ssh-keygen generates, manages and converts authentication keys for
ssh(1). ssh-keygen can create keys for use by SSH protocol versions 1
and 2. Protocol 1 should not be used and is only offered to support
legacy devices. It suffers from a number of cryptographic weaknesses
and doesn't support many of the advanced features available for proto‐
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
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Lists, and to test whether given keys have been revoked by one.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
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
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, num‐
bers, 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 key copied to other machines.
For RSA1 keys and keys stored in the newer OpenSSH format, 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
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 /etc/rc
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
specifies 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 1024 bits and the default is 2048
bits. Generally, 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 ignored.
Provides a new comment.
-c Requests changing the comment in the private and public key
files. This operation is only supported for RSA1 keys and keys
stored in the newer OpenSSH format. 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).
Specifies the hash algorithm used when displaying key finger‐
prints. Valid options are: “md5” and “sha256”. The default is
-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, list‐
ing any occurrences found. This option is useful to find
hashed host names or addresses and may also be used in conjunc‐
tion 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 suf‐
fix. 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”.
Exit after screening the specified number of lines while per‐
forming 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 per‐
forming DH candidate screening using the -T option. This will
be used to skip lines in the input file that have already been
processed if the job is restarted.
-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 one or more certificates.
-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 finger‐
print. If combined with -v, a visual ASCII art representation
of the key is supplied with the fingerprint.
Specify the amount of memory to use (in megabytes) when gener‐
ating 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 cer‐
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
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 certifi‐
cate is considered valid. 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.
-o Causes ssh-keygen to save private keys using the new OpenSSH
format rather than the more compatible PEM format. The new
format has increased resistance to brute-force password crack‐
ing 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
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.
-t dsa | ecdsa | ed25519 | rsa | rsa1
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
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 indi‐
cate an explicit time interval. The start time may be speci‐
fied 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 gen‐
eration. Multiple -v options increase the verbosity. The max‐
imum is 3.
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
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 suit‐
ability (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 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 speci‐
fied 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/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 sim‐
pler, format to the X.509 certificates used in ssl(8).
ssh-keygen supports two types of certificates: user and host. User
certificates 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
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 provid‐
ing its public half as an argument to -s:
$ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_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
$ 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 host_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 specific
command. For a list of valid certificate options, see the documenta‐
tion for the -O option above.
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
considered valid. By default, certificates are valid from UNIX Epoch
to the distant future.
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.
ssh-keygen is able to manage OpenSSH format Key Revocation Lists
(KRLs). These binary files specify keys or certificates to be revoked
using a compact format, taking as little as one bit per certificate if
they are being revoked by serial number.
KRLs may be generated using the -k flag. This option reads one or more
files from the command line and generates a new KRL. The files may
either contain a KRL specification (see below) or public keys, listed
one per line. Plain public keys are revoked by listing their hash or
contents in the KRL and certificates revoked by serial number or key ID
(if the serial is zero or not available).
Revoking keys using a KRL specification offers explicit control over
the types of record used to revoke keys and may be used to directly
revoke certificates by serial number or key ID without having the com‐
plete original certificate on hand. A KRL specification consists of
lines containing one of the following directives followed by a colon
and some directive-specific information.
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 num‐
bers including and between each is revoked. The CA key must
have been specified on the ssh-keygen command line using the -s
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 particu‐
lar key (or keys). The -Q flag will query an existing KRL, testing
each key specified on the command line. If any key listed on the com‐
mand 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 was revoked.
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
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 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 2 DSA, ECDSA, Ed25519 or RSA pub‐
lic 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 for‐
mat is described in moduli(5).
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.
This page is part of the openssh (Portable OpenSSH) project. Informa‐
tion about the project can be found at
http://www.openssh.com/portable.html. If you have a bug report for
this manual page, see http://www.openssh.com/report.html. This page
was obtained from the tarball openssh-7.5p1.tar.gz fetched from
http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/ on 2017-04-25.
If you discover any rendering problems in this HTML version of the
page, or you believe there is a better or more up-to-date source for
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