Credits to: https://github.com/quarantin/rsakey-cracker
Tool to brute-force passphrase of RSA private keys in PEM format.
$ cat dict.txt | rsakey-cracker ./private-key.pem
OR
$ john -i --stdout | rsakey-cracker ./private-key.pem
$ ./rsakey-cracker out.pem < ~/rockyou.txt &
[1] 83397
$ kill -INFO %1
key: out.pem, tasks: 10
time since started: 1 minutes, 3 seconds