openssh/ssh-keygen.1

.\"	$OpenBSD: ssh-keygen.1,v 1.193 2020/01/18 21:16:43 naddy Exp $
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
.\"                    All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose.  Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\"
.\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    notice, this list of conditions and the following disclaimer.
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.Dd $Mdocdate: January 18 2020 $
.Dt SSH-KEYGEN 1
.Os
.Sh NAME
.Nm ssh-keygen
.Nd OpenSSH authentication key utility
.Sh SYNOPSIS
.Nm ssh-keygen
.Op Fl q
.Op Fl b Ar bits
.Op Fl C Ar comment
.Op Fl f Ar output_keyfile
.Op Fl m Ar format
.Op Fl t Cm dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
.Op Fl N Ar new_passphrase
.Op Fl O Ar option
.Op Fl w Ar provider
.Nm ssh-keygen
.Fl p
.Op Fl f Ar keyfile
.Op Fl m Ar format
.Op Fl N Ar new_passphrase
.Op Fl P Ar old_passphrase
.Nm ssh-keygen
.Fl i
.Op Fl f Ar input_keyfile
.Op Fl m Ar key_format
.Nm ssh-keygen
.Fl e
.Op Fl f Ar input_keyfile
.Op Fl m Ar key_format
.Nm ssh-keygen
.Fl y
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl c
.Op Fl C Ar comment
.Op Fl f Ar keyfile
.Op Fl P Ar passphrase
.Nm ssh-keygen
.Fl l
.Op Fl v
.Op Fl E Ar fingerprint_hash
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl B
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl D Ar pkcs11
.Nm ssh-keygen
.Fl F Ar hostname
.Op Fl lv
.Op Fl f Ar known_hosts_file
.Nm ssh-keygen
.Fl H
.Op Fl f Ar known_hosts_file
.Nm ssh-keygen
.Fl K
.Op Fl w Ar provider
.Nm ssh-keygen
.Fl R Ar hostname
.Op Fl f Ar known_hosts_file
.Nm ssh-keygen
.Fl r Ar hostname
.Op Fl g
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl M Cm generate
.Op Fl O Ar option
.Ar output_file
.Nm ssh-keygen
.Fl M Cm screen
.Op Fl f Ar input_file
.Op Fl O Ar option
.Ar output_file
.Nm ssh-keygen
.Fl I Ar certificate_identity
.Fl s Ar ca_key
.Op Fl hU
.Op Fl D Ar pkcs11_provider
.Op Fl n Ar principals
.Op Fl O Ar option
.Op Fl V Ar validity_interval
.Op Fl z Ar serial_number
.Ar
.Nm ssh-keygen
.Fl L
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl A
.Op Fl f Ar prefix_path
.Nm ssh-keygen
.Fl k
.Fl f Ar krl_file
.Op Fl u
.Op Fl s Ar ca_public
.Op Fl z Ar version_number
.Ar
.Nm ssh-keygen
.Fl Q
.Fl f Ar krl_file
.Ar
.Nm ssh-keygen
.Fl Y Cm check-novalidate
.Fl n Ar namespace
.Fl s Ar signature_file
.Nm ssh-keygen
.Fl Y Cm sign
.Fl f Ar key_file
.Fl n Ar namespace
.Ar
.Nm ssh-keygen
.Fl Y Cm verify
.Fl f Ar allowed_signers_file
.Fl I Ar signer_identity
.Fl n Ar namespace
.Fl s Ar signature_file
.Op Fl r Ar revocation_file
.Sh DESCRIPTION
.Nm
generates, manages and converts authentication keys for
.Xr ssh 1 .
.Nm
can create keys for use by SSH protocol version 2.
.Pp
The type of key to be generated is specified with the
.Fl t
option.
If invoked without any arguments,
.Nm
will generate an RSA key.
.Pp
.Nm
is also used to generate groups for use in Diffie-Hellman group
exchange (DH-GEX).
See the
.Sx MODULI GENERATION
section for details.
.Pp
Finally,
.Nm
can be used to generate and update Key Revocation Lists, and to test whether
given keys have been revoked by one.
See the
.Sx KEY REVOCATION LISTS
section for details.
.Pp
Normally each user wishing to use SSH
with public key authentication runs this once to create the authentication
key in
.Pa ~/.ssh/id_dsa ,
.Pa ~/.ssh/id_ecdsa ,
.Pa ~/.ssh/id_ecdsa_sk ,
.Pa ~/.ssh/id_ed25519 ,
.Pa ~/.ssh/id_ed25519_sk
or
.Pa ~/.ssh/id_rsa .
Additionally, the system administrator may use this to generate host keys,
as seen in
.Pa /etc/rc .
.Pp
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
.Dq .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
.Fl p
option.
.Pp
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.
.Pp
.Nm
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
.Dq user@host
when the key is created, but can be changed using the
.Fl c
option.
.Pp
It is still possible for
.Nm
to write the previously-used PEM format private keys using the
.Fl 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
.Fl p
(change passphrase) flag.
.Pp
After a key is generated, instructions below detail where the keys
should be placed to be activated.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl A
For each of the key types (rsa, dsa, ecdsa and ed25519)
for which host keys
do not exist, generate the host keys with the default key file path,
an empty passphrase, default bits for the key type, and default comment.
If
.Fl 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
.Pa /etc/rc
to generate new host keys.
.It Fl a Ar rounds
When saving a private key, 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).
.It Fl B
Show the bubblebabble digest of specified private or public key file.
.It Fl b Ar 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
.Fl 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
.Fl b
flag will be ignored.
.It Fl C Ar comment
Provides a new comment.
.It Fl 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.
.It Fl D Ar pkcs11
Download the public keys provided by the PKCS#11 shared library
.Ar pkcs11 .
When used in combination with
.Fl s ,
this option indicates that a CA key resides in a PKCS#11 token (see the
.Sx CERTIFICATES
section for details).
.It Fl E Ar fingerprint_hash
Specifies the hash algorithm used when displaying key fingerprints.
Valid options are:
.Dq md5
and
.Dq sha256 .
The default is
.Dq sha256 .
.It Fl 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
.Fl m
option.
The default export format is
.Dq RFC4716 .
This option allows exporting OpenSSH keys for use by other programs, including
several commercial SSH implementations.
.It Fl F Ar hostname | [hostname]:port
Search for the specified
.Ar hostname
(with optional port number)
in a
.Pa 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
.Fl H
option to print found keys in a hashed format.
.It Fl f Ar filename
Specifies the filename of the key file.
.It Fl g
Use generic DNS format when printing fingerprint resource records using the
.Fl r
command.
.It Fl H
Hash a
.Pa 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
.Nm ssh
and
.Nm 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.
.It Fl h
When signing a key, create a host certificate instead of a user
certificate.
Please see the
.Sx CERTIFICATES
section for details.
.It Fl I Ar certificate_identity
Specify the key identity when signing a public key.
Please see the
.Sx CERTIFICATES
section for details.
.It Fl i
This option will read an unencrypted private (or public) key file
in the format specified by the
.Fl 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
.Dq RFC4716 .
.It Fl K
Download resident keys from a FIDO authenticator.
Public and private key files will be written to the current directory for
each downloaded key.
.It Fl k
Generate a KRL file.
In this mode,
.Nm
will generate a KRL file at the location specified via the
.Fl 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
.Sx KEY REVOCATION LISTS
section.
.It Fl L
Prints the contents of one or more certificates.
.It Fl l
Show fingerprint of specified public key file.
For RSA and DSA keys
.Nm
tries to find the matching public key file and prints its fingerprint.
If combined with
.Fl v ,
a visual ASCII art representation of the key is supplied with the
fingerprint.
.It Fl M Cm generate
Generate candidate Diffie-Hellman Group Exchange (DH-GEX) parameters for
eventual use by the
.Sq diffie-hellman-group-exchange-*
key exchange methods.
The numbers generated by this operation must be further screened before
use.
See the
.Sx MODULI GENERATION
section for more information.
.It Fl M Cm 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
.Pa /etc/moduli
file.
See the
.Sx MODULI GENERATION
section for more information.
.It Fl m Ar key_format
Specify a key format for key generation, the
.Fl i
(import),
.Fl e
(export) conversion options, and the
.Fl 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:
.Dq RFC4716
(RFC 4716/SSH2 public or private key),
.Dq PKCS8
(PKCS8 public or private key)
or
.Dq 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
.Dq RFC4716 .
Setting a format of
.Dq PEM
when generating or updating a supported private key type will cause the
key to be stored in the legacy PEM private key format.
.It Fl N Ar new_passphrase
Provides the new passphrase.
.It Fl n Ar 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.
Please see the
.Sx CERTIFICATES
section for details.
.It Fl O Ar option
Specify a key/value option.
These are specific to the operation that
.Nm
has been requested to perform.
.Pp
When signing certificates, one of the options listed in the
.Sx CERTIFICATES
section may be specified here.
.Pp
When performing moduli generation or screening, one of the options
listed in the
.Sx MODULI GENERATION
section may be specified.
.Pp
When generating a key that will be hosted on a FIDO authenticator,
this flag may be used to specify key-specific options.
Those supported at present are:
.Bl -tag -width Ds
.It Cm application
Override the default FIDO application/origin string of
.Dq ssh: .
This may be useful when generating host or domain-specific resident keys.
.It Cm device
Explicitly specify a
.Xr fido 4
device to use, rather than letting the token middleware select one.
.It Cm no-touch-required
Indicate that the generated private key should not require touch
events (user presence) when making signatures.
Note that
.Xr sshd 8
will refuse such signatures by default, unless overridden via
an authorized_keys option.
.It Cm resident
Indicate that the key should be stored on the FIDO authenticator itself.
Resident keys may be supported on FIDO2 tokens and typically require that
a PIN be set on the token prior to generation.
Resident keys may be loaded off the token using
.Xr ssh-add 1 .
.It Cm 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.
.El
.Pp
The
.Fl O
option may be specified multiple times.
.It Fl P Ar passphrase
Provides the (old) passphrase.
.It Fl 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.
.It Fl Q
Test whether keys have been revoked in a KRL.
.It Fl q
Silence
.Nm ssh-keygen .
.It Fl R Ar hostname | [hostname]:port
Removes all keys belonging to the specified
.Ar hostname
(with optional port number)
from a
.Pa known_hosts
file.
This option is useful to delete hashed hosts (see the
.Fl H
option above).
.It Fl r Ar hostname
Print the SSHFP fingerprint resource record named
.Ar hostname
for the specified public key file.
.It Fl s Ar ca_key
Certify (sign) a public key using the specified CA key.
Please see the
.Sx CERTIFICATES
section for details.
.Pp
When generating a KRL,
.Fl s
specifies a path to a CA public key file used to revoke certificates directly
by key ID or serial number.
See the
.Sx KEY REVOCATION LISTS
section for details.
.It Fl t Cm dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
Specifies the type of key to create.
The possible values are
.Dq dsa ,
.Dq ecdsa ,
.Dq ecdsa-sk ,
.Dq ed25519 ,
.Dq ed25519-sk ,
or
.Dq rsa .
.Pp
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
.Dq ssh-rsa
(SHA1 signatures, not recommended),
.Dq rsa-sha2-256 ,
and
.Dq rsa-sha2-512
(the default).
.It Fl U
When used in combination with
.Fl s ,
this option indicates that a CA key resides in a
.Xr ssh-agent 1 .
See the
.Sx CERTIFICATES
section for more information.
.It Fl u
Update a KRL.
When specified with
.Fl k ,
keys listed via the command line are added to the existing KRL rather than
a new KRL being created.
.It Fl V Ar 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.
.Pp
The start time may be specified as the string
.Dq always
to indicate the certificate has no specified start time,
a date in YYYYMMDD format, a time in YYYYMMDDHHMM[SS] format,
a relative time (to the current time) consisting of a minus sign followed by
an interval in the format described in the
TIME FORMATS section of
.Xr sshd_config 5 .
.Pp
The end time may be specified as a YYYYMMDD date, a YYYYMMDDHHMM[SS] time,
a relative time starting with a plus character or the string
.Dq forever
to indicate that the certificate has no expirty date.
.Pp
For example:
.Dq +52w1d
(valid from now to 52 weeks and one day from now),
.Dq -4w:+4w
(valid from four weeks ago to four weeks from now),
.Dq 20100101123000:20110101123000
(valid from 12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011),
.Dq -1d:20110101
(valid from yesterday to midnight, January 1st, 2011).
.Dq -1m:forever
(valid from one minute ago and never expiring).
.It Fl v
Verbose mode.
Causes
.Nm
to print debugging messages about its progress.
This is helpful for debugging moduli generation.
Multiple
.Fl v
options increase the verbosity.
The maximum is 3.
.It Fl w Ar 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.
.It Fl Y Cm check-novalidate
Checks that a signature generated using
.Nm
.Fl Y Cm sign
has a valid structure.
This does not validate if a signature comes from an authorized signer.
When testing a signature,
.Nm
accepts a message on standard input and a signature namespace using
.Fl n .
A file containing the corresponding signature must also be supplied using the
.Fl s
flag.
Successful testing of the signature is signalled by
.Nm
returning a zero exit status.
.It Fl Y Cm sign
Cryptographically sign a file or some data using a SSH key.
When signing,
.Nm
accepts zero or more files to sign on the command-line - if no files
are specified then
.Nm
will sign data presented on standard input.
Signatures are written to the path of the input file with
.Dq .sig
appended, or to standard output if the message to be signed was read from
standard input.
.Pp
The key used for signing is specified using the
.Fl f
option and may refer to either a private key, or a public key with the private
half available via
.Xr 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
.Fl n
flag.
Namespaces are arbitrary strings, and may include:
.Dq file
for file signing,
.Dq email
for email signing.
For custom uses, it is recommended to use names following a
NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces.
.It Fl Y Cm verify
Request to verify a signature generated using
.Nm
.Fl Y Cm sign
as described above.
When verifying a signature,
.Nm
accepts a message on standard input and a signature namespace using
.Fl n .
A file containing the corresponding signature must also be supplied using the
.Fl s
flag, along with the identity of the signer using
.Fl I
and a list of allowed signers via the
.Fl f
flag.
The format of the allowed signers file is documented in the
.Sx ALLOWED SIGNERS
section below.
A file containing revoked keys can be passed using the
.Fl 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
.Nm
returning a zero exit status.
.It Fl y
This option will read a private
OpenSSH format file and print an OpenSSH public key to stdout.
.It Fl z Ar serial_number
Specifies a serial number to be embedded in the certificate to distinguish
this certificate from others from the same CA.
If the
.Ar serial_number
is prefixed with a
.Sq +
character, then the serial number will be incremented for each certificate
signed on a single command-line.
The default serial number is zero.
.Pp
When generating a KRL, the
.Fl z
flag is used to specify a KRL version number.
.El
.Sh MODULI GENERATION
.Nm
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).
.Pp
Generation of primes is performed using the
.Fl M Cm generate
option.
The desired length of the primes may be specified by the
.Fl O Cm bits
option.
For example:
.Pp
.Dl # ssh-keygen -M generate -O bits=2048 moduli-2048.candidates
.Pp
By default, the search for primes begins at a random point in the
desired length range.
This may be overridden using the
.Fl O Cm start
option, which specifies a different start point (in hex).
.Pp
Once a set of candidates have been generated, they must be screened for
suitability.
This may be performed using the
.Fl M Cm screen
option.
In this mode
.Nm
will read candidates from standard input (or a file specified using the
.Fl f
option).
For example:
.Pp
.Dl # ssh-keygen -M screen -f moduli-2048.candidates moduli-2048
.Pp
By default, each candidate will be subjected to 100 primality tests.
This may be overridden using the
.Fl O Cm 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
.Fl O Cm generator
option.
Valid generator values are 2, 3, and 5.
.Pp
Screened DH groups may be installed in
.Pa /etc/moduli .
It is important that this file contains moduli of a range of bit lengths and
that both ends of a connection share common moduli.
.Pp
A number of options are available for moduli generation and screening via the
.Fl O
flag:
.Bl -tag -width Ds
.It Ic lines Ns = Ns Ar number
Exit after screening the specified number of lines while performing DH
candidate screening.
.It Ic start-line Ns = Ns Ar line-number
Start screening at the specified line number while performing DH candidate
screening.
.It Ic checkpoint Ns = Ns Ar 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.
.It Ic memory Ns = Ns Ar mbytes
Specify the amount of memory to use (in megabytes) when generating
candidate moduli for DH-GEX.
.It Ic start Ns = Ns Ar hex-value
Specify start point (in hex) when generating candidate moduli for DH-GEX.
.It Ic generator Ns = Ns Ar value
Specify desired generator (in decimal) when testing candidate moduli for DH-GEX.
.El
.Sh CERTIFICATES
.Nm
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
.Xr ssl 8 .
.Pp
.Nm
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:
.Pp
.Dl $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
.Pp
The resultant certificate will be placed in
.Pa /path/to/user_key-cert.pub .
A host certificate requires the
.Fl h
option:
.Pp
.Dl $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
.Pp
The host certificate will be output to
.Pa /path/to/host_key-cert.pub .
.Pp
It is possible to sign using a CA key stored in a PKCS#11 token by
providing the token library using
.Fl D
and identifying the CA key by providing its public half as an argument
to
.Fl s :
.Pp
.Dl $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub
.Pp
Similarly, it is possible for the CA key to be hosted in a
.Xr ssh-agent 1 .
This is indicated by the
.Fl U
flag and, again, the CA key must be identified by its public half.
.Pp
.Dl $ ssh-keygen -Us ca_key.pub -I key_id user_key.pub
.Pp
In all cases,
.Ar key_id
is a "key identifier" that is logged by the server when the certificate
is used for authentication.
.Pp
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:
.Pp
.Dl $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
.Dl "$ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub"
.Pp
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.
.Pp
The options that are valid for user certificates are:
.Pp
.Bl -tag -width Ds -compact
.It Ic clear
Clear all enabled permissions.
This is useful for clearing the default set of permissions so permissions may
be added individually.
.Pp
.It Ic critical : Ns Ar name Ns Op Ns = Ns Ar contents
.It Ic extension : Ns Ar name Ns Op Ns = Ns Ar contents
Includes an arbitrary certificate critical option or extension.
The specified
.Ar name
should include a domain suffix, e.g.\&
.Dq name@example.com .
If
.Ar 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.
.Pp
.It Ic force-command Ns = Ns Ar command
Forces the execution of
.Ar command
instead of any shell or command specified by the user when
the certificate is used for authentication.
.Pp
.It Ic no-agent-forwarding
Disable
.Xr ssh-agent 1
forwarding (permitted by default).
.Pp
.It Ic no-port-forwarding
Disable port forwarding (permitted by default).
.Pp
.It Ic no-pty
Disable PTY allocation (permitted by default).
.Pp
.It Ic no-user-rc
Disable execution of
.Pa ~/.ssh/rc
by
.Xr sshd 8
(permitted by default).
.Pp
.It Ic no-x11-forwarding
Disable X11 forwarding (permitted by default).
.Pp
.It Ic permit-agent-forwarding
Allows
.Xr ssh-agent 1
forwarding.
.Pp
.It Ic permit-port-forwarding
Allows port forwarding.
.Pp
.It Ic permit-pty
Allows PTY allocation.
.Pp
.It Ic permit-user-rc
Allows execution of
.Pa ~/.ssh/rc
by
.Xr sshd 8 .
.Pp
.It Ic permit-X11-forwarding
Allows X11 forwarding.
.Pp
.It Ic no-touch-required
Do not require signatures made using this key require demonstration
of user presence (e.g. by having the user touch the authenticator).
This option only makes sense for the FIDO authenticator algorithms
.Cm ecdsa-sk
and
.Cm ed25519-sk .
.Pp
.It Ic source-address Ns = Ns Ar address_list
Restrict the source addresses from which the certificate is considered valid.
The
.Ar address_list
is a comma-separated list of one or more address/netmask pairs in CIDR
format.
.El
.Pp
At present, no standard options are valid for host keys.
.Pp
Finally, certificates may be defined with a validity lifetime.
The
.Fl 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
.Ux
Epoch to the distant future.
.Pp
For certificates to be used for user or host authentication, the CA
public key must be trusted by
.Xr sshd 8
or
.Xr ssh 1 .
Please refer to those manual pages for details.
.Sh KEY REVOCATION LISTS
.Nm
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.
.Pp
KRLs may be generated using the
.Fl 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).
.Pp
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.
.Bl -tag -width Ds
.It Cm serial : Ar serial_number Ns Op - Ns Ar 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
.Nm
command line using the
.Fl s
option.
.It Cm id : Ar key_id
Revokes a certificate with the specified key ID string.
The CA key must have been specified on the
.Nm
command line using the
.Fl s
option.
.It Cm key : Ar public_key
Revokes the specified key.
If a certificate is listed, then it is revoked as a plain public key.
.It Cm sha1 : Ar public_key
Revokes the specified key by including its SHA1 hash in the KRL.
.It Cm sha256 : Ar 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.
.It Cm hash : Ar fingerprint
Revokes a key using a fingerprint hash, as obtained from a
.Xr sshd 8
authentication log message or the
.Nm
.Fl l
flag.
Only SHA256 fingerprints are supported here and resultant KRLs are
not supported by OpenSSH versions prior to 7.9.
.El
.Pp
KRLs may be updated using the
.Fl u
flag in addition to
.Fl k .
When this option is specified, keys listed via the command line are merged into
the KRL, adding to those already there.
.Pp
It is also possible, given a KRL, to test whether it revokes a particular key
(or keys).
The
.Fl 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
.Nm
will exit with a non-zero exit status.
A zero exit status will only be returned if no key was revoked.
.Sh ALLOWED SIGNERS
When verifying signatures,
.Nm
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
.Xr 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
.Ql #
are ignored as comments.
.Pp
The principals field is a pattern-list (See PATTERNS in
.Xr 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
.Fl I
option must match a principals pattern in order for the corresponding key to be
considered acceptable for verification.
.Pp
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):
.Bl -tag -width Ds
.It Cm 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.
.It Cm 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.
.El
.Pp
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.
.Pp
An example allowed signers file:
.Bd -literal -offset 3n
# 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...
.Ed
.Sh ENVIRONMENT
.Bl -tag -width Ds
.It Ev SSH_SK_PROVIDER
Specifies the path to a library used to interact with FIDO authenticators.
.El
.Sh FILES
.Bl -tag -width Ds -compact
.It Pa ~/.ssh/id_dsa
.It Pa ~/.ssh/id_ecdsa
.It Pa ~/.ssh/id_ecdsa_sk
.It Pa ~/.ssh/id_ed25519
.It Pa ~/.ssh/id_ed25519_sk
.It Pa ~/.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
.Nm
but it is offered as the default file for the private key.
.Xr ssh 1
will read this file when a login attempt is made.
.Pp
.It Pa ~/.ssh/id_dsa.pub
.It Pa ~/.ssh/id_ecdsa.pub
.It Pa ~/.ssh/id_ecdsa_sk.pub
.It Pa ~/.ssh/id_ed25519.pub
.It Pa ~/.ssh/id_ed25519_sk.pub
.It Pa ~/.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
.Pa ~/.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.
.Pp
.It Pa /etc/moduli
Contains Diffie-Hellman groups used for DH-GEX.
The file format is described in
.Xr moduli 5 .
.El
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr moduli 5 ,
.Xr sshd 8
.Rs
.%R RFC 4716
.%T "The Secure Shell (SSH) Public Key File Format"
.%D 2006
.Re
.Sh 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.