How to share OpenSSH keys with WSL in Windows 10

The SSH config directive IdentityFile specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, authenticator hosted Ed25519 or RSA authentication identity is read. You can use this to share your Windows OpenSSH key with WSL. Assuming you've created a key pair in Windows using ssh-keygen.

Manually install OpenSSH in Windows Server
Retrieve SSH public key from Active Directory for SSH authentication

First start with creating a symlink in WSL from ~/.ssh to /mnt/c/Users/exampleuser/.ssh/. Substitute "exampleuser" with your user name:

ln -s /mnt/c/Users/exampleuser/.ssh ~/.ssh

Next, create a file /etc/wsl.conf, and add the following contents:

[automount]
options = "metadata,mask=22,fmask=11"

This is required because Linux uses a different user/group system than Windows does, and using this automount option adds specific metadata. Source: Chmod/Chown WSL Improvements. You also have to fix file permissions on ~/.ssh, the keys and config files:

chmod 600 ~/.ssh
chmod 644 ~/.ssh/id_ed25519
chmod 644 ~/.ssh/config

Last, open up your ssh config file in Windows, which is located in C:\Users\exampleuser\.ssh, and add two IdentityFile lines. One using your Windows path and one the WSL Linux path:

Host *.example.org
  IdentityFile	c:/users/exampleuser/.ssh/id_ed25519
  IdentityFile	/mnt/c/Users/exampleuser/.ssh/id_ed25519
  User exampleuser
  ForwardAgent yes

ForwardAgent specifies whether the connection to the authentication agent (if any) will be forwarded to the remote machine. If you have the ssh-agent service running in Windows, you can use this to connect to a host where you don't have an authorized_keys available yet by ssh'ing into a jump host. Use with caution.

Tunnel RDP through SSH & PuTTY
Set up Monit monitoring in Windows Subsystem for Linux WSL