Puttygen convert PPK to id_rsa
How to convert a ppk to an openSSH private key encrypted with a passphrase
The Windows PuTTYgen's "Export OpenSSH key" does encrypt the key with 3DES-CBC. If the passphrase is non-empty, the output file says so:
-BEGIN RSA PRIVATE KEY-
Proc-Type: 4, ENCRYPTED
DEK-Info: DES-EDE3-CBC, 157A04D5AE43F45B
NiGUXnTOhATzg4dGvyXs8rzetF7KpplJJIKrZvQunXuVcZhVS+NTpnTgwJb+zOCm
...
I've tested this on various versions over the past 4 years. If the passphrase is empty, I even get an "Are you sure?" prompt.The Linux puttygen also always uses the same passphrase when converting.For RSA & DSA keys, OpenSSH uses the same 'raw' key format as OpenSSL. So if 3DES-CBC is not sufficient, you can use the openssl command-line tool to reencrypt them:openssl rsa -aes-128-cbc < old.key > new.key
On Unix of course OpenSSH's own ssh-keygen is better: