|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | SYNOPSIS | DESCRIPTION | MODULI GENERATION | CERTIFICATES | FIDO AUTHENTICATOR | KEY REVOCATION LISTS | ALLOWED SIGNERS | ENVIRONMENT | FILES | SEE ALSO | AUTHORS | COLOPHON |
|
|
|
SSH-KEYGEN(1) General Commands Manual SSH-KEYGEN(1)
ssh-keygen — OpenSSH authentication key utility
ssh-keygen [-q] [-a rounds] [-b bits] [-C comment] [-f
output_keyfile] [-m format] [-N new_passphrase] [-O option] [-t
ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa] [-w provider] [-Z
cipher] ssh-keygen -p [-a rounds] [-f keyfile] [-m format] [-N
new_passphrase] [-P old_passphrase] [-Z cipher] ssh-keygen -i [-f
input_keyfile] [-m key_format] ssh-keygen -e [-f input_keyfile]
[-m key_format] ssh-keygen -y [-f input_keyfile] ssh-keygen -c [-a
rounds] [-C comment] [-f keyfile] [-P passphrase] ssh-keygen -l
[-v] [-E fingerprint_hash] [-f input_keyfile] ssh-keygen -B [-f
input_keyfile] ssh-keygen -D pkcs11 ssh-keygen -F hostname [-lv]
[-f known_hosts_file] ssh-keygen -H [-f known_hosts_file]
ssh-keygen -K [-a rounds] [-w provider] ssh-keygen -R hostname [-f
known_hosts_file] ssh-keygen -r hostname [-g] [-f input_keyfile]
ssh-keygen -M generate [-O option] output_file ssh-keygen -M
screen [-f input_file] [-O option] output_file ssh-keygen -I
certificate_identity -s ca_key [-hU] [-D pkcs11_provider] [-n
principals] [-O option] [-V validity_interval] [-z serial_number]
file ... ssh-keygen -L [-f input_keyfile] ssh-keygen -A [-a
rounds] [-f prefix_path] ssh-keygen -k -f krl_file [-u] [-s
ca_public] [-z version_number] file ... ssh-keygen -Q [-l] -f
krl_file file ... ssh-keygen -Y find-principals [-O option] -s
signature_file -f allowed_signers_file ssh-keygen -Y
match-principals -I signer_identity -f allowed_signers_file
ssh-keygen -Y check-novalidate [-O option] -n namespace -s
signature_file ssh-keygen -Y sign [-O option] -f key_file -n
namespace file ... ssh-keygen -Y verify [-O option] -f
allowed_signers_file -I signer_identity -n namespace -s
signature_file [-r revocation_file]
ssh-keygen generates, manages and converts authentication keys for
ssh(1). ssh-keygen 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 will generate an
Ed25519 key.
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/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
~/.ssh/id_ed25519_sk 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.
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
-c option.
It is still possible for ssh-keygen to 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)
flag.
After a key is generated, ssh-keygen will ask where the keys
should be placed to be activated.
The options are as follows:
-A Generate host keys of all default key types (rsa, ecdsa,
and ed25519) if they do not already exist. The host keys
are generated 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.
-a rounds
When saving a private key, this option specifies the
number of KDF (key derivation function, currently
bcrypt_pbkdf(3)) rounds used. Higher numbers result in
slower passphrase verification and increased resistance to
brute-force password cracking (should the keys be stolen).
The default is 16 rounds.
-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 1024 bits and the default is
3072 bits. Generally, 3072 bits is considered sufficient.
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. ECDSA-SK,
Ed25519 and Ed25519-SK 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. 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 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 fingerprint_hash
Specifies the hash algorithm used when displaying key
fingerprints. Valid options are: “md5” and “sha256”. The
default is “sha256”.
-e This option will read a private or public OpenSSH key file
and print to stdout a public 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 | [hostname]:port
Search for the specified hostname (with optional port
number) 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 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. See the “CERTIFICATES” section for
details.
-I certificate_identity
Specify the key identity when signing a public key. 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”.
-K Download resident keys from a FIDO authenticator. Public
and private key files will be written to the current
directory for each downloaded key. If multiple FIDO
authenticators are attached, keys will be downloaded from
the first touched authenticator. See the “FIDO
AUTHENTICATOR” section for more information.
-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 one or more certificates.
-l Show fingerprint of specified public key file. ssh-keygen
will try 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.
-M generate
Generate candidate Diffie-Hellman Group Exchange (DH-GEX)
parameters for eventual use by the
‘diffie-hellman-group-exchange-*’ key exchange methods.
The numbers generated by this operation must be further
screened before use. See the “MODULI GENERATION” section
for more information.
-M screen
Screen candidate parameters for Diffie-Hellman Group
Exchange. This will accept a list of candidate numbers
and test that they are safe (Sophie Germain) primes with
acceptable group generators. The results of this
operation may be added to the /etc/moduli file. See the
“MODULI GENERATION” section for more information.
-m key_format
Specify a key format for key generation, the -i (import),
-e (export) conversion options, and the -p change
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” (PKCS8 public or private
key) or “PEM” (PEM public key). By default OpenSSH will
write newly-generated private keys in its own format, but
when converting public keys for export the default 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.
-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. See the
“CERTIFICATES” section for details.
-O option
Specify a key/value option. These are specific to the
operation that ssh-keygen has been requested to perform.
When signing certificates, one of the options listed in
the “CERTIFICATES” section may be specified here.
When performing moduli generation or screening, one of the
options listed in the “MODULI GENERATION” section may be
specified.
When generating FIDO authenticator-backed keys, the
options listed in the “FIDO AUTHENTICATOR” section may be
specified.
When performing signature-related options using the -Y
flag, the following options are accepted:
hashalg=algorithm
Selects the hash algorithm to use for hashing the
message to be signed. Valid algorithms are
“sha256” and “sha512.” The default is “sha512.”
print-pubkey
Print the full public key to standard output after
signature verification.
verify-time=timestamp
Specifies a time to use when validating signatures
instead of the current time. The time may be
specified as a date or time in the YYYYMMDD[Z] or
in YYYYMMDDHHMM[SS][Z] formats. Dates and times
will be interpreted in the current system time
zone unless suffixed with a Z character, which
causes them to be interpreted in the UTC time
zone.
When generating SSHFP DNS records from public keys using
the -r flag, the following options are accepted:
hashalg=algorithm
Selects a hash algorithm to use when printing
SSHFP records using the -D flag. Valid algorithms
are “sha1” and “sha256”. The default is to print
both.
The -O option may be specified multiple times.
-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. If the -l
option is also specified then the contents of the KRL will
be printed.
-q Silence ssh-keygen.
-R hostname | [hostname]:port
Removes all keys belonging to the specified hostname (with
optional port number) 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 ca_key
Certify (sign) a public key using the specified CA key.
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 ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
Specifies the type of key to create. The possible values
are “ecdsa”, “ecdsa-sk”, “ed25519 (the default),”
“ed25519-sk”, or “rsa”.
This flag may also be used to specify the desired
signature type when signing certificates using an RSA CA
key. The available RSA signature variants are “ssh-rsa”
(SHA1 signatures, not recommended), “rsa-sha2-256”, and
“rsa-sha2-512” (the default for RSA keys).
-U When used in combination with -s or -Y sign, this option
indicates that a CA key resides in a ssh-agent(1). See
the “CERTIFICATES” section for more information.
-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:
• The string “always” to indicate the certificate has no
specified start time.
• A date or time in the system time zone formatted as
YYYYMMDD or YYYYMMDDHHMM[SS].
• A date or time in the UTC time zone as YYYYMMDDZ or
YYYYMMDDHHMM[SS]Z.
• A relative time before the current system time
consisting of a minus sign followed by an interval in
the format described in the TIME FORMATS section of
sshd_config(5).
• A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as
a hexadecimal number beginning with “0x”.
The end time may be specified similarly to the start time:
• The string “forever” to indicate the certificate has
no specified end time.
• A date or time in the system time zone formatted as
YYYYMMDD or YYYYMMDDHHMM[SS].
• A date or time in the UTC time zone as YYYYMMDDZ or
YYYYMMDDHHMM[SS]Z.
• A relative time after the current system time
consisting of a plus sign followed by an interval in
the format described in the TIME FORMATS section of
sshd_config(5).
• A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as
a hexadecimal number beginning with “0x”.
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.
20100101123000Z:20110101123000Z
Similar, but interpreted in the UTC time zone
rather than the system time zone.
-1d:20110101
Valid from yesterday to midnight, January 1st,
2011.
0x1:0x2000000000
Valid from roughly early 1970 to May 2033.
-1m:forever
Valid from one minute ago and never expiring.
-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 provider
Specifies a path to a library that will be used when
creating FIDO authenticator-hosted keys, overriding the
default of using the internal USB HID support.
-Y find-principals
Find the principal(s) associated with the public key of a
signature, provided using the -s flag in an authorized
signers file provided using the -f flag. The format of
the allowed signers file is documented in the “ALLOWED
SIGNERS” section below. If one or more matching
principals are found, they are returned on standard
output.
-Y match-principals
Find principal matching the principal name provided using
the -I flag in the authorized signers file specified using
the -f flag. If one or more matching principals are
found, they are returned on standard output.
-Y check-novalidate
Checks that a signature generated using ssh-keygen -Y sign
has a valid structure. This does not validate if a
signature comes from an authorized signer. When testing a
signature, ssh-keygen accepts a message on standard input
and a signature namespace using -n. A file containing the
corresponding signature must also be supplied using the -s
flag. Successful testing of the signature is signalled by
ssh-keygen returning a zero exit status.
-Y sign
Cryptographically sign a file or some data using an SSH
key. When signing, ssh-keygen accepts zero or more files
to sign on the command-line - if no files are specified
then ssh-keygen will sign data presented on standard
input. Signatures are written to the path of the input
file with “.sig” appended, or to standard output if the
message to be signed was read from standard input.
The key used for signing is specified using the -f option
and may refer to either a private key, or a public key
with the private half available via ssh-agent(1). An
additional signature namespace, used to prevent signature
confusion across different domains of use (e.g. file
signing vs email signing) must be provided via the -n
flag. Namespaces are arbitrary strings, and may include:
“file” for file signing, “email” for email signing. For
custom uses, it is recommended to use names following a
NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous
namespaces.
-Y verify
Request to verify a signature generated using ssh-keygen
-Y sign as described above. When verifying a signature,
ssh-keygen accepts a message on standard input and a
signature namespace using -n. A file containing the
corresponding signature must also be supplied using the -s
flag, along with the identity of the signer using -I and a
list of allowed signers via the -f flag. The format of
the allowed signers file is documented in the “ALLOWED
SIGNERS” section below. A file containing revoked keys
can be passed using the -r flag. The revocation file may
be a KRL or a one-per-line list of public keys.
Successful verification by an authorized signer is
signalled by ssh-keygen returning a zero exit status.
-y This option will read a private OpenSSH format file and
print an OpenSSH public key to stdout.
-Z cipher
Specifies the cipher to use for encryption when writing an
OpenSSH-format private key file. The list of available
ciphers may be obtained using "ssh -Q cipher". The
default is “aes256-ctr”.
-z serial_number
Specifies a serial number to be embedded in the
certificate to distinguish this certificate from others
from the same CA. If the serial_number is prefixed with a
‘+’ character, then the serial number will be incremented
for each certificate signed on a single command-line. The
default serial number is zero.
When 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 -M generate option.
The desired length of the primes may be specified by the -O bits
option. For example:
# ssh-keygen -M generate -O bits=2048 moduli-2048.candidates
By default, the search for primes begins at a random point in the
desired length range. This may be overridden using the -O start
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 -M
screen option. In this mode ssh-keygen will read candidates from
standard input (or a file specified using the -f option). For
example:
# ssh-keygen -M screen -f moduli-2048.candidates moduli-2048
By default, each candidate will be subjected to 100 primality
tests. This may be overridden using the -O prime-tests 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 -O generator 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.
A number of options are available for moduli generation and
screening via the -O flag:
lines=number
Exit after screening the specified number of lines while
performing DH candidate screening.
start-line=line-number
Start screening at the specified line number while
performing DH candidate screening.
checkpoint=filename
Write the last line processed to the specified file while
performing DH candidate screening. This will be used to
skip lines in the input file that have already been
processed if the job is restarted.
memory=mbytes
Specify the amount of memory to use (in megabytes) when
generating candidate moduli for DH-GEX.
start=hex-value
Specify start point (in hex) when generating candidate
moduli for DH-GEX.
generator=value
Specify desired generator (in decimal) when testing
candidate moduli for DH-GEX.
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
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.
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.
critical:name[=contents]
extension:name[=contents]
Includes an arbitrary certificate critical option or
extension. The specified name should include a domain
suffix, e.g. “name@example.com”. 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.
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.
no-touch-required
Do not require signatures made using this key include
demonstration of user presence (e.g. by having the user
touch the authenticator). This option only makes sense
for the FIDO authenticator algorithms ecdsa-sk and
ed25519-sk.
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.
verify-required
Require signatures made using this key indicate that the
user was first verified. This option only makes sense for
the FIDO authenticator algorithms ecdsa-sk and ed25519-sk.
Currently PIN authentication is the only supported
verification method, but other methods may be supported in
the future.
At present, no standard options are valid for host keys.
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 the 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). Refer to
those manual pages for details.
ssh-keygen is able to generate FIDO authenticator-backed keys,
after which they may be used much like any other key type
supported by OpenSSH, so long as the hardware authenticator is
attached when the keys are used. FIDO authenticators generally
require the user to explicitly authorise operations by touching or
tapping them. FIDO keys consist of two parts: a key handle part
stored in the private key file on disk, and a per-device private
key that is unique to each FIDO authenticator and that cannot be
exported from the authenticator hardware. These are combined by
the hardware at authentication time to derive the real key that is
used to sign authentication challenges. Supported key types are
ecdsa-sk and ed25519-sk.
The options that are valid for FIDO keys are:
application
Override the default FIDO application/origin string of
“ssh:”. This may be useful when generating host or
domain-specific resident keys. The specified application
string must begin with “ssh:”.
challenge=path
Specifies a path to a challenge string that will be passed
to the FIDO authenticator during key generation. The
challenge string may be used as part of an out-of-band
protocol for key enrollment (a random challenge is used by
default).
device Explicitly specify a fido(4) device to use, rather than
letting the authenticator middleware select one.
no-touch-required
Indicate that the generated private key should not require
touch events (user presence) when making signatures. Note
that sshd(8) will refuse such signatures by default,
unless overridden via an authorized_keys option.
resident
Indicate that the key handle should be stored on the FIDO
authenticator itself. This makes it easier to use the
authenticator on multiple computers. Resident keys may be
supported on FIDO2 authenticators and typically require
that a PIN be set on the authenticator prior to
generation. Resident keys may be loaded off the
authenticator using ssh-add(1). Storing both parts of a
key on a FIDO authenticator increases the likelihood of an
attacker being able to use a stolen authenticator device.
user A username to be associated with a resident key,
overriding the empty default username. Specifying a
username may be useful when generating multiple resident
keys for the same application name.
verify-required
Indicate that this private key should require user
verification for each signature. Not all FIDO
authenticators support this option. Currently PIN
authentication is the only supported verification method,
but other methods may be supported in the future.
write-attestation=path
May be used at key generation time to record the
attestation data returned from FIDO authenticators during
key generation. This information is potentially
sensitive. By default, this information is discarded.
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 including its SHA1 hash in
the KRL.
sha256: public_key
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.
hash: fingerprint
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 -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 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.
When verifying signatures, ssh-keygen uses a simple list of
identities and keys to determine whether a signature comes from an
authorized source. This "allowed signers" file uses a format
patterned after the AUTHORIZED_KEYS FILE FORMAT described in
sshd(8). Each line of the file contains the following space-
separated fields: principals, options, keytype, base64-encoded
key. Empty lines and lines starting with a ‘#’ are ignored as
comments.
The principals field is a pattern-list (see PATTERNS in
ssh_config(5)) consisting of one or more comma-separated
USER@DOMAIN identity patterns that are accepted for signing. When
verifying, the identity presented via the -I option must match a
principals pattern in order for the corresponding key to be
considered acceptable for verification.
The options (if present) consist of comma-separated option
specifications. No spaces are permitted, except within double
quotes. The following option specifications are supported (note
that option keywords are case-insensitive):
cert-authority
Indicates that this key is accepted as a certificate
authority (CA) and that certificates signed by this CA may
be accepted for verification.
namespaces=namespace-list
Specifies a pattern-list of namespaces that are accepted
for this key. If this option is present, the signature
namespace embedded in the signature object and presented
on the verification command-line must match the specified
list before the key will be considered acceptable.
valid-after=timestamp
Indicates that the key is valid for use at or after the
specified timestamp, which may be a date or time in the
YYYYMMDD[Z] or YYYYMMDDHHMM[SS][Z] formats. Dates and
times will be interpreted in the current system time zone
unless suffixed with a Z character, which causes them to
be interpreted in the UTC time zone.
valid-before=timestamp
Indicates that the key is valid for use at or before the
specified timestamp.
When verifying signatures made by certificates, the expected
principal name must match both the principals pattern in the
allowed signers file and the principals embedded in the
certificate itself.
An example allowed signers file:
# Comments allowed at start of line
user1@example.com,user2@example.com ssh-rsa AAAAX1...
# A certificate authority, trusted for all principals in a domain.
*@example.com cert-authority ssh-ed25519 AAAB4...
# A key that is accepted only for file signing.
user2@example.com namespaces="file" ssh-ed25519 AAA41...
SSH_SK_PROVIDER
Specifies a path to a library that will be used when
loading any FIDO authenticator-hosted keys, overriding the
default of using the built-in USB HID support.
~/.ssh/id_ecdsa
~/.ssh/id_ecdsa_sk
~/.ssh/id_ed25519
~/.ssh/id_ed25519_sk
~/.ssh/id_rsa
Contains the ECDSA, authenticator-hosted ECDSA, Ed25519,
authenticator-hosted 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_ecdsa.pub
~/.ssh/id_ecdsa_sk.pub
~/.ssh/id_ed25519.pub
~/.ssh/id_ed25519_sk.pub
~/.ssh/id_rsa.pub
Contains the ECDSA, authenticator-hosted ECDSA, Ed25519,
authenticator-hosted 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/moduli
Contains Diffie-Hellman groups used for DH-GEX. The file
format is described in moduli(5).
ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8) The Secure
Shell (SSH) Public Key File Format, RFC 4716, 2006.
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.
This page is part of the openssh (Portable OpenSSH) project.
Information about the project can be found at
http://www.openssh.com/portable.html. If you have a bug report
for this manual page, see ⟨http://www.openssh.com/report.html⟩.
This page was obtained from the tarball openssh-9.9p2.tar.gz
fetched from
⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
GNU August 17, 2024 SSH-KEYGEN(1)
Pages that refer to this page: systemd-vmspawn(1)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|