8332 RSA SHA-2 signature algorithms rsa-sha2-256/512. These

   algorithms have the advantage of using the same key type as

   "ssh-rsa" but use the safe SHA-2 hash algorithms. These have been

   supported since OpenSSH 7.2 and are already used by default if the

   client and server support them.

 * The ssh-ed25519 signature algorithm. It has been supported in

   OpenSSH since release 6.5.

 * The RFC5656 ECDSA algorithms: ecdsa-sha2-nistp256/384/521. These

   have been supported by OpenSSH since release 5.7.

To check whether a server is using the weak ssh-rsa public key

algorithm, for host authentication, try to connect to it after

removing the ssh-rsa algorithm from ssh(1)'s allowed list:

    ssh -oHostKeyAlgorithms=-ssh-rsa user@host

If the host key verification fails and no other supported host key

types are available, the server software on that host should be

upgraded.

We intend to enable UpdateHostKeys by default in the next OpenSSH

release. This will assist the client by automatically migrating to

better algorithms. Users may consider enabling this option manually.

[1] "SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and

    Application to the PGP Web of Trust" Leurent, G and Peyrin, T

    (2020) https://eprint.iacr.org/2020/014.pdf