OpenSSH 8.8, RSA keys, SFTP and duplicacy: error on connection, unable to make backup

I run backups from various linux devices to a microserver on the LAN using sftp to a chroot jail on the server. It’s been working well for years.

The server and devices all run manjaro, yesterday an update to OpenSSH 8.8p1 seems to have triggered this error which prevents making backups.

My usual backup:

duplicacy backup -stats -threads 16
# which uses this:
"storage": "sftp://duplicacy@"

Please describe what you expect to happen (but doesn’t):
Duplicacy runs a backup, progress is displayed in the terminal.

Please describe what actually happens (the wrong behaviour):
An error is thrown:

Storage set to sftp://duplicacy@
Failed to load the SFTP storage at sftp://duplicacy@
ssh: handshake failed: ssh: unable to authenticate, attempted methods
[publickey none], no supported methods remain

This is presumably happening because this version of OpenSSH deprecates SHA1 RSA keys: OpenSSH: Release Notes

And indeed sshd on the server side logs an error:

sshd[94018]: userauth_pubkey: key type ssh-rsa not in PubkeyAcceptedAlgorithms [preauth]
sshd[94018]: Connection closed by authenticating user duplicacy port 44932 [preauth]

However I believe I’m using a 3000+ bit SHA256 key which should be valid:

ssh-keygen -l -f id_rsa
# gives
3072 SHA256:[fingerprint-removed] root@computer (RSA)

And I’m able to access the repo when running sftp manually, including specifying the same key with -i:

sftp -i /root/.ssh/id_rsa duplicacy@
Connected to
sftp> ls -al
drwxr-xr-x    3 root     968             3 Dec 19  2019 .
drwxr-xr-x    3 root     968             3 Dec 19  2019 ..
drwxr-xr-x    4 969      968             5 Dec 19  2019 repo
sftp> exit

I’m able to make backups using the workaround of allowing ssh-rsa, by adding this to /etc/ssh/sshd_config and restarting the sshd service


On my LAN this is obviously not too much of a security worry. I’ll generate some new keys that don’t use RSA and then remove this.

But I’m posting this as a bug because I don’t see why manual sftp should work, but duplicacy running over sftp doesn’t.

This seems to be an issue in the go ssh library: x/crypto/ssh: support RSA SHA-2 host key signatures · Issue #37278 · golang/go · GitHub

1 Like

Hard to know when that issue will get patched in Go.

A different fix: I generated some ed25519 based keys:

ssh-keygen -t ed25519

And copied them to the relevant places, authorized_keys files etc. Then updated my .duplicacy/preferences file with the new key name.

Removed PubkeyAcceptedAlgorithms=+ssh-rsa from my sshd_config and restarted sshd.

And Duplicacy is now running as normal.