How to Register SSH Public Keys

Let's take a look at how to register a public key after installing SSH on Windows.

Scenario

Even after creating the .ssh\authorized_keys file and registering the public key, a problem arose where a password was requested when accessing with the registered key. The cause of this issue was a special setting for admin group users in Windows OpenSSH.

In the content of the $env:PROGRAMDATA\ssh\sshd_config file:

Match Group administrators
       AuthorizedKeysFile __PROGRAMDATA__/ssh/administrators_authorized_keys

Users in the admin group are configured to reference the administrators_authorized_keys file.

Solution

You can resolve the issue by creating a dedicated administrators_authorized_keys file for admin group users and registering the public key.

Creating the File

If you only need a file with the correct permissions, run the following command with PowerShell as an administrator.

Add-Content -Force -Path $env:ProgramData\ssh\administrators_authorized_keys -Value $null;
icacls.exe $env:ProgramData\ssh\administrators_authorized_keys /inheritance:r /grant "Administrators:F" /grant "SYSTEM:F"

Add-Content: This PowerShell cmdlet creates a new file and adds content if the file does not exist at the specified path.
- -Force: Ignores any errors that may occur during the operation and forces the command to execute, creating a new file if it does not exist.
icacls.exe: A Windows command-line utility that modifies the access control list (ACL) of files or directories.
- /inheritance:r: Removes inheritance from the file or folder, retaining only explicitly set permissions.
- /grant "Administrators:F": Grants "Full control" permission to the "Administrators" group.
- /grant "SYSTEM:F": Grants "Full control" permission to the "SYSTEM" account.

Registering the Key

To create the authorization file and immediately register the public key, run the following command with PowerShell as an administrator.

$authorizedKey = Get-Content -Path $env:USERPROFILE\.ssh\id_ed25519.pub
Add-Content -Force -Path $env:ProgramData\ssh\administrators_authorized_keys -Value $authorizedKey
icacls.exe $env:ProgramData\ssh\administrators_authorized_keys /inheritance:r /grant "Administrators:F" /grant "SYSTEM:F"

Troubleshooting

The ssh-copy-id command from Linux systems is not supported for registering public keys on Windows systems. Therefore, you must register them manually.
If you need to paste the contents of the clipboard into the administrators_authorized_keys file, install Visual Studio Code and run the following command with PowerShell as an administrator.
```
code $env:ProgramData\ssh\administrators_authorized_keys
```

References

Microsoft Documentation