— Authentication key
generation, management and conversion
[-t dsa | ecdsa | ed25519 | rsa]
[-z serial_number] file
-k -f krl_file
[-z version_number] file
krl_file file ...
generates, manages and converts authentication keys
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
generate an RSA key.
By default key is stored in PEM format. Option
] could be used to convert private key from
proprietary in more portable and widely used PEM format. Note if key contain
X.509 certificate PEM format is required. For those keys request to store in
proprietary format is ignored.
is also used to generate groups for use in
Diffie-Hellman group exchange (DH-GEX). See the
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Revocation Lists, and to test whether given keys have been revoked by one.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
. 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
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 proprietary 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
After a key is generated, instructions below detail where the keys should be
placed to be activated.
The options are as follows:
- 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.
- When saving a private key in OpenSSH proprietary format
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
When screening DH-GEX candidates (using the -T command).
This option specifies the number of primality tests to perform.
- Show the bubblebabble digest of specified private or public
- 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
- Provides a new comment.
- Requests changing the comment in the private and public key
files. This operation is only supported for keys stored in the proprietary
OpenSSH format. The program will prompt for the file containing the
private keys, for the passphrase if the key has one, and for the new
- Download the X.509 certificate provided by the PKCS#11
shared library pkcs11.
- Specifies the hash algorithm used when displaying key
fingerprints. Valid options are: “md5” and
“sha256”. The default is “sha256”.
- This option will read a private or public 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 keys for use by other
programs, including several commercial SSH implementations.
- 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.
- Specifies the filename of the key file.
- Generate candidate primes for DH-GEX. These primes must be
screened for safety (using the -T option) before
- Use generic DNS format when printing fingerprint resource
records using the -r command.
- 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.
- When signing a key, create a host certificate instead of a
user certificate. Please see the
CERTIFICATES section for
- Specify the key identity when signing a public key. Please
see the CERTIFICATES section for
- This option will read an unencrypted private (or public)
key file in the format specified by the -m option and
print an 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”.
- Exit after screening the specified number of lines while
performing DH candidate screening using the -T
- Start screening at the specified line number while
performing DH candidate screening using the -T
- 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
- 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
- Prints the contents of one or more certificates.
- Show fingerprint of public key read from specified file.
For all key types ssh-keygen tries to find file with the
matching public key and prints its fingerprint. When identity contain
X.509 certificate public key is extracted from it and fingerprint is
printed for public key. If combined with -v, a visual
ASCII art representation of the key is supplied with the fingerprint.
- Specify the amount of memory to use (in megabytes) when
generating candidate moduli for DH-GEX.
- 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 portable
PEM private key format.
- Provides the new passphrase.
- 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
- Specify a certificate option when signing a key. This
option may be specified multiple times. See also the
CERTIFICATES section for further
At present, no standard options are valid for host keys. The options that
are valid for user certificates are:
- Clear all enabled permissions. This is useful for
clearing the default set of permissions so permissions may be added
- Includes an arbitrary certificate critical option or
extension. The specified name should include a
domain suffix, e.g. “firstname.lastname@example.org”. 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.
- Forces the execution of command
instead of any shell or command specified by the user when the
certificate is used for authentication.
- Disable ssh-agent(1) forwarding
(permitted by default).
- Disable port forwarding (permitted by default).
- Disable PTY allocation (permitted by default).
- Disable execution of ~/.ssh/rc by
sshd(8) (permitted by default).
- Disable X11 forwarding (permitted by default).
- Allows ssh-agent(1) forwarding.
- Allows port forwarding.
- Allows PTY allocation.
- Allows execution of ~/.ssh/rc by
- Allows X11 forwarding.
- 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.
- Causes ssh-keygen to save private keys
using the proprietary OpenSSH format rather than the more compatible PEM
format. Ed25519 keys always use the proprietary key format.
- Provides the (old) passphrase.
- 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
- Test whether keys have been revoked in a KRL.
- Silence ssh-keygen.
- Removes all keys belonging to
hostname from a known_hosts file.
This option is useful to delete hashed hosts (see the -H
- Print the SSHFP fingerprint resource record named
hostname for the specified public key file. In case
of X.509 certificates print CERT resource record.
- Specify start point (in hex) when generating candidate
moduli for DH-GEX.
- Certify (sign) a public key using the specified CA key.
Please see the CERTIFICATES section
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.
- Test DH group exchange candidate primes (generated using
the -G option) for safety.
dsa | ecdsa | ed25519
- Specifies the type of key to create. The possible values
are “dsa”, “ecdsa”, “ed25519”, or
- 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
- 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.
- 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
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
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
- 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.
- Specify desired generator when testing candidate moduli for
- This option will read a private key file and print to
stdout an public key in OpenSSH format. For this option keyfile name could
use prefixes “engine:” or “store:” to load
identifies using engine or store functionality provided by cryptographic
library. For more details see configuration option
IdentityFile in ssh_config(5).
- 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.
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
desired length of the primes may be specified by the -b
option. For example:
# ssh-keygen -G moduli-2048.candidates -b
By default, the search for primes begins at a random point in the desired length
range. This may be overridden using the -S
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
this mode ssh-keygen
will read candidates from standard
input (or a file specified using the -f
# ssh-keygen -T moduli-2048 -f
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 [APPDATA]/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.
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 limited, format to the X.509
certificates used in ssl(8)
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
The resultant certificate will be placed in
. A host certificate requires the
$ ssh-keygen -s /path/to/ca_key -I key_id -h
The host certificate will be output to
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
$ ssh-keygen -s ca_key -I key_id -h -n
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
Finally, certificates may be defined with a validity lifetime. The
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
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)
Please refer to those manual pages for details.
KEY REVOCATION LISTS
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.
- 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
- 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.
- Revokes the specified key. If a certificate is listed, then
it is revoked as a plain public key.
- Revokes the specified key by including its SHA1 hash in the
- 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.
- 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
. 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.
- Contains the DSA, ECDSA, Ed25519 or RSA authentication
identity of the user.
This file should not be readable by anyone but the user.
For RSA, ECDSA or DSA identity file may contain X.509 certificate that match
it. In addition file may contain extra X.509 certificates. Extra
certificates along with certificates from X.509 store are used to build
chain of certificates leading to a trusted certificate authority if
required by public key algorithm format.
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.
- 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. If file ~/.ssh/id_rsa,
~/.ssh/id_ecdsa or ~/.ssh/id_dsa
contain RSA/ECDSA/DSA private key and X.509 certificates public key file
must contain certificate that match private key! Use
ssh-keygen with option -y to regenerate its content.
Note in case with X.509 certificate you can append content to
~/.ssh/authorized_keys or to add certificate
“Distinguished Name” / “Subject” in corresponding
format to “authorized keys” file. See
sshd(8). Using distinguished name is preffered as X.509
certificate could be renewed and after renewal there is no need to update
“authorized keys” file.
- Contains Diffie-Hellman groups used for DH-GEX. The file
format is described in moduli(5).
J. Galbraith and R.
Thayer, The Secure Shell (SSH) Public Key File
Format, RFC 4716, November
PKIX-SSH 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.
Roumen Petrov contributed support for X.509 certificates.