Host certificates signed with ed25519 fails with old clients

[Jakub Jelen]

Hello all,
recently Fedora infrastructure deployed OpenSSH RSA certificates signed 
with ed25519 CA on server with GIT repositories and we encounter 
problems when connecting from old clients (openssh-5.3p1 + certificates) 
as described in the following bug [1].

There is a known workaround (using only the raw key) and after reading 
some more code around the key exchange and certificates specification, I 
don't see a simple way how to prevent it
  * the client does not know what CA key will be used
  * the server can not select raw RSA (different than would be selected 
by client)

The question is, can/should be the ED25519 keys be used for CA? The 
specification (The line 196 [2]) does not list them or is outdated.  If 
it is a bug, can this be fixed? If it is intended, how to prevent using 
ED25519 keys as CA?

Also reading through the gssgex code I noticed duplicate conditions on 
lines 168 and 172 [3]. Can this be fixed too?

Any more ideas to the current problem?

Attached patches to the minor issues, but not resolving the original 
problem.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1450609
[2] 
https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.certkeys#L196
[3] https://github.com/openssh/openssh-portable/blob/master/kexgexc.c#L172

Thanks,
-- 
Jakub Jelen
Software Engineer
Security Technologies
Red Hat
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