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
[-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 ]
[-n principals ]
[-O option ]
[-V validity_interval ]
[-z serial_number ] file
... |
ssh-keygen |
-L
[-f input_keyfile ] |
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 RSA keys for use by
SSH protocol version 1 and DSA, ECDSA, ED25519 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 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.
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and update Key Revocation Lists, and to test whether given keys have been
revoked by one.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.
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, 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 system administration scripts 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 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 768 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. 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.
-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
- 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 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 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. 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. 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.
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.
At present, no options are valid for host keys.
-o
- Causes
ssh-keygen
to save SSH protocol
2 private keys using the new OpenSSH format rather than the more
compatible PEM format. 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
- 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
|
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, 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 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, 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 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.
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:
# 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/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.
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 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 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.
KEY REVOCATION LISTS¶
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 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 commandline. 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 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 public key authentication. There is
no need to keep the contents of this file secret.
- /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 versions 1.5 and
2.0.