When a ppk file (version 3) created using new version of PUTTYgen 0.75 (or newer) is used in older version of PUTTY or WinSCP, an error is thrown as PUTTY key format too low.
The solution is so simple, import ppk version 3 in PUTTYgen and export to ppk version 2
- Open PUTTYgen and load ppk file
- Go to Key > Parameters for saving key files and choose ppk version 2. Now, you save the private key in version 2
- Try with ppk version 2 in older version of PUTTY or WinSCP to successfully authenticate