ssh-keygen —
authentication key generation, management and
conversion
ssh-keygen |
[-q]
[-b bits]
[-t dsa | ecdsa | ed25519 | rsa]
[-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 |
-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]
[-U]
[-D pkcs11_provider]
[-n principals]
[-O option]
[-V validity_interval]
[-z serial_number] file
... |
ssh-keygen |
-L
[-f input_keyfile] |
ssh-keygen |
-A
[-f prefix_path] |
ssh-keygen |
-k -f
krl_file
[-u]
[-s ca_public]
[-z version_number] file
... |
ssh-keygen |
-Q -f
krl_file file
... |
ssh-keygen generates, manages and converts
authentication keys for
ssh(1).
ssh-keygen can create 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.
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update Key Revocation Lists, and to test whether given keys have been revoked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/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, 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 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 (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. If -f has also been specified, its
argument is used as a prefix to the default path for the resulting host
key files. This is used by /etc/rc to
generate new host keys.
- -a
rounds
- When saving a private key 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 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 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.
- -C
comment
- Provides a new comment.
- -c
- Requests changing the comment in the private and public key
files. 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 commercial SSH implementations.
- -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 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”.
- -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
performing DH candidate screening using the
-T option.
- -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. 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”. Setting a format of “PEM” when
generating or updating a supported private key type will cause the key to
be stored in the legacy PEM private key format.
- -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. Multiple 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. See also the
CERTIFICATES section for
further details.
At present, no standard options are valid for host keys. 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.
- critical:name[=contents]
-
- extension:name[=contents]
- Includes an arbitrary certificate critical option or
extension. The specified name should
include a domain suffix, e.g. “name@example.com”. If
contents is specified then it is
included as the contents of the extension/option encoded as a string,
otherwise the extension/option is created with no contents (usually
indicating a flag). Extensions may be ignored by a client or server
that does not recognise them, whereas unknown critical options will
cause the certificate to be refused.
- force-command=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=address_list
- 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.
- -P
passphrase
- 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
hostname 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 public 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
- Specifies the type of key to create. The possible values
are “dsa”, “ecdsa”, “ed25519”,
or “rsa”.
- -U
- When used in combination with
-s, this option indicates that a CA key
resides in a
ssh-agent(1). See the
CERTIFICATES section for
more information.
- -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, 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 the string “always” to
indicate the certificate has no specified start time, a date in YYYYMMDD
format, a time in YYYYMMDDHHMM[SS] format, a relative time (to the current
time) consisting of a minus sign followed by an interval in the format
described in the TIME FORMATS section of
sshd_config(5).
The end time may be specified as a YYYYMMDD date, a YYYYMMDDHHMM[SS] time, a
relative time starting with a plus character or the string
“forever” to indicate that the certificate has no expirty
date.
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, 2011). “-1m:forever” (valid from one minute ago and
never expiring).
- -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 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 version number.
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:
# 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 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/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 simpler, 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
/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
-s:
$ ssh-keygen -s ca_key.pub -D libpkcs11.so -I
key_id user_key.pub
Similarly, it is possible for the CA key to be hosted in a
ssh-agent(1). This is
indicated by the
-U flag and, again, the CA key
must be identified by its public half.
$ ssh-keygen -Us ca_key.pub -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 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 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 documentation 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 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 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 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 including its SHA1 hash in the
KRL.
- sha256:
public_key
- Revokes the specified key by including its SHA256 hash in
the KRL. KRLs that revoke keys by SHA256 hash are not supported by OpenSSH
versions prior to 7.9.
- hash:
fingerprint
- Revokes a key using a fingerprint hash, as obtained from a
sshd(8) authentication log
message or the ssh-keygen
-l flag. Only SHA256 fingerprints are
supported here and resultant KRLs are not supported by OpenSSH versions
prior to 7.9.
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.
- ~/.ssh/id_dsa
-
- ~/.ssh/id_ecdsa
-
- ~/.ssh/id_ed25519
-
- ~/.ssh/id_rsa
- Contains the 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 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.
- /etc/moduli
- 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 created OpenSSH.
Markus Friedl contributed the support for SSH protocol versions 1.5 and
2.0.