Convert a .ppk file to .pub with PuttyGen

1. Run “puttygen.exe”. The “PuTTY Key Generator” dialog will appear.
2. Use the “Load” button to load the .ppk file that contains your private key. If your private key file is protected with a passphrase, you will be prompted to supply the passphrase in order to complete the loading of the private key.
3. Once the private key has been loaded properly, choose “Export OpenSSH key” from the main “Conversions” pull-down menu that is part of the “PuTTY Key Generator” window.
4. When the “Save private key as:” window appears, specify a filename for the private key file such as “Identity” and choose “Save”.

   NOTE: The private key file name should not have any extension associated with it. For example, if SecureCRT is configured to use a public key named “Identity.pub”, it expects to find the private key in the same folder as the public key file, with the name of “Identity”.

   Similarly, if SecureCRT is confgured to use a private key named “Identity”, it expects to find a the public key in a file named “Identity.pub” located in the same folder as the private key file.

   Now you will need to export the public key to a file format that follows the IETF SecSH internet-draft.

5. Choose “Save public key”. When the “Save public key as:” window appears, specify a filename for the public key file, such as “Identity.pub”, and choose “Save”.

 引自：http://www.tallgeorge.net/?page_id=162