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ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
ssh-keygen -i [-f input_keyfile]
ssh-keygen -e [-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 [-n] [-D smartcard]
ssh-keygen -s ca_key -I certificate_identity [-h] [-Z 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, 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. 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
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.
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 stored in the pkcs11 provider.
-e This option will read a private or public OpenSSH key file and
print the key in RFC 4716 SSH Public Key File Format to stdout.
This option allows exporting keys for use by several commercial
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 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-
-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.
-n Extract the public key from smartcard.
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
Disable X11 forwarding. (permitted by default)
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
clear Clear all enabled permissions. This is useful for clear-
ing the default set of permissions so permissions may be
Allows X11 forwarding.
used for authentication.
Restrict the source addresses from which the certificate
is considered valid from. The address_list is a comma-
separated list of one or more address/netmask pairs in
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 /etc/rc when creating a new key.
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.
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 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
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 pro-
cess: 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 tested 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
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 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
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. In
both 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 -Z user1,user2 user_key.pub
$ ssh-keygen -s ca_key -I key_id -h -Z 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 have a maximum validity inter-
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.
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.
~/.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).
The reseeding of the OpenSSL random generator is usually done
from /dev/urandom. If the SSH_USE_STRONG_RNG environment vari-
able is set to value other than 0 the OpenSSL random generator is
reseeded from /dev/random. The number of bytes read is defined
by the SSH_USE_STRONG_RNG value. Minimum is 14 bytes. This set-
ting is not recommended on the computers without the hardware
random generator because insufficient entropy causes the connec-
tion to be blocked until enough entropy is available.
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 March 23, 2017 BSD