Ssh-keygen
generates and manages authentication keys for
ssh (1).
Normally each user wishing to use
ssh
with RSA authentication runs this once to create the authentication
key in
$HOME/\s+2.\s0ssh/identity .
Additionally, the system administrator may use this to generate host keys.
Normally this program generates the key and asks for a file in which
to store the private key. The public key is stored in a file with the
same name but ".pub" appended. The program also asks for a
passphrase. The passphrase may be empty to indicate no passphrase
(host keys must have empty passphrase), or it may be a string of
arbitrary length. Good passphrases are 10-30 characters long and are
not simple sentences or otherwise easily guessable (English
prose has only 1-2 bits of entropy per word, and provides very bad
passphrases). The passphrase can be changed later by using the
-p
option.
There is no way to recover a lost passphrase. If the passphrase is
lost or forgotten, you will have to generate a new key and copy the
corresponding public key to other machines.
USING GOOD, UNGUESSABLE PASSPHRASES IS STRONGLY RECOMMENDED. EMPTY
PASSPHRASES SHOULD NOT BE USED UNLESS YOU UNDERSTAND WHAT YOU ARE DOING.
There is also a comment field in the key file that is only for
convenience to the user to help identify the key. The comment can
tell what the key is for, or whatever is useful. The comment is
initialized to user@host when the key is created, but can be changed
using the
-c
option.
The cipher to be used when encrypting keys with a passphrase is
defined in ssh.h. Using the
-u
option, keys encrypted in any supported cipher can be updated to
use this default cipher.