PuTTY's private SSH key can't be used interchangeably with OpenSSH clients because they both use and support a different key format.

You need to convert PuTTY's key file, which uses the .ppk extension and stands for PuTTY Private Key, to the OpenSSH Private Key Format before using it with OpenSSH.

PuTTYGen or PuTTY Key Generator is a tool you can use to convert the PuTTY Private Key (.ppk) file to the OpenSSH private key format.

Step-by-step video guide:

Steps to convert PuTTY Private Key to OpenSSH private key using PuTTYgen:

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

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

  8. Select the location and file name for your OpenSSH private key and click on the Save button.
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