WSL: ssh localhost Permission denied (publickey)

If you're running Ubuntu on Windows Subsystem for Linux, there will not be a preinstalled public key or authorized keys list, so you'll need to generate your own.

If you don't already have openssh-server installed:

1. sudo apt-get upgrade
2. sudo apt-get update
3. sudo apt-get install openssh-server
4. sudo service ssh start

Then take the following steps to enable sshing to localhost:

1. cd ~/.ssh
2. ssh-keygen to generate a public/private rsa key pair; use the default options
3. cat id_rsa.pub >> authorized_keys to append the key to the authorized_keys file
4. chmod 640 authorized_keys to set restricted permissions
5. sudo service ssh restart to pickup recent changes
6. ssh localhost