MySQL Workbench SSH private key connection error

I had a problem getting connections to MySQL using TCP/IP over SSH working. I wanted to use a private key instead of saving a password for the SSH connection, but I kept getting an error message:

Authentication error. Please check that your username and password are correct and try again.
Details (Original exception message): Authentication failed, please check credentials.
Please refer to logs for details

The problem was that I was using a PPK file instead of an OpenSSH file. The solution is to open PuttyGen, load your PPK file and convert it to an OpenSSH key.

Problem solved!