ssh-keygen —
authentication key generation
ssh-keygen |
[-q] [-b
bits] [-t
type] [-N
new_passphrase] [-C
comment] [-f
output_keyfile] |
ssh-keygen |
-p [-P
old_passphrase] [-N
new_passphrase] [-f
keyfile] |
ssh-keygen |
-x [-f
input_keyfile] |
ssh-keygen |
-X [-f
input_keyfile] |
ssh-keygen |
-y [-f
input_keyfile] |
ssh-keygen |
-c [-P
passphrase] [-C
comment] [-f
keyfile] |
ssh-keygen |
-l [-f
input_keyfile] |
ssh-keygen generates and manages authentication keys for
ssh(1). ssh-keygen defaults to
generating an RSA key for use by protocols 1.3 and 1.5; specifying the
-t option allows you to create a key for use by
protocol 2.0.
Normally each user wishing to use SSH with RSA or DSA
authentication runs this once to create the authentication key in
$HOME/.ssh/identity or
$HOME/.ssh/id_dsa. Additionally, the system
administrator may use this to generate host keys, as seen in
/etc/rc.
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 an 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.
For RSA, 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.
After a key is generated, instructions below detail where the keys
should be placed to be activated.
The options are as follows:
-b
bits- Specifies the number of bits in the key to create. Minimum is 512 bits.
Generally 1024 bits is considered sufficient, and key sizes above that no
longer improve security but make things slower. The default is 1024
bits.
-c- Requests changing the comment in the private and public key files. The
program will prompt for the file containing the private keys, for
passphrase if the key has one, and for the new comment.
-f- Specifies the filename of the key file.
-l- Show fingerprint of specified private or public key file.
-p- Requests changing the passphrase of a private key file instead of creating
a new private key. The program will prompt for the file containing the
private key, for the old passphrase, and twice for the new
passphrase.
-q- Silence
ssh-keygen. Used by
/etc/rc when creating a new key.
-t
type- Specifies the type of the key to create. The possible values are
“rsa1” for protocol version 1 and “rsa” or
“dsa” for protocol version 2. The default is
“rsa”.
-C
comment- Provides the new comment.
-N
new_passphrase- Provides the new passphrase.
-P
passphrase- Provides the (old) passphrase.
-x- This option will read a private OpenSSH DSA format file and print a
SSH2-compatible public key to stdout.
-X- This option will read a unencrypted SSH2-compatible private (or public)
key file and print an OpenSSH compatible private (or public) key to
stdout.
-y- This option will read a private OpenSSH format file and print an OpenSSH
public key to stdout.
- $HOME/.ssh/identity
- Contains the RSA authentication identity of the user. This file should not
be readable by anyone but the user. It is possible to specify a passphrase
when generating the key; that passphrase will be used to encrypt the
private part of this file using 3DES. This file is not automatically
accessed by
ssh-keygen but it is offered as the
default file for the private key. sshd(8) will read this
file when a login attempt is made.
- $HOME/.ssh/identity.pub
- Contains the public key for authentication. The contents of this file
should be added to $HOME/.ssh/authorized_keys on
all machines where you wish to log in using RSA authentication. There is
no need to keep the contents of this file secret.
- $HOME/.ssh/id_dsa
- Contains the DSA authentication identity of the user. This file should not
be readable by anyone but the user. It is possible to specify a passphrase
when generating the key; that passphrase will be used to encrypt the
private part of this file using 3DES. This file is not automatically
accessed by
ssh-keygen but it is offered as the
default file for the private key. sshd(8) will read this
file when a login attempt is made.
- $HOME/.ssh/id_dsa.pub
- Contains the public key for authentication. The contents of this file
should be added to $HOME/.ssh/authorized_keys2 on
all machines where you wish to log in using public key authentication.
There is no need to keep the contents of this file secret.
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu
Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt
and Dug Song removed many bugs, re-added newer features and created OpenSSH.
Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.