| SSH-KEYGEN(1) | General Commands Manual | SSH-KEYGEN(1) |
ssh-keygen —
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 principals]
[-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 ... |
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have been revoked by one.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, 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, 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-a
rounds-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-b
bits-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-c-D
pkcs11-s, this option indicates that a CA key resides in
a PKCS#11 token (see the
CERTIFICATES section for
details).-E
fingerprint_hash-e-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-H option to print
found keys in a hashed format.-f
filename-G
output_file-T option) before use.-g-r command.-Hssh 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-I
certificate_identity-i-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-T option.-j
start_line-T option.-K
checkpt-T option. This will be used to skip lines in the
input file that have already been processed if the job is restarted.-kssh-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-lssh-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-m
key_format-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-n
principals-O
optionclearforce-command=commandno-agent-forwardingno-port-forwardingno-ptyno-user-rcno-x11-forwardingpermit-agent-forwardingpermit-port-forwardingpermit-ptypermit-user-rcpermit-x11-forwardingsource-address=address_listAt present, no options are valid for host keys.
-ossh-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-p-Q-qssh-keygen.-R
hostname-H option above).-r
hostname-S
start-s
ca_keyWhen 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-G option) for safety.-t
dsa |
ecdsa
|
ed25519
| rsa
|
rsa1-u-k, keys listed
via the command line are added to the existing KRL rather than a new KRL
being created.-V
validity_intervalFor 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).
-vssh-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-y-z
serial_numberWhen 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 2048By 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.candidatesBy 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.pubThe 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.pubThe 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.pubIn 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.pubAdditional 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]ssh-keygen command
line using the -s option.id:
key_idssh-keygen command line
using the -s option.key:
public_keysha1:
public_keyKRLs 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.
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-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.
The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
| July 3, 2015 | BSD |