How do you convert an SSH private key to a .ppk on the Windows command line?

I have generated a key pair with ssh-keygen. I now want to use the same key pair with PuTTY-based applications.

Because I'm working in a CLI environment, I want to convert the key to the .ppk format from within this environment.

Apparently, you can use puttygen openssh_private_key -o output_filename.ppk on UNIX-like environments. But the Windows version of puttygen will just load the key and prompt you to use the GUI to export the key.

Batch mode for Windows PuTTYgen suggests that this simply is not currently possible, at least with puttygen. I'd gladly accept and option that uses other means though.

How can I convert the key from the command line?

WinSCP supports command-line conversion of private keys from the OpenSSH (or ssh.com) format to the PuTTY .ppk format.

Use the /keygen switch:

winscp.com /keygen mykey.pem /output=mykey.ppk

(I'm the author of WinSCP)

Or, you can compile/run the Unix command-line puttygen using WSL or Cygwin.

Or build your own tool from PuTTY code. It's open-source. It is rather easy (that's basically what WinSCP does).

Use the import_ssh2 to load the .pem:

ssh2_userkey *import_ssh2(const Filename *filename, int type,
                          char *passphrase, const char **errmsg_p);

Use the ssh2_save_userkey to save it as the .ppk:

bool ssh2_save_userkey(
    const Filename *filename, ssh2_userkey *key, char *passphrase);

NOTE: The OP requires Command line. This answer uses a graphical application... Will leave the answer here just in case it is useful.

There is a possibly simpler solution.

1) Download (if you do not already have it) the PuTTY Key Generator (PuTTYgen) application by going here or here.

2) Open PuTTYgen

3) Click Conversions, then click import key

4) Locate your key file, then click open.

You can now save your key as a PPK file by clicking the Save private key button

You can also save a public version of your key by clicking the Save public key button

https://devops.profitbricks.com/tutorials/use-ssh-keys-with-putty-on-windows/

https://kb.site5.com/shell-access-ssh/how-to-convert-ssh-keys-to-ppk-format/