SSH-KEYGEN(1) BSD General Commands Manual SSH-KEYGEN(1)
ssh-keygen -- authentication key generation, management and conversion
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 -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] [-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 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 without any arguments, ssh-keygen will generate an RSA key for use in SSH protocol 2
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 cre-
ate the authentication key in ~/.ssh/identity, ~/.ssh/id_ecdsa, ~/.ssh/id_dsa or
~/.ssh/id_rsa. 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 pro-
gram 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, punc-
tuation, numbers, whitespace, or any string of characters 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 copied to the corresponding public key to other machines.
For RSA1 keys, 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
The options are as follows:
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 file.
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. Generally, 2048 bits is considered suffi-
cient. DSA keys must be exactly 1024 bits as specified by FIPS 186-2.
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 program 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,
including 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.
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 com-
-H Hash a known_hosts file. This replaces all hostnames and addresses with hashed rep-
resentations 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-hashed names.
-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 sup-
ported. 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 supplied with the fingerprint.
Specify the amount of memory to use (in megabytes) when generating candidate moduli
Specify a key format for the -i (import) or -e (export) conversion 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 conver-
sion format is ``RFC4716''.
Provides the new passphrase.
Specify one or more principals (user or host names) to be included in a certificate
when signing a key. Multiple principals 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 mul-
tiple times. Please see the CERTIFICATES section for details. The options that are
valid for user certificates are:
clear Clear all enabled permissions. This is useful for clearing the default set
of permissions so permissions may be added individually.
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 default).
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-separated list of one or more
address/netmask pairs in CIDR format.
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. Used by system administration scripts when creating a new key.
Removes all keys belonging to hostname from a known_hosts file. This option is use-
ful to delete hashed hosts (see the -H option above).
Print the SSHFP fingerprint resource record named hostname for the specified public
Specify start point (in hex) when generating candidate moduli for DH-GEX.
Certify (sign) a public key using the specified CA key. Please see the CERTIFICATES
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'' or ``rsa'' for protocol version 2.
Specify a validity interval when signing a certificate. A validity 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 charac-
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
-v Verbose mode. Causes ssh-keygen to print debugging messages about its progress.
This is helpful for debugging moduli generation. Multiple -v options increase the
verbosity. The maximum is 3.
Specify desired generator when testing candidate moduli for DH-GEX.
-y This option will read a private OpenSSH format file and print an OpenSSH public key
Specifies a serial number to be embedded in the certificate to distinguish this cer-
tificate 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 gener-
ated using a fast, but memory intensive process. These candidate primes are then tested for
suitability (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 specifies a different start point (in
Once a set of candidates have been generated, they must be tested for suitability. This may
be performed using the -T option. In this mode ssh-keygen will read candidates from stan-
dard 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 generator is desired, it may be requested using the -W
option. Valid generator values are 2, 3, and 5.
Screened DH groups may be installed in /etc/ssh/moduli. It is important that this file con-
tains moduli of a range of bit lengths and that both ends of a connection share common mod-
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 Certifica-
tion 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 cer-
tificates are a different, and much simpler, format to the X.509 certificates used in
ssh-keygen supports two types of certificates: user and host. User certificates authenti-
cate 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 providing the token
library using -D and identifying the CA key by providing its public half as an argument to
$ 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 principals:
$ 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 disable features of the SSH session, may be
valid only when presented from particular source addresses or may force the use of a spe-
cific 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 speci-
fication of certificate start and end times. A certificate that is presented at a time out-
side this range will not be considered valid. By default, certificates have a maximum
For certificates to be used for user or host authentication, the CA public key must be
trusted by sshd(8) or ssh(1). Please refer to those manual pages for details.
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 pri-
vate 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 authentication. 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 generating the key; that passphrase will be used to
encrypt the private part of this file using 128-bit AES. This file is not automati-
cally 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
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 created OpenSSH. Markus Friedl contributed the support
for SSH protocol versions 1.5 and 2.0.
BSD October 17, 2017 BSD