SSH-KEYGEN(1) User Commands SSH-KEYGEN(1)

NAME

ssh-keygen - OpenSSH authentication key utility

SYNOPSIS

ssh-keygen [-q] [-a rounds] [-b bits] [-C comment] [-f output_keyfile]
[-m format] [-N new_passphrase] [-O option]
[-t dsa | 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]

DESCRIPTION

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Lists, and to test whether given keys have been revoked by one.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_dsa,
~/.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 /lib/svc/method/sshd
to generate new host keys.

-a rounds
When saving a new-format private key (i.e. an ed25519 key or 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). 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. 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.
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.
This operation is only supported for keys stored in the newer
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 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. For RSA and DSA
keys ssh-keygen tries 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
/usr/share/lib/ssh/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".

-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.
Causes ssh-keygen to save 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.
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 new passphrase.
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.
Silence ssh-keygen.
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).
Print the SSHFP fingerprint resource record named hostname for the
specified public key file.
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.
Specifies the type of key to create. The possible values are "dsa",
"ecdsa", "ecdsa-sk", "ed25519", "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).
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.
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 interval.

The start time may be specified as:
+o The string "always" to indicate the certificate has no specified start
time.
+o A date or time in the system time zone formatted as YYYYMMDD or
YYYYMMDDHHMM[SS].
+o A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z.
+o 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).
+o 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:
+o The string "forever" to indicate the certificate has no specified end
time.
+o A date or time in the system time zone formatted as YYYYMMDD or
YYYYMMDDHHMM[SS].
+o A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z.
+o 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).
+o 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.
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.
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.
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.
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.
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.
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.
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.
This option will read a private OpenSSH format file and print an OpenSSH
public key to stdout.
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".
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.

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 -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 /usr/share/lib/ssh/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.

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 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.

FIDO AUTHENTICATOR

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.

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 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.

ALLOWED SIGNERS

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...

ENVIRONMENT

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.

FILES

~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ecdsa_sk
~/.ssh/id_ed25519
~/.ssh/id_ed25519_sk
~/.ssh/id_rsa
Contains the DSA, 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_dsa.pub
~/.ssh/id_ecdsa.pub
~/.ssh/id_ecdsa_sk.pub
~/.ssh/id_ed25519.pub
~/.ssh/id_ed25519_sk.pub
~/.ssh/id_rsa.pub
Contains the DSA, 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.

/usr/share/lib/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.

illumos September 4, 2023 illumos