Ssh-keygen (command)

ssh-keygen is an OpenSSH software command used to generate, manage, and convert authentication keys. It support at least four different key types RSA, DSA, ECDSA and ed25519.

Commands

Generate a keypar:

• ssh-keygen -t ed25519 (There is no need to set the key size, as all Ed25519 keys are 256 bits) other options: [-t dsa | ecdsa | ed25519 | rsa]

Two files will be generated, one your private key and a second file containing second key (.pub extension)

• ssh-keygen -t ed25519 -f your_new_ed25519_key

• ssh-keygen -t rsa -f your_new_rsa_key

• Generate FIDO key:

 ssh-keygen -t ecdsa-sk -f ~/.ssh/id_ecdsa_sk
 Generating public/private ecdsa-sk key pair.

• Convert:

ssh-keygen -e -m PEM -f private_key_in_ed25519_format
do_convert_to_pem: unsupported key type ED25519

• ssh-keygen -l -f ~/.ssh/ssh_host_XXXXkey.pub

 -l      Show fingerprint of specified public key file.

.ssh_host_XXXXkey.pub is not a public key file.

Legacy format:

• ssh-keygen -l -E md5 -f ~/.ssh/ssh_host_XXXXkey.pub

See also: puttygen -O fingerprint

• ssh-keygen -vF host (-v flag added in OpenSSH 8.1)

Activities

• Generate a new public private key using ed25519 key format using the following command:

ssh-keygen -t ed25519

• Solve" "WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!" warning:

ssh-keygen -R SERVER_NAME -R Removes all keys belonging to hostname from a known_hosts file

ssh -oStrictHostKeyChecking=no SERVER_NAME Temporarily turning off host key checking

Both solutions have security implications.

• Understand different key types: dsa, ecdsa, ed25519 and rsa
• Generate public key from private key:

ssh-keygen -y -f ~/.ssh/id_rsa > ~./.ssh/id_rsa.pub

• Generate a key par with old PEM format using:

ssh-keygen -m PEM

• Changing the private key's passphrase without changing the key

ssh-keygen -f ~/.ssh/id_rsa -p

-p change the passphrase of a private key file

Related commands

• ssh-copy-id
• openssl
• puttygen (PuTTY)

See also

• Certification Authority
• passwd, ssh-keygen, chage, /etc/passwd, Password policy, Passwd (package), mkpasswd, cracklib-check, Password cracking, Phone to sign in, Passkey, htpasswd
• OpenSSH (changelog): /etc/ssh/sshd_config | /etc/ssh/ssh_config | ~/.ssh/ | openSSL | sshd logs | sftp | scp | authorized_keys | ssh-keygen | ssh-keyscan | ssh-add | ssh-agent | ssh | Ssh -O stop | ssh-copy-id | CheckHostIP | UseKeychain, OpenSSF
• Kerberos
• Cisco IOS/Configure public RSA key authentication

http://man7.org/linux/man-pages/man1/ssh-keygen.1.html

https://superuser.com/questions/421997/what-is-a-ssh-key-fingerprint-and-how-is-it-generated

https://www.openssh.com/txt/release-8.1

https://wiki.archlinux.org/index.php/SSH_keys#Changing_the_private_key's_passphrase_without_changing_the_key