|SSH-KEYGEN(1)||General Commands Manual||SSH-KEYGEN(1)|
ssh-keygengenerates, manages and converts authentication keys for ssh(1).
ssh-keygencan 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
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have been revoked by one.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
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
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.
ssh-keygen will by default write keys in
an OpenSSH-specific format. This format is preferred as it offers better
protection for keys at rest as well as allowing storage of key comments
within the private key file itself. The key comment may be useful to help
identify the key. The comment is initialized to “user@host”
when the key is created, but can be changed using the
It is still possible for
write the previously-used PEM format private keys using the
-m flag. This may be used when generating new keys,
and existing new-format keys may be converted using this option in
conjunction with the
-p (change passphrase)
After a key is generated, instructions below detail where the keys should be placed to be activated.
The options are as follows:
-fhas 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.
When screening DH-GEX candidates (using the
-T command). This option specifies the number of
primality tests to perform.
-bflag 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
-bflag will be ignored.
-s, this option indicates that a CA key resides in a PKCS#11 token (see the CERTIFICATES section for details).
-moption. The default export format is “RFC4716”. This option allows exporting OpenSSH keys for use by other programs, including several commercial SSH implementations.
-Fhostname | [hostname]:port
-Hoption to print found keys in a hashed format.
-Toption) before use.
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.
-moption 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”.
-Toption. This will be used to skip lines in the input file that have already been processed if the job is restarted.
ssh-keygenwill generate a KRL file at the location specified via the
-fflag 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.
ssh-keygentries 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.
-e(export) conversion options, and the
-pchange passphrase operation. The latter may be used to convert between OpenSSH private key and PEM private key formats. 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.
At present, no standard options are valid for host keys. The options that are valid for user certificates are:
-Rhostname | [hostname]:port
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.
-Goption) for safety.
-s, this option indicates that a CA key resides in a ssh-agent(1). See the CERTIFICATES section for more information.
-k, keys listed via the command line are added to the existing KRL rather than a new KRL being created.
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).
ssh-keygento print debugging messages about its progress. This is helpful for debugging moduli generation. Multiple
-voptions increase the verbosity. The maximum is 3.
When generating a KRL, the
-z flag is
used to specify a KRL version number.
ssh-keygenmay 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
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
# 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
The DH generator value will be chosen automatically for the prime under
consideration. If a specific generator is desired, it may be requested using
-W option. Valid generator values are 2, 3, and
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-keygensupports 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
$ 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
$ 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.
ssh-keygenis 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
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.
ssh-keygencommand line using the
ssh-keygencommand line using the
-lflag. 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
-k. When this option is specified, keys
listed via the command line are merged into the KRL, adding to those already
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-keygenbut 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.
|January 23, 2019||OpenBSD-current|