XPost: alt.os.linux

As per subject, I have a number of Windows 7 PCs which are running an
old-ish 32-bit version of ssh via CygWin and PuTTy. Several of these
machines dual-boot between Windows and Ubuntu22. I also have a number
of servers and network media players, 4 pretty old, 2 much newer.

The machines running under Windows can log into the old servers via the
old-ish 32-bit ssh and Cygwin using a public key file, no password is
required, but to log into the newer servers, I have to use PuTTy.

However, when booted into Ubuntu 22 none of the machines, even though
they're using the *SAME* key files, can login in using just these public
keys, a password is requested for both old and new servers. I never had
this problem with Ubuntu 18.

I've checked all the usual suspects:

+ The id_rsa* copied into ~/.ssh are identical to those used by the
Windows builds.

+ The ~/.ssh directory (700) and the files within it (600) all have
the correct ownership and permissions.

Suitably anonymised, trying to force using a key file with the following command ...

ssh -i /user/.ssh/id_rsa.pub -v -E ssh.log -o 'PubkeyAuthentication yes'
-o 'PasswordAuthentication no' user@server

... results in the output appended. Can anyone help explain what is
going wrong and help fix the problem? My first suspicion is that a
newer version of ssh in the Ubuntu 22 builds is not accepting the older
keys accepted by Ubuntu 18, so, if this is true, I need to know what configuration changes to make in /etc/ssh_config to get backward
compatibility. Nothing leaps out at me from reading the online manual
and so far searches have produced the usual advice about how to set up
public key use, and the standard advice to try when it goes wrong, all
of which I've done. I've not yet found anything covering this exact
situation.

Debug output:

debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: include
/etc/ssh/ssh_config.d/*.conf matched no files
debug1: /etc/ssh/ssh_config line 21: Applying options for *
debug1: Connecting to server [IP Address] port 22.
debug1: Connection established.
debug1: identity file /user/.ssh/id_rsa.pub type 0
debug1: identity file /user/.ssh/id_rsa.pub-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.9p1 Ubuntu-3ubuntu0.10
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.9 debug1: compat_banner: match: OpenSSH_5.9 pat OpenSSH_5* compat 0x0c000002 debug1: Authenticating to server:22 as 'user'
debug1: load_hostkeys: fopen /user/.ssh/known_hosts2: No such file or
directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: ecdh-sha2-nistp256
debug1: kex: host key algorithm: ecdsa-sha2-nistp521
debug1: kex: server->client cipher: aes128-ctr MAC: umac-64@openssh.com compression: none
debug1: kex: client->server cipher: aes128-ctr MAC: umac-64@openssh.com compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: SSH2_MSG_KEX_ECDH_REPLY received
debug1: Server host key: <anonymised> SHA256:<anonymised>
debug1: load_hostkeys: fopen /user/.ssh/known_hosts2: No such file or
directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug1: Host 'server' is known and matches the ECDSA host key.
debug1: Found key in /user/.ssh/known_hosts:7
debug1: rekey out after 4294967296 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: rekey in after 4294967296 blocks
debug1: get_agent_identities: bound agent to hostkey
debug1: get_agent_identities: ssh_fetch_identitylist: agent contains no identities
debug1: Will attempt key: /user/.ssh/id_rsa.pub RSA SHA256:<an unknown
key, not the one in id_rsa or id_rsa.pub> explicit
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Offering public key: /user/.ssh/id_rsa.pub RSA SHA256:<same
unknown key, not the one in id_rsa or id_rsa.pub> explicit
debug1: send_pubkey_test: no mutual signature algorithm
debug1: Next authentication method: keyboard-interactive
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: No more authentication methods to try.
user@server: Permission denied (publickey,password,keyboard-interactive).



However, the same key file
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