.ppk(PuTTY private key).
.ppkfile extension indicates that the private key is in PuTTY's proprietary format. You must use a key of this format when using PuTTY as your SSH client. It cannot be used with other SSH client tools. Refer to the PuTTY documentation to convert a private key in this format to a different format.
.pubextension to indicate that the file contains a public key.
sshutility on Linux), export the private key:
.ppkformat, using an extension such as
.opensshto indicate the file's content.