NAME¶
ssh-keygen —
authentication key
generation, management and conversion
SYNOPSIS¶
ssh-keygen |
[-q]
[-b bits]
[-t type]
[-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Revocation Lists, and to test whether given keys have been revoked by one.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.
- -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.
This option allows importing keys from other software, including several
commercial SSH implementations. The default import format is
“RFC4716”.
- -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
type
- 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.