ssh keys

OpenSSH format

ssh-keygen -t rsa -f filename

PEM format

ssh-keygen -m pem -t rsa -f filename

this creates two files by the names

filename
filename.pub

Private key created is OpenSSH format by default, it starts and ends with

-----BEGIN OPENSSH PRIVATE KEY-----
...
...
-----END OPENSSH PRIVATE KEY-----

to convert this to PEM format

ssh-keygen -p -N "" -m pem -f filename

PEM format starts and ends with

-----BEGIN RSA PRIVATE KEY-----
...
...
-----END RSA PRIVATE KEY-----