I created a new Ubuntu 22 VM a few weeks ago and noticed when trying a git pull or git push to a GCP Cloud Source Repo, I wasn’t having any luck when using SSH:

cd myrepo/
git pull
myusername@myorg.com@source.developers.google.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

The SSH key was a standard RSA with the public key uploaded to Cloud Source SSH Keys, so there was no obvious reason why it wasn’t working.

Next step was try and get some type of debug or error message as to why the public key exchange wasn’t working. Newer versions of Git can turn on SSH debugging by setting the GIT_SSH_COMMAND environment variable, so I did that:

export GIT_SSH_COMMAND="ssh -vvv"

When re-running the git pull request, I get some somewhat useful debugs back:

debug1: Authentications that can continue: publickey
debug3: start over, passed a different list publickey
debug3: preferred gssapi-with-mic,publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /home/j5/.ssh/id_rsa RSA SHA256:JBgC+R4Ozel+YI+7oEv1UOf9/jLqGBhysN8bpoEDbPU
debug1: send_pubkey_test: no mutual signature algorithm

The ‘no mutual signature algorithm’ indicated one side didn’t like the signing algorithm. I did a Google and found this article which indicates that Ubuntu 22 doesn’t allow RSA by default. I can’t change the setting on the Cloud Source side, so on the Ubuntu 22 client, I did this as a quick work-around:

echo "PubkeyAcceptedKeyTypes +ssh-rsa" > /etc/ssh/ssh_config.d/enable_rsa.conf

And now the git pull/push works without issue.

The better solution is instead use an Elliptic Curve DSA. To generate a new ECDSA key:

ssh-keygen -t ecdsa
cat ~/.ssh/id_ecdsa.pub

Then copy/paste the key in to the SSH Key Manager. This will be easier to copy/paste then RSA since it’s shorter.