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-...@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-...@openssh.com and rsa-sha2-256-cert-...@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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