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 rounds] [-J num_lines]
[-j start_line] [-K checkpt] [-W generator]
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 -k -f krl_file [-u] [-s ca_public] [-z version_number]
ssh-keygen -Q -f krl_file file ...
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, ED25519 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 without 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.
Finally, ssh-keygen can be used to generate and update Key Revocation
Lists, and to test whether given keys have been revoked by one. See the
KEY REVOCATION LISTS 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_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 or ~/.ssh/id_rsa.
Additionally, the system administrator may use this to generate host
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
tell what the key is for, or whatever is useful. The comment is initial-
ized 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:
-A For each of the key types (rsa1, rsa, dsa, ecdsa and ed25519) 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 adminis-
tration scripts to generate new host keys.
When saving a new-format private key (i.e. an ed25519 key or any
SSH protocol 2 key when the -o flag is set), this option speci-
fies the number of KDF (key derivation function) rounds used.
Higher numbers result in slower passphrase verification and
increased resistance to brute-force password cracking (should the
keys be stolen).
When screening DH-GEX candidates ( using the -T command). This
option specifies the number of primality tests to perform.
-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 the 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
fail. ED25519 keys have a fixed length and the -b flag will be
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).
Specifies the filename of the key file.
Generate candidate primes for DH-GEX. These primes must be
screened for safety (using the -T option) before use.
-g Use generic DNS format when printing fingerprint resource records
using the -r command.
-H Hash a known_hosts file. This replaces all hostnames and
addresses with hashed representations within the specified file;
the original content is moved to a file with a .old suffix.
These hashes may be used normally by ssh and sshd, but they do
not reveal identifying information should the file's contents be
disclosed. This option will not modify existing hashed hostnames
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.
Exit after screening the specified number of lines while perform-
ing DH candidate screening using the -T option.
Start screening at the specified line number while performing DH
candidate screening using the -T option.
Write the last line processed to the file checkpt while perform-
ing DH candidate screening using the -T option. This will be
used to skip lines in the input file that have already been pro-
cessed if the job is restarted. This option allows importing
keys from other software, including several commercial SSH imple-
mentations. The default import format is ``RFC4716''.
-k Generate a KRL file. In this mode, ssh-keygen will generate a
KRL file at the location specified via the -f flag that revokes
every key or certificate presented on the command line.
Keys/certificates to be revoked may be specified by public key
file or using the format described in the KEY REVOCATION LISTS
-L Prints the contents of a certificate.
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
Forces the execution of command instead of any shell or
command specified by the user when the certificate is
used for authentication.
Disable ssh-agent(1) forwarding (permitted by default).
Disable port forwarding (permitted by default).
no-pty Disable PTY allocation (permitted by default).
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.
At present, no options are valid for host keys.
-o Causes ssh-keygen to save SSH protocol 2 private keys using the
new OpenSSH format rather than the more compatible PEM format.
The new format has increased resistance to brute-force password
cracking but is not supported by versions of OpenSSH prior to
6.5. Ed25519 keys always use the new private key format.
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 Test whether keys have been revoked in a KRL.
-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
Certify (sign) a public key using the specified CA key. Please
see the CERTIFICATES section for details.
When generating a KRL, -s specifies a path to a CA public key
file used to revoke certificates directly by key ID or serial
number. See the KEY REVOCATION LISTS section for details.
Test DH group exchange candidate primes (generated using the -G
option) for safety.
Specifies the type of key to create. The possible values are
``rsa1'' for protocol version 1 and ``dsa'', ``ecdsa'',
``ed25519'', or ``rsa'' for protocol version 2.
-u Update a KRL. When specified with -k, keys listed via the com-
mand line are added to the existing KRL rather than a new KRL
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.
When generating a KRL, the -z flag is used to specify a KRL ver-
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).
Once a set of candidates have been generated, they must be screened for
suitability. This may be performed using the -T option. In this mode
ssh-keygen will read candidates from standard input (or a file specified
using the -f option). For example:
# ssh-keygen -T moduli-2048 -f moduli-2048.candidates
By default, each candidate will be subjected to 100 primality tests.
This may be overridden using the -a option. The DH generator value will
be chosen automatically for the prime under consideration. If a specific
(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-
tificate that is presented at a time outside this range will not be con-
sidered valid. By default, certificates are valid from UNIX Epoch to the
For certificates to be used for user or host authentication, the CA pub-
lic key must be trusted by sshd(8) or ssh(1). Please refer to those man-
ual pages for details.
KEY REVOCATION LISTS
ssh-keygen is able to manage OpenSSH format Key Revocation Lists (KRLs).
types of record used to revoke keys and may be used to directly revoke
certificates by serial number or key ID without having the complete orig-
inal certificate on hand. A KRL specification consists of lines contain-
ing one of the following directives followed by a colon and some direc-
Revokes a certificate with the specified serial number. Serial
numbers are 64-bit values, not including zero and may be
expressed in decimal, hex or octal. If two serial numbers are
specified separated by a hyphen, then the range of serial numbers
including and between each is revoked. The CA key must have been
specified on the ssh-keygen command line using the -s option.
Revokes a certificate with the specified key ID string. The CA
key must have been specified on the ssh-keygen command line using
the -s option.
Revokes the specified key. If a certificate is listed, then it
is revoked as a plain public key.
Revokes the specified key by its SHA1 hash.
KRLs may be updated using the -u flag in addition to -k. When this
option is specified, keys listed via the command line are merged into the
KRL, adding to those already there.
It is also possible, given a KRL, to test whether it revokes a particular
key (or keys). The -Q flag will query an existing KRL, testing each key
specified on the commandline. If any key listed on the command line has
been revoked (or an error encountered) then ssh-keygen will exit with a
non-zero exit status. A zero exit status will only be returned if no key
Contains the protocol version 1 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. 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, ED25519 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), 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 January 23, 2019 BSD
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