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).