[Bug 1790963] Re: Unable to connect with openssh 7.8 client and certificates

[Etienne CHAMPETIER]

Hi all,

This only affect RSA certificates, not ECDSA or ED25519 certs
Even if your CA is an RSA key, you can sign ECDSA or ED25519 public keys so you get ECDSA/ED25519 certificates which allow you to work around the issue without changing anything server-side
(and without deploying a new CA)

Exemple of working cert (7.8 client, <7.8 server):
$ ssh-keygen -Lf ~/.ssh/id_ed25519-cert.pub
~/.ssh/id_ed25519-cert.pub:
        Type: ssh-ed25519-cert-v01 at openssh.com user certificate
        Public key: ED25519-CERT SHA256:<...>
        Signing CA: RSA SHA256:<...>
        Key ID: "..."

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https://bugs.launchpad.net/bugs/1790963

Title:
  Unable to connect with openssh 7.8 client and certificates

Status in openssh package in Ubuntu:
  Confirmed

Bug description:
  Users are unable to connect to Ubuntu when using openssh client 7.8
  and certificates. We have seen this with both xenial and bionic, but
  this affects connecting to ANY host running openssh server <7.8.

  It appears to be specific to using certificate authentication.

  The only known recourse at this time is either downgrade clients to
  7.7 or a previous version of openssh, or create new keys/certificates
  with a different alg that is acceptable for both the older server and
  newer client.

  The error message via ssh -vvv is:
  debug1: Next authentication method: publickey
  debug1: Offering public key: RSA SHA256:REDACTED
  debug1: send_pubkey_test: no mutual signature algorithm

  When comparing the list returned from a 7.6 server and a 7.8 server
  via "ssh -Q key", we find that 7.8 returns rsa-
  sha2-512-cert-v01 at openssh.com and rsa-sha2-256-cert-v01 at openssh.com
  which are not present (or valid) for the earlier version server.

  It appears that the change noted here in the release notes[1] for 7.8 is related:
   * sshd(8): the semantics of PubkeyAcceptedKeyTypes and the similar
     HostbasedAcceptedKeyTypes options have changed. These now specify
     signature algorithms that are accepted for their respective
     authentication mechanism, where previously they specified accepted
     key types. This distinction matters when using the RSA/SHA2
     signature algorithms "rsa-sha2-256", "rsa-sha2-512" and their
     certificate counterparts. Configurations that override these
     options but omit these algorithm names may cause unexpected
     authentication failures (no action is required for configurations
     that accept the default for these options).


  This is also affecting other Linux distributions as well:
  https://bugzilla.redhat.com/show_bug.cgi?id=1623929
  https://bugs.archlinux.org/task/59838

  [1] https://www.openssh.com/txt/release-7.8

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