ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C comment]
ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
ssh-keygen -i [-m key_format] [-f input_keyfile]
ssh-keygen -e [-m key_format] [-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 -B [-f input_keyfile]
ssh-keygen -D pkcs11
ssh-keygen -F hostname [-f known_hosts_file] [-l]
ssh-keygen -H [-f known_hosts_file]
ssh-keygen -R hostname [-f known_hosts_file]
ssh-keygen -r hostname [-f input_keyfile] [-g]
ssh-keygen -G output_file [-v] [-b bits] [-M memory] [-S start_point]
ssh-keygen -T output_file -f input_file [-v] [-a num_trials]
ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals]
[-O option] [-V validity_interval] [-z serial_number] file ...
ssh-keygen -L [-f input_keyfile]
ssh-keygen generates, manages and converts authentication keys for
ssh(1). ssh-keygen can create RSA keys for use by SSH protocol version 1
and DSA, ECDSA or RSA keys for use by SSH protocol version 2. The type
of key to be generated is specified with the -t option. If invoked with-
out any arguments, ssh-keygen will generate an RSA key for use in SSH
protocol 2 connections.
ssh-keygen is also used to generate groups for use in Diffie-Hellman
group exchange (DH-GEX). See the MODULI GENERATION section for details.
Normally each user wishing to use SSH with public key authentication runs
this once to create the authentication key in ~/.ssh/identity,
~/.ssh/id_ecdsa, ~/.ssh/id_dsa or ~/.ssh/id_rsa. Additionally, the sys-
tem 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 an
empty passphrase), or it may be a string of arbitrary length. A
passphrase is similar to a password, except it can be a phrase with a
series of words, punctuation, numbers, whitespace, or any string of char-
acters you want. Good passphrases are 10-30 characters long, are not
simple sentences or otherwise easily guessable (English prose has only
1-2 bits of entropy per character, and provides very bad passphrases),
and contain a mix of upper and lowercase letters, numbers, and non-
alphanumeric characters. 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, a new key must be generated and the corresponding public
-A For each of the key types (rsa1, rsa, dsa and ecdsa) for which
host keys do not exist, generate the host keys with the default
key file path, an empty passphrase, default bits for the key
type, and default comment. This is used by system administration
scripts to generate new host keys.
Specifies the number of primality tests to perform when screening
DH-GEX candidates using the -T command.
-B Show the bubblebabble digest of specified private or public key
Specifies the number of bits in the key to create. For RSA keys,
the minimum size is 768 bits and the default is 2048 bits. Gen-
erally, 2048 bits is considered sufficient. DSA keys must be
exactly 1024 bits as specified by FIPS 186-2. For ECDSA keys,
the -b flag determines they key length by selecting from one of
three elliptic curve sizes: 256, 384 or 521 bits. Attempting to
use bit lengths other than these three values for ECDSA keys will
Provides a new comment.
-c Requests changing the comment in the private and public key
files. This operation is only supported for RSA1 keys. The pro-
gram will prompt for the file containing the private keys, for
the passphrase if the key has one, and for the new comment.
Download the RSA public keys provided by the PKCS#11 shared
library pkcs11. When used in combination with -s, this option
indicates that a CA key resides in a PKCS#11 token (see the
CERTIFICATES section for details).
-e This option will read a private or public OpenSSH key file and
print to stdout the key in one of the formats specified by the -m
option. The default export format is ``RFC4716''. This option
allows exporting OpenSSH keys for use by other programs, includ-
ing several commercial SSH implementations.
Search for the specified hostname in a known_hosts file, listing
any occurrences found. This option is useful to find hashed host
names or addresses and may also be used in conjunction with the
-H option to print found keys in a hashed format.
Specifies the filename of the key file.
and is therefore safe to use on files that mix hashed and non-
-h When signing a key, create a host certificate instead of a user
certificate. Please see the CERTIFICATES section for details.
Specify the key identity when signing a public key. Please see
the CERTIFICATES section for details.
-i This option will read an unencrypted private (or public) key file
in the format specified by the -m option and print an OpenSSH
compatible private (or public) key to stdout. This option allows
importing keys from other software, including several commercial
SSH implementations. The default import format is ``RFC4716''.
-L Prints the contents of a certificate.
-l Show fingerprint of specified public key file. Private RSA1 keys
are also supported. For RSA and DSA keys ssh-keygen tries to
find the matching public key file and prints its fingerprint. If
combined with -v, an ASCII art representation of the key is sup-
plied with the fingerprint.
Specify the amount of memory to use (in megabytes) when generat-
ing candidate moduli for DH-GEX.
Specify a key format for the -i (import) or -e (export) conver-
sion options. The supported key formats are: ``RFC4716'' (RFC
4716/SSH2 public or private key), ``PKCS8'' (PEM PKCS8 public
key) or ``PEM'' (PEM public key). The default conversion format
Provides the new passphrase.
Specify one or more principals (user or host names) to be
included in a certificate when signing a key. Multiple princi-
pals may be specified, separated by commas. Please see the
CERTIFICATES section for details.
Specify a certificate option when signing a key. This option may
be specified multiple times. Please see the CERTIFICATES section
for details. The options that are valid for user certificates
clear Clear all enabled permissions. This is useful for clear-
ing the default set of permissions so permissions may be
Disable execution of ~/.ssh/rc by sshd(8) (permitted by
Disable X11 forwarding (permitted by default).
Allows ssh-agent(1) forwarding.
Allows port forwarding.
Allows PTY allocation.
Allows execution of ~/.ssh/rc by sshd(8).
Allows X11 forwarding.
Restrict the source addresses from which the certificate
is considered valid. The address_list is a comma-sepa-
rated list of one or more address/netmask pairs in CIDR
At present, no options are valid for host keys.
Provides the (old) passphrase.
-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.
Removes all keys belonging to hostname from a known_hosts file.
This option is useful to delete hashed hosts (see the -H option
Print the SSHFP fingerprint resource record named hostname for
the specified public key file.
Specify start point (in hex) when generating candidate moduli for
Specify a validity interval when signing a certificate. A valid-
ity interval may consist of a single time, indicating that the
certificate is valid beginning now and expiring at that time, or
may consist of two times separated by a colon to indicate an
explicit time interval. The start time may be specified as a
date in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a
relative time (to the current time) consisting of a minus sign
followed by a relative time in the format described in the TIME
FORMATS section of sshd_config(5). The end time may be specified
as a YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time
starting with a plus character.
For example: ``+52w1d'' (valid from now to 52 weeks and one day
from now), ``-4w:+4w'' (valid from four weeks ago to four weeks
from now), ``20100101123000:20110101123000'' (valid from 12:30
PM, January 1st, 2010 to 12:30 PM, January 1st, 2011),
``-1d:20110101'' (valid from yesterday to midnight, January 1st,
-v Verbose mode. Causes ssh-keygen to print debugging messages
about its progress. This is helpful for debugging moduli genera-
tion. Multiple -v options increase the verbosity. The maximum
Specify desired generator when testing candidate moduli for DH-
-y This option will read a private OpenSSH format file and print an
OpenSSH public key to stdout.
Specifies a serial number to be embedded in the certificate to
distinguish this certificate from others from the same CA. The
default serial number is zero.
ssh-keygen may be used to generate groups for the Diffie-Hellman Group
Exchange (DH-GEX) protocol. Generating these groups is a two-step
process: first, candidate primes are generated using a fast, but memory
intensive process. These candidate primes are then tested for suitabil-
ity (a CPU-intensive process).
Generation of primes is performed using the -G option. The desired
length of the primes may be specified by the -b option. For example:
# ssh-keygen -G moduli-2048.candidates -b 2048
By default, the search for primes begins at a random point in the desired
length range. This may be overridden using the -S option, which speci-
fies a different start point (in hex).
Screened DH groups may be installed in /etc/ssh/moduli. It is important
that this file contains moduli of a range of bit lengths and that both
ends of a connection share common moduli.
ssh-keygen supports signing of keys to produce certificates that may be
used for user or host authentication. Certificates consist of a public
key, some identity information, zero or more principal (user or host)
names and a set of options that are signed by a Certification Authority
(CA) key. Clients or servers may then trust only the CA key and verify
its signature on a certificate rather than trusting many user/host keys.
Note that OpenSSH certificates are a different, and much simpler, format
to the X.509 certificates used in ssl(8).
ssh-keygen supports two types of certificates: user and host. User cer-
tificates authenticate users to servers, whereas host certificates
authenticate server hosts to users. To generate a user certificate:
$ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
The resultant certificate will be placed in /path/to/user_key-cert.pub.
A host certificate requires the -h option:
$ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
The host certificate will be output to /path/to/host_key-cert.pub.
It is possible to sign using a CA key stored in a PKCS#11 token by pro-
viding the token library using -D and identifying the CA key by providing
its public half as an argument to -s:
$ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id host_key.pub
In all cases, key_id is a "key identifier" that is logged by the server
when the certificate is used for authentication.
Certificates may be limited to be valid for a set of principal
(user/host) names. By default, generated certificates are valid for all
users or hosts. To generate a certificate for a specified set of princi-
$ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
$ ssh-keygen -s ca_key -I key_id -h -n host.domain user_key.pub
Additional limitations on the validity and use of user certificates may
be specified through certificate options. A certificate option may dis-
able features of the SSH session, may be valid only when presented from
particular source addresses or may force the use of a specific command.
For a list of valid certificate options, see the documentation for the -O
Finally, certificates may be defined with a validity lifetime. The -V
option allows specification of certificate start and end times. A cer-
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. ssh(1) will read this file when a login attempt is made.
Contains the protocol version 1 RSA public key for authentica-
tion. The contents of this file should be added to
~/.ssh/authorized_keys on all machines where the user wishes to
log in using RSA authentication. There is no need to keep the
contents of this file secret.
Contains the protocol version 2 DSA, ECDSA or 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 gener-
ating the key; that passphrase will be used to encrypt the pri-
vate part of this file using 128-bit AES. This file is not auto-
matically accessed by ssh-keygen but it is offered as the default
file for the private key. ssh(1) will read this file when a
login attempt is made.
Contains the protocol version 2 DSA, ECDSA or RSA public key for
authentication. The contents of this file should be added to
~/.ssh/authorized_keys on all machines where the user wishes to
log in using public key authentication. There is no need to keep
the contents of this file secret.
Contains Diffie-Hellman groups used for DH-GEX. The file format
is described in moduli(5).
ssh(1), ssh-add(1), ssh-agent(1), ssh-vulnkey(1), moduli(5), sshd(8)
The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
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 cre-
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.
BSD February 27, 2017 BSD
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