When you try to use a private key to connect to a box using SSH2. You may get the following error "Unable to use key file "xxx" (OpenSSH SSH-2 private key)".

The reason is because the OpenSSH private key is not compatible with Putty. You need to download PuttyGen (which also can be used to generate a public/private key pair for Putty) and convert your OpenSSH private key to a proprietary Putty private key.

To do that, open your PuttyGen, go to the Conversion - "Import key". Then after you load your OpenSSH private key, you can click on "Save private key" and it will create a new private key specifically for Putty with extension .ppk (Putty Private Key file). Then, you can load the key by open up Putty - Connection - SSH - Auth and browse for "Private key file authentication:". I hope this help with your issue.