putty-key

https://github.com/philr/putty-key

PuTTY::Key is a Ruby implementation of the PuTTY private key (ppk) format, handling reading and writing .ppk files. It includes a refinement to Ruby's OpenSSL library to add support for converting DSA, EC and RSA private keys to and from PuTTY private key files. This allows OpenSSH ecdsa, ssh-dss and ssh-rsa private keys to be converted to and from PuTTY's private key format.