Ssh-keygen

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 | ecdsa-sk | ed25519 | ed25519-sk | 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

• ssh-keygen -A Generate all (-A) rsa, dsa, ecdsa and ed25519 key types.

• 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)

Changelog

• OpenSSH 7.8, released in August 2018 Incompatible changes: ssh-keygen write OpenSSH format private keys by default instead of using OpenSSL's PEM format.

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

• Generate fingerprint or ID of a key

ssh-keygen -lf your_key.pub
2048 SHA256:u6IaFqRcwp0QX0nPBa/HHB2k/g73tH+YkoaE0riGRAT NAME@XX (RSA)

ssh-keygen -lf your_key
your_key is not a key file.

Related commands

• ssh-copy-id
• openssl: openssl rsa -in mykey.pem -pubout > mykey.pub, openssl genrsa
• puttygen (PuTTY)

See also

• CA, FreeIPA, PKI, OpenCA, Wildcard certificate, certtool, certbot (Let's Encrypt), certinfo (Cloudflare), ACME, Boulder, cfssl (Cloudflare), Public key certificate, public key, TLS and X.509, OCSP, Subject Alternative Name (SAN), openssl ca, Self signed certificate
• SSH: Ssh (OpenSSH client), TLS, .ppk, .key, .pem, .crt, .pub, ED25519, ssh-copy-id
• passwd, ssh-keygen, chage, /etc/passwd, Password policy, Passwd (package)
• OpenSSH (changelog), /etc/ssh/sshd_config, /etc/ssh/ssh_config, ~/.ssh/, openSSL, sshd logs, sftp, scp, authorized_keys, ssh-keyscan, ssh-add, ssh-keygen, ssh, Ssh -O stop
• 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