If you use a newer SSH key generated with the ED25519 algorithm instead of RSA (see Create a new SSH key pair), the deployment with Capistrano may fail with the following message:
The deploy has failed with an error: unsupported key type `ssh-ed25519'
net-ssh requires the following gems for ed25519 support:
* ed25519 (>= 1.2, < 2.0)
* bcrypt_pbkdf (>= 1.0, < 2.0)
See https://github.com/net-ssh/net-ssh/issues/565 for more information
Gem::LoadError : "ed25519 is not part of the bundle. Add it to your Gemfile."
Solution
As described in the error message, the ed25519 and bcrypt_pbkdf gems must be added to the Gemfile to solve the problem. Now, the deployment should work faultlessly again.
Fixing a (consecutive) fingerprint mismatch
After changing to ed25519, existing hostname fingerprints might no longer match and you will see an error like this:
Exception while executing as USER@HOSTNAME: fingerprint SHA256:HASH does not match for "HOSTNAME,IP" (SSHKit::Runner::ExecuteError)
This is generally an indicator of a man-in-the-middle attack. You can test this: if an SSH shell (e.g. geordi shell production) opens without fingerprint error, you should be fine. You may then proceed to remove your old (e.g. RSA based) fingerprints from ~/.ssh/known_hosts. Here are instructions.
