This article talks about the secret known as the private key in a asymmetric cryptographic scheme.
A private key is a cryptographic key that is private (that you should be kept secret).
This page is about how to manage/create a Asymmetric private key.
An Asymmetric private key is categorized as something you have authentication.
There is two asymmetric cryptographic scheme that defines how to create a private key.
There is therefore two types of key. The most known being RSA
Different file formats are used to store keys.
The creation of the private key is format dependent.
See Rsa Private Key
For a ppk private key (Putty), see ppk create
With a:
-----BEGIN OPENSSH PRIVATE KEY-----
xxxxxxxxxx
-----END OPENSSH PRIVATE KEY-----
# with prompt of the passphrase
# where N is the new passphrase (ie empty)
# and f is the file
ssh-keygen -p -N "" -f /path/keyfile.pem
chmod 400 /path/keyfile.pem