Key Formats

• RFC4716: SSH2 public or private key. Begins with -----BEGIN OPENSSH PRIVATE KEY-----
• PKCS#8: PKCS8 public or private key. Use the -m PKCS8 option of ssh-keygen. Begins with -----BEGIN PUBLIC KEY-----
• PKCS#1
• DER: Binary format
• PEM: PEM public key. This refers to PKCS#8 when it's used in openssl commands.

PKCS8/PKCS1 are specifically for RSA keys.

Command Of ssh-keygen

• -l Show fingerprint of specified public key file
• -c Requests changing the comment in the private and public key files.
• -e This option will read a private or public OpenSSH key file and print the key in RFC 4716 SSH Public Key File Format to stdout.
• -y This option will read a private OpenSSH format file and print an OpenSSH public key to stdout.

Common Commands

Export the SSH public key in PEM format.

ssh-keygen -e -f ~/.ssh/id_rsa.pub -m PEM

note, the output key is a RSA public key, not a generic public key required for openssl commands like openssl rsautl -pubin

To be more specific, a RSA public key begins with a line like -----BEGIN RSA PUBLIC KEY----- while a PKCS8 starts with a line like -----BEGIN PUBLIC KEY-----

You'll need to convert it to a generic public key before running those commands, or you'll fail

openssl rsa -in ~/.ssh/pub.pem -RSAPublicKey_in -out -

Import an PEM format public key:

ssh-keygen -i -m PEM -f /tmp/pub.pem

Generate a public openssh key from a private key

ssh-keygen -y  -f ~/.ssh/id_rsa_1

show fingle print of SSH key

ssh-keygen -l -f ~/.ssh/id_rsa.pub 

References

https://linux.die.net/man/1/ssh-keygen