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 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Lists, and to test whether given keys have been revoked by one.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_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/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". 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:
+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.

-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.

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 /etc/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_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/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 August 17, 2024 illumos