<!-- language-all: lang-powershell -->

This seems to be related to the version of OpenSSH you're running: 

 - `where ssh` returns:
    ```
    %WinDir%\System32\OpenSSH\ssh.exe
    %ProgramFiles%\Git\usr\bin\ssh.exe
    ```
   `ssh -V` returns:
    ```
    # %WinDir%\System32\OpenSSH\ssh.exe
      OpenSSH_7.5p1, without OpenSSL

    # %ProgramFiles%\Git\usr\bin\ssh.exe
      OpenSSH_7.3p1, OpenSSL 1.0.2k  26 Jan 2017
    ```

When running `..\Git\usr\bin\ssh.exe`, it works fine and doesn't complain about the permissions, but running `..\OpenSSH\ssh.exe` comes back with the following, even though key ACLs are Full Access for myself and nothing else:
```
load key "t:\mykeys\rich-private.ppk": invalid format
  [email protected]: Permission denied (publickey).
```