How to Convert PuTTY’s private key (.ppk) to SSH key
1. Launch PuTTY Key Generator (puttygen.exe)
2. Click on the File → Load private key and select your PuTTY’s key file, which normally ends with .ppk extension.
3. Enter the key’s passphrase if prompted.
4. Detail of the imported key will be displayed.
5. Go to Conversions → Export OpenSSH key from the menu.
6. Save your converted key to a new file and location.