How to convert ppk to OpenSSH key using PuTTYGen

You won't be able to directly use your PuTTY's key in Linux's OpenSSH because the keys are of different format.

You'll need to first convert PuTTY's key to OpenSSH's key format to be able to use the key.

You can convert PuTTY Private Key (ppk) file to OpenSSH private key using PuTTY Key Generator (PuTTYGen).

1. Download PuTTYGen (puttygen.exe) from the official website and launch the program.

   https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html

2. Click on the Load button on PuTTYGen's main interface.
3. Select your PuTTY's private key file which normally ends with .ppk extension and click on the Open button.
4. Enter the key's passphrase if prompted and then click OK.
5. Details of the imported key will be displayed on the main interface and edit if necessary.
6. Go to Conversions → Export OpenSSH key from the menu.

   There's an old and new types of SSH key file format and will be automatically be determined based on the key's type except if you choose Export OpenSSH key (force new file format).

7. Select the location and file name for your OpenSSH private key and click on the save button.