SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
NAME
ssh-keygen - authentication key generation, management and
conversion
SYNOPSIS
ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa |
rsa1] [-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 [-v] [-E fingerprint_hash] [-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 prin-
cipals] [-O option] [-V validity_interval] [-z
serial_number] file ...
ssh-keygen -L [-f input_keyfile]
ssh-keygen -A
ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z
version_number] file ...
ssh-keygen -Q -f krl_file file ...
DESCRIPTION
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 sup-
port many of the advanced features available for protocol 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 GEN-
ERATION 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Revocation Lists, and to test whether given keys have been
revoked by one.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
Last change: June 16 2016 1
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
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 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 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 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 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 /etc/rc to generate new host
keys.
-a rounds
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
Last change: June 16 2016 2
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
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 com-
mand). This option specifies the number of primality
tests to perform.
-B Show the bubblebabble digest of specified private or
public key file.
-b bits
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 suf-
ficient. DSA keys must be exactly 1024 bits as speci-
fied by FIPS 186-2. For ECDSA keys, the -b flag deter-
mines 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.
-C comment
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.
-D pkcs11
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 fingerprint_hash
Specifies the hash algorithm used when displaying key
fingerprints. Valid options are: ``md5'' and
``sha256''. The default is ``sha256''.
-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 com-
mercial SSH implementations.
Last change: June 16 2016 3
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
-F hostname
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.
-f filename
Specifies the filename of the key file.
-G output_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-hashed names.
-h When signing a key, create a host certificate instead
of a user certificate. Please see the CERTIFICATES
section for details.
-I certificate_identity
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 pub-
lic) 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 imple-
mentations. The default import format is ``RFC4716''.
-J num_lines
Exit after screening the specified number of lines
while performing DH candidate screening using the -T
option.
-j start_line
Start screening at the specified line number while per-
forming DH candidate screening using the -T option.
Last change: June 16 2016 4
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
-K checkpt
Write the last line processed to the file checkpt while
performing 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 section.
-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 fingerprint. If combined with -v, a
visual ASCII art representation of the key is supplied
with the fingerprint.
-M memory
Specify the amount of memory to use (in megabytes) when
generating candidate moduli for DH-GEX.
-m key_format
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 conversion format is ``RFC4716''.
-N new_passphrase
Provides the new passphrase.
-n principals
Specify one or more principals (user or host names) to
be included in a certificate when signing a key. Mul-
tiple principals may be specified, separated by commas.
Please see the CERTIFICATES section for details.
-O option
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 are:
clear
Clear all enabled permissions. This is useful for
clearing the default set of permissions so permissions
may be added individually.
Last change: June 16 2016 5
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
force-command Ns = Ns command
Forces the execution of command instead of any shell or
command specified by the user when the certificate is
used for authentication.
no-agent-forwarding
Disable ssh-agent(1) forwarding (permitted by default).
no-port-forwarding
Disable port forwarding (permitted by default).
no-pty
Disable PTY allocation (permitted by default).
no-user-rc
Disable execution of ~/.ssh/rc by sshd(8) (permitted by
default).
no-x11-forwarding
Disable X11 forwarding (permitted by default).
permit-agent-forwarding
Allows ssh-agent(1) forwarding.
permit-port-forwarding
Allows port forwarding.
permit-pty
Allows PTY allocation.
permit-user-rc
Allows execution of ~/.ssh/rc by sshd(8).
permit-x11-forwarding
Allows X11 forwarding.
source-address Ns = Ns address_list
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 CIDR format.
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 for-
mat. 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.
-P passphrase
Last change: June 16 2016 6
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
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.
-R hostname
Removes all keys belonging to hostname from a
known_hosts file. This option is useful to delete
hashed hosts (see the -H option above).
-r hostname
Print the SSHFP fingerprint resource record named host-
name for the specified public key file.
-S start
Specify start point (in hex) when generating candidate
moduli for DH-GEX.
-s ca_key
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 pub-
lic key file used to revoke certificates directly by
key ID or serial number. See the KEY REVOCATION LISTS
section for details.
-T output_file
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 command line are added to the existing KRL rather
than a new KRL being created.
-V validity_interval
Specify a validity interval when signing a certificate.
A validity interval may consist of a single time, indi-
cating that the certificate is valid beginning now and
Last change: June 16 2016 7
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
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, Janu-
ary 1st, 2011).
-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.
-W generator
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 to stdout.
-z serial_number
Specifies a serial number to be embedded in the certi-
ficate 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 version number.
MODULI GENERATION
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 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:
Dl # ssh-keygen -G moduli-2048.candidates -b 2048
Last change: June 16 2016 8
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
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 specified using the -f
option). For example:
Dl # ssh-keygen -T moduli-2048 -f moduli-2048.candidates
By default, each candidate will be subjected to 100 primal-
ity 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.
CERTIFICATES
ssh-keygen supports signing of keys to produce certificates
that may be used for user or host authentication. Certifi-
cates 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 certifi-
cates 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:
Dl $ 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:
Dl $ ssh-keygen -s /path/to/ca_key -I key_id -h
/path/to/host_key.pub
Last change: June 16 2016 9
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
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 identify-
ing the CA key by providing its public half as an argument
to -s :
Dl $ 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 princi-
pal (user/host) names. By default, generated certificates
are valid for all users or hosts. To generate a certificate
for a specified set of principals:
Dl $ ssh-keygen -s ca_key -I key_id -n user1,user2
user_key.pub Dl "$ ssh-keygen -s ca_key -I key_id -h -n
host.domain host_key.pub"
Additional limitations on the validity and use of user cer-
tificates 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 documentation for
the -O option above.
Finally, certificates may be defined with a validity life-
time. The -V option allows specification of certificate
start and end times. A certificate that is presented at a
time outside this range will not be considered valid. By
default, certificates are valid from UNIX Epoch to the dis-
tant 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.
KEY REVOCATION LISTS
ssh-keygen is able to manage OpenSSH format Key Revocation
Lists (KRLs). These binary files specify keys or certifi-
cates 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
Last change: June 16 2016 10
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
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 con-
trol 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 complete 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.
serial: serial_number[-serial_number]
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.
id: key_id
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.
key: public_key
Revokes the specified key. If a certificate is listed,
then it is revoked as a plain public key.
sha1: 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 command line. 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 was
revoked.
FILES
~/.ssh/identity
Contains the protocol version 1 RSA authentication
identity of the user. This file should not be readable
Last change: June 16 2016 11
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
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.
~/.ssh/identity.pub
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.
~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ed25519
~/.ssh/id_rsa
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.
~/.ssh/id_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ed25519.pub
~/.ssh/id_rsa.pub
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 pub-
lic key authentication. There is no need to keep the
contents of this file secret.
Last change: June 16 2016 12
SSH-KEYGEN(1) USER COMMANDS SSH-KEYGEN(1)
/etc/ssh/moduli
Contains Diffie-Hellman groups used for DH-GEX. The
file format is described in moduli(5).
SEE ALSO
ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)
The Secure Shell (SSH) Public Key File Format, RFC 4716,
2006.
AUTHORS
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 ver-
sions 1.5 and 2.0.
Last change: June 16 2016 13
Man(1) output converted with
man2html