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Description of problem:
It turns out that LEGACY crypto policy prefers weaker algorithms (ssh-rsa) to SHA2 based (rsa-sha2-256) in public keys algorithms for SSH, which causes it failing to authenticate to FIPS servers, which do not allow these weaker algorithms.
OpenSSH is trying the first supported algorithm (from the RSA variants) that is in the list and as there is no way to negotiate what the server supports, it fails after one attempt.
This problem does not happen in DEFAULT policy, as it already has the SHA2 variants in front of the SHA1.
Version-Release number of selected component (if applicable):
all RHEL8 versions
How reproducible:
deterministic
Steps to Reproduce:
1. Configure FIPS system as a SSH server
2. Configure client with LEGACY crypto policy
3. Generate rsa key on client, copy the public key to the server
4. Try to connect using publickey authentication
Actual results:
the server rejects our key:
sshd[19281]: userauth_pubkey: key type ssh-rsa not in PubkeyAcceptedKeyTypes [preauth]
Expected results:
The key is sent with rsa-sha2-256/512 sig type and is accepted by server.
Will changing the order for LEGACY policy cause any issues trying to connect to older servers that support only sha-rsa and no the (now preferred) rsa-sha2-256/512 algorithms ?
(In reply to Simo Sorce from comment #1)
> Will changing the order for LEGACY policy cause any issues trying to connect
> to older servers that support only sha-rsa and no the (now preferred)
> rsa-sha2-256/512 algorithms ?
As far as I know, it should not, at least for OpenSSH.
The servers supporting rsa-sha2 need to advertise these algorithms in the server-sig-alg extension [1] and the client should send the SHA2 signatures only to the servers that support it and fall back to legacy ssh-rsa algorithm otherwise.
[1] https://tools.ietf.org/html/rfc8308#section-3.1
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (crypto-policies bug fix and enhancement update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHBA-2020:4536