1 .\" $OpenBSD: ssh-keygen.1,v 1.228 2023/02/10 06:40:48 jmc Exp $
3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
14 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
15 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
16 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
18 .\" Redistribution and use in source and binary forms, with or without
19 .\" modification, are permitted provided that the following conditions
21 .\" 1. Redistributions of source code must retain the above copyright
22 .\" notice, this list of conditions and the following disclaimer.
23 .\" 2. Redistributions in binary form must reproduce the above copyright
24 .\" notice, this list of conditions and the following disclaimer in the
25 .\" documentation and/or other materials provided with the distribution.
27 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
28 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
29 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
30 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
31 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
32 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
33 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
34 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
35 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
36 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
38 .Dd $Mdocdate: February 10 2023 $
43 .Nd OpenSSH authentication key utility
50 .Op Fl f Ar output_keyfile
52 .Op Fl N Ar new_passphrase
54 .Op Fl t Cm dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
62 .Op Fl N Ar new_passphrase
63 .Op Fl P Ar old_passphrase
67 .Op Fl f Ar input_keyfile
68 .Op Fl m Ar key_format
71 .Op Fl f Ar input_keyfile
72 .Op Fl m Ar key_format
75 .Op Fl f Ar input_keyfile
81 .Op Fl P Ar passphrase
85 .Op Fl E Ar fingerprint_hash
86 .Op Fl f Ar input_keyfile
89 .Op Fl f Ar input_keyfile
95 .Op Fl f Ar known_hosts_file
98 .Op Fl f Ar known_hosts_file
105 .Op Fl f Ar known_hosts_file
109 .Op Fl f Ar input_keyfile
116 .Op Fl f Ar input_file
120 .Fl I Ar certificate_identity
123 .Op Fl D Ar pkcs11_provider
124 .Op Fl n Ar principals
126 .Op Fl V Ar validity_interval
127 .Op Fl z Ar serial_number
131 .Op Fl f Ar input_keyfile
135 .Op Fl f Ar prefix_path
140 .Op Fl s Ar ca_public
141 .Op Fl z Ar version_number
149 .Fl Y Cm find-principals
151 .Fl s Ar signature_file
152 .Fl f Ar allowed_signers_file
154 .Fl Y Cm match-principals
155 .Fl I Ar signer_identity
156 .Fl f Ar allowed_signers_file
158 .Fl Y Cm check-novalidate
161 .Fl s Ar signature_file
171 .Fl f Ar allowed_signers_file
172 .Fl I Ar signer_identity
174 .Fl s Ar signature_file
175 .Op Fl r Ar revocation_file
178 generates, manages and converts authentication keys for
181 can create keys for use by SSH protocol version 2.
183 The type of key to be generated is specified with the
186 If invoked without any arguments,
188 will generate an RSA key.
191 is also used to generate groups for use in Diffie-Hellman group
194 .Sx MODULI GENERATION
199 can be used to generate and update Key Revocation Lists, and to test whether
200 given keys have been revoked by one.
202 .Sx KEY REVOCATION LISTS
205 Normally each user wishing to use SSH
206 with public key authentication runs this once to create the authentication
209 .Pa ~/.ssh/id_ecdsa ,
210 .Pa ~/.ssh/id_ecdsa_sk ,
211 .Pa ~/.ssh/id_ed25519 ,
212 .Pa ~/.ssh/id_ed25519_sk
215 Additionally, the system administrator may use this to generate host keys,
219 Normally this program generates the key and asks for a file in which
220 to store the private key.
221 The public key is stored in a file with the same name but
224 The program also asks for a passphrase.
225 The passphrase may be empty to indicate no passphrase
226 (host keys must have an empty passphrase), or it may be a string of
228 A passphrase is similar to a password, except it can be a phrase with a
229 series of words, punctuation, numbers, whitespace, or any string of
231 Good passphrases are 10-30 characters long, are
232 not simple sentences or otherwise easily guessable (English
233 prose has only 1-2 bits of entropy per character, and provides very bad
234 passphrases), and contain a mix of upper and lowercase letters,
235 numbers, and non-alphanumeric characters.
236 The passphrase can be changed later by using the
240 There is no way to recover a lost passphrase.
241 If the passphrase is lost or forgotten, a new key must be generated
242 and the corresponding public key copied to other machines.
245 will by default write keys in an OpenSSH-specific format.
246 This format is preferred as it offers better protection for
247 keys at rest as well as allowing storage of key comments within
248 the private key file itself.
249 The key comment may be useful to help identify the key.
250 The comment is initialized to
252 when the key is created, but can be changed using the
256 It is still possible for
258 to write the previously-used PEM format private keys using the
261 This may be used when generating new keys, and existing new-format
262 keys may be converted using this option in conjunction with the
264 (change passphrase) flag.
266 After a key is generated,
268 will ask where the keys
269 should be placed to be activated.
271 The options are as follows:
274 Generate host keys of all default key types (rsa, ecdsa, and
275 ed25519) if they do not already exist.
276 The host keys are generated with the default key file path,
277 an empty passphrase, default bits for the key type, and default comment.
280 has also been specified, its argument is used as a prefix to the
281 default path for the resulting host key files.
284 to generate new host keys.
286 When saving a private key, this option specifies the number of KDF
287 (key derivation function, currently
290 Higher numbers result in slower passphrase verification and increased
291 resistance to brute-force password cracking (should the keys be stolen).
292 The default is 16 rounds.
294 Show the bubblebabble digest of specified private or public key file.
296 Specifies the number of bits in the key to create.
297 For RSA keys, the minimum size is 1024 bits and the default is 3072 bits.
298 Generally, 3072 bits is considered sufficient.
299 DSA keys must be exactly 1024 bits as specified by FIPS 186-2.
302 flag determines the key length by selecting from one of three elliptic
303 curve sizes: 256, 384 or 521 bits.
304 Attempting to use bit lengths other than these three values for ECDSA keys
306 ECDSA-SK, Ed25519 and Ed25519-SK keys have a fixed length and the
308 flag will be ignored.
310 Provides a new comment.
312 Requests changing the comment in the private and public key files.
313 The program will prompt for the file containing the private keys, for
314 the passphrase if the key has one, and for the new comment.
316 Download the public keys provided by the PKCS#11 shared library
318 When used in combination with
320 this option indicates that a CA key resides in a PKCS#11 token (see the
322 section for details).
323 .It Fl E Ar fingerprint_hash
324 Specifies the hash algorithm used when displaying key fingerprints.
332 This option will read a private or public OpenSSH key file and
333 print to stdout a public key in one of the formats specified by the
336 The default export format is
338 This option allows exporting OpenSSH keys for use by other programs, including
339 several commercial SSH implementations.
340 .It Fl F Ar hostname | [hostname]:port
341 Search for the specified
343 (with optional port number)
346 file, listing any occurrences found.
347 This option is useful to find hashed host names or addresses and may also be
348 used in conjunction with the
350 option to print found keys in a hashed format.
352 Specifies the filename of the key file.
354 Use generic DNS format when printing fingerprint resource records using the
361 This replaces all hostnames and addresses with hashed representations
362 within the specified file; the original content is moved to a file with
364 These hashes may be used normally by
368 but they do not reveal identifying information should the file's contents
370 This option will not modify existing hashed hostnames and is therefore safe
371 to use on files that mix hashed and non-hashed names.
373 When signing a key, create a host certificate instead of a user
378 .It Fl I Ar certificate_identity
379 Specify the key identity when signing a public key.
384 This option will read an unencrypted private (or public) key file
385 in the format specified by the
387 option and print an OpenSSH compatible private
388 (or public) key to stdout.
389 This option allows importing keys from other software, including several
390 commercial SSH implementations.
391 The default import format is
394 Download resident keys from a FIDO authenticator.
395 Public and private key files will be written to the current directory for
397 If multiple FIDO authenticators are attached, keys will be downloaded from
398 the first touched authenticator.
400 .Sx FIDO AUTHENTICATOR
401 section for more information.
406 will generate a KRL file at the location specified via the
408 flag that revokes every key or certificate presented on the command line.
409 Keys/certificates to be revoked may be specified by public key file or
410 using the format described in the
411 .Sx KEY REVOCATION LISTS
414 Prints the contents of one or more certificates.
416 Show fingerprint of specified public key file.
419 tries to find the matching public key file and prints its fingerprint.
422 a visual ASCII art representation of the key is supplied with the
425 Generate candidate Diffie-Hellman Group Exchange (DH-GEX) parameters for
427 .Sq diffie-hellman-group-exchange-*
428 key exchange methods.
429 The numbers generated by this operation must be further screened before
432 .Sx MODULI GENERATION
433 section for more information.
435 Screen candidate parameters for Diffie-Hellman Group Exchange.
436 This will accept a list of candidate numbers and test that they are
437 safe (Sophie Germain) primes with acceptable group generators.
438 The results of this operation may be added to the
442 .Sx MODULI GENERATION
443 section for more information.
444 .It Fl m Ar key_format
445 Specify a key format for key generation, the
449 (export) conversion options, and the
451 change passphrase operation.
452 The latter may be used to convert between OpenSSH private key and PEM
454 The supported key formats are:
456 (RFC 4716/SSH2 public or private key),
458 (PKCS8 public or private key)
462 By default OpenSSH will write newly-generated private keys in its own
463 format, but when converting public keys for export the default format is
467 when generating or updating a supported private key type will cause the
468 key to be stored in the legacy PEM private key format.
469 .It Fl N Ar new_passphrase
470 Provides the new passphrase.
471 .It Fl n Ar principals
472 Specify one or more principals (user or host names) to be included in
473 a certificate when signing a key.
474 Multiple principals may be specified, separated by commas.
479 Specify a key/value option.
480 These are specific to the operation that
482 has been requested to perform.
484 When signing certificates, one of the options listed in the
486 section may be specified here.
488 When performing moduli generation or screening, one of the options
490 .Sx MODULI GENERATION
491 section may be specified.
493 When generating FIDO authenticator-backed keys, the options listed in the
494 .Sx FIDO AUTHENTICATOR
495 section may be specified.
497 When performing signature-related options using the
499 flag, the following options are accepted:
501 .It Cm hashalg Ns = Ns Ar algorithm
502 Selects the hash algorithm to use for hashing the message to be signed.
510 Print the full public key to standard output after signature verification.
511 .It Cm verify-time Ns = Ns Ar timestamp
512 Specifies a time to use when validating signatures instead of the current
514 The time may be specified as a date or time in the YYYYMMDD[Z] or
515 in YYYYMMDDHHMM[SS][Z] formats.
516 Dates and times will be interpreted in the current system time zone unless
517 suffixed with a Z character, which causes them to be interpreted in the
521 When generating SSHFP DNS records from public keys using the
523 flag, the following options are accepted:
525 .It Cm hashalg Ns = Ns Ar algorithm
526 Selects a hash algorithm to use when printing SSHFP records using the
533 The default is to print both.
538 option may be specified multiple times.
539 .It Fl P Ar passphrase
540 Provides the (old) passphrase.
542 Requests changing the passphrase of a private key file instead of
543 creating a new private key.
544 The program will prompt for the file
545 containing the private key, for the old passphrase, and twice for the
548 Test whether keys have been revoked in a KRL.
551 option is also specified then the contents of the KRL will be printed.
555 .It Fl R Ar hostname | [hostname]:port
556 Removes all keys belonging to the specified
558 (with optional port number)
562 This option is useful to delete hashed hosts (see the
566 Print the SSHFP fingerprint resource record named
568 for the specified public key file.
570 Certify (sign) a public key using the specified CA key.
575 When generating a KRL,
577 specifies a path to a CA public key file used to revoke certificates directly
578 by key ID or serial number.
580 .Sx KEY REVOCATION LISTS
582 .It Fl t Cm dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
583 Specifies the type of key to create.
584 The possible values are
593 This flag may also be used to specify the desired signature type when
594 signing certificates using an RSA CA key.
595 The available RSA signature variants are
597 (SHA1 signatures, not recommended),
603 When used in combination with
607 this option indicates that a CA key resides in a
611 section for more information.
616 keys listed via the command line are added to the existing KRL rather than
617 a new KRL being created.
618 .It Fl V Ar validity_interval
619 Specify a validity interval when signing a certificate.
620 A validity interval may consist of a single time, indicating that the
621 certificate is valid beginning now and expiring at that time, or may consist
622 of two times separated by a colon to indicate an explicit time interval.
624 The start time may be specified as:
629 to indicate the certificate has no specified start time.
631 A date or time in the system time zone formatted as YYYYMMDD or
634 A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z.
636 A relative time before the current system time consisting of a minus sign
637 followed by an interval in the format described in the
638 TIME FORMATS section of
641 A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a hexadecimal
642 number beginning with
646 The end time may be specified similarly to the start time:
651 to indicate the certificate has no specified end time.
653 A date or time in the system time zone formatted as YYYYMMDD or
656 A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z.
658 A relative time after the current system time consisting of a plus sign
659 followed by an interval in the format described in the
660 TIME FORMATS section of
663 A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a hexadecimal
664 number beginning with
671 Valid from now to 52 weeks and one day from now.
673 Valid from four weeks ago to four weeks from now.
674 .It 20100101123000:20110101123000
675 Valid from 12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011.
676 .It 20100101123000Z:20110101123000Z
677 Similar, but interpreted in the UTC time zone rather than the system time zone.
679 Valid from yesterday to midnight, January 1st, 2011.
681 Valid from roughly early 1970 to May 2033.
683 Valid from one minute ago and never expiring.
689 to print debugging messages about its progress.
690 This is helpful for debugging moduli generation.
693 options increase the verbosity.
696 Specifies a path to a library that will be used when creating
697 FIDO authenticator-hosted keys, overriding the default of using
698 the internal USB HID support.
699 .It Fl Y Cm find-principals
700 Find the principal(s) associated with the public key of a signature,
703 flag in an authorized signers file provided using the
706 The format of the allowed signers file is documented in the
709 If one or more matching principals are found, they are returned on
711 .It Fl Y Cm match-principals
712 Find principal matching the principal name provided using the
714 flag in the authorized signers file specified using the
717 If one or more matching principals are found, they are returned on
719 .It Fl Y Cm check-novalidate
720 Checks that a signature generated using
723 has a valid structure.
724 This does not validate if a signature comes from an authorized signer.
725 When testing a signature,
727 accepts a message on standard input and a signature namespace using
729 A file containing the corresponding signature must also be supplied using the
732 Successful testing of the signature is signalled by
734 returning a zero exit status.
736 Cryptographically sign a file or some data using a SSH key.
739 accepts zero or more files to sign on the command-line - if no files
742 will sign data presented on standard input.
743 Signatures are written to the path of the input file with
745 appended, or to standard output if the message to be signed was read from
748 The key used for signing is specified using the
750 option and may refer to either a private key, or a public key with the private
753 An additional signature namespace, used to prevent signature confusion across
754 different domains of use (e.g. file signing vs email signing) must be provided
758 Namespaces are arbitrary strings, and may include:
763 For custom uses, it is recommended to use names following a
764 NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces.
766 Request to verify a signature generated using
770 When verifying a signature,
772 accepts a message on standard input and a signature namespace using
774 A file containing the corresponding signature must also be supplied using the
776 flag, along with the identity of the signer using
778 and a list of allowed signers via the
781 The format of the allowed signers file is documented in the
784 A file containing revoked keys can be passed using the
787 The revocation file may be a KRL or a one-per-line list of public keys.
788 Successful verification by an authorized signer is signalled by
790 returning a zero exit status.
792 This option will read a private
793 OpenSSH format file and print an OpenSSH public key to stdout.
795 Specifies the cipher to use for encryption when writing an OpenSSH-format
797 The list of available ciphers may be obtained using
801 .It Fl z Ar serial_number
802 Specifies a serial number to be embedded in the certificate to distinguish
803 this certificate from others from the same CA.
808 character, then the serial number will be incremented for each certificate
809 signed on a single command-line.
810 The default serial number is zero.
812 When generating a KRL, the
814 flag is used to specify a KRL version number.
816 .Sh MODULI GENERATION
818 may be used to generate groups for the Diffie-Hellman Group Exchange
820 Generating these groups is a two-step process: first, candidate
821 primes are generated using a fast, but memory intensive process.
822 These candidate primes are then tested for suitability (a CPU-intensive
825 Generation of primes is performed using the
828 The desired length of the primes may be specified by the
833 .Dl # ssh-keygen -M generate -O bits=2048 moduli-2048.candidates
835 By default, the search for primes begins at a random point in the
836 desired length range.
837 This may be overridden using the
839 option, which specifies a different start point (in hex).
841 Once a set of candidates have been generated, they must be screened for
843 This may be performed using the
848 will read candidates from standard input (or a file specified using the
853 .Dl # ssh-keygen -M screen -f moduli-2048.candidates moduli-2048
855 By default, each candidate will be subjected to 100 primality tests.
856 This may be overridden using the
859 The DH generator value will be chosen automatically for the
860 prime under consideration.
861 If a specific generator is desired, it may be requested using the
864 Valid generator values are 2, 3, and 5.
866 Screened DH groups may be installed in
868 It is important that this file contains moduli of a range of bit lengths.
870 A number of options are available for moduli generation and screening via the
874 .It Ic lines Ns = Ns Ar number
875 Exit after screening the specified number of lines while performing DH
877 .It Ic start-line Ns = Ns Ar line-number
878 Start screening at the specified line number while performing DH candidate
880 .It Ic checkpoint Ns = Ns Ar filename
881 Write the last line processed to the specified file while performing DH
883 This will be used to skip lines in the input file that have already been
884 processed if the job is restarted.
885 .It Ic memory Ns = Ns Ar mbytes
886 Specify the amount of memory to use (in megabytes) when generating
887 candidate moduli for DH-GEX.
888 .It Ic start Ns = Ns Ar hex-value
889 Specify start point (in hex) when generating candidate moduli for DH-GEX.
890 .It Ic generator Ns = Ns Ar value
891 Specify desired generator (in decimal) when testing candidate moduli for DH-GEX.
895 supports signing of keys to produce certificates that may be used for
896 user or host authentication.
897 Certificates consist of a public key, some identity information, zero or
898 more principal (user or host) names and a set of options that
899 are signed by a Certification Authority (CA) key.
900 Clients or servers may then trust only the CA key and verify its signature
901 on a certificate rather than trusting many user/host keys.
902 Note that OpenSSH certificates are a different, and much simpler, format to
903 the X.509 certificates used in
907 supports two types of certificates: user and host.
908 User certificates authenticate users to servers, whereas host certificates
909 authenticate server hosts to users.
910 To generate a user certificate:
912 .Dl $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
914 The resultant certificate will be placed in
915 .Pa /path/to/user_key-cert.pub .
916 A host certificate requires the
920 .Dl $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
922 The host certificate will be output to
923 .Pa /path/to/host_key-cert.pub .
925 It is possible to sign using a CA key stored in a PKCS#11 token by
926 providing the token library using
928 and identifying the CA key by providing its public half as an argument
932 .Dl $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub
934 Similarly, it is possible for the CA key to be hosted in a
936 This is indicated by the
938 flag and, again, the CA key must be identified by its public half.
940 .Dl $ ssh-keygen -Us ca_key.pub -I key_id user_key.pub
944 is a "key identifier" that is logged by the server when the certificate
945 is used for authentication.
947 Certificates may be limited to be valid for a set of principal (user/host)
949 By default, generated certificates are valid for all users or hosts.
950 To generate a certificate for a specified set of principals:
952 .Dl $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
953 .Dl "$ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub"
955 Additional limitations on the validity and use of user certificates may
956 be specified through certificate options.
957 A certificate option may disable features of the SSH session, may be
958 valid only when presented from particular source addresses or may
959 force the use of a specific command.
961 The options that are valid for user certificates are:
963 .Bl -tag -width Ds -compact
965 Clear all enabled permissions.
966 This is useful for clearing the default set of permissions so permissions may
967 be added individually.
969 .It Ic critical : Ns Ar name Ns Op Ns = Ns Ar contents
970 .It Ic extension : Ns Ar name Ns Op Ns = Ns Ar contents
971 Includes an arbitrary certificate critical option or extension.
974 should include a domain suffix, e.g.\&
975 .Dq name@example.com .
978 is specified then it is included as the contents of the extension/option
979 encoded as a string, otherwise the extension/option is created with no
980 contents (usually indicating a flag).
981 Extensions may be ignored by a client or server that does not recognise them,
982 whereas unknown critical options will cause the certificate to be refused.
984 .It Ic force-command Ns = Ns Ar command
985 Forces the execution of
987 instead of any shell or command specified by the user when
988 the certificate is used for authentication.
990 .It Ic no-agent-forwarding
993 forwarding (permitted by default).
995 .It Ic no-port-forwarding
996 Disable port forwarding (permitted by default).
999 Disable PTY allocation (permitted by default).
1002 Disable execution of
1006 (permitted by default).
1008 .It Ic no-x11-forwarding
1009 Disable X11 forwarding (permitted by default).
1011 .It Ic permit-agent-forwarding
1016 .It Ic permit-port-forwarding
1017 Allows port forwarding.
1020 Allows PTY allocation.
1022 .It Ic permit-user-rc
1028 .It Ic permit-X11-forwarding
1029 Allows X11 forwarding.
1031 .It Ic no-touch-required
1032 Do not require signatures made using this key include demonstration
1033 of user presence (e.g. by having the user touch the authenticator).
1034 This option only makes sense for the FIDO authenticator algorithms
1039 .It Ic source-address Ns = Ns Ar address_list
1040 Restrict the source addresses from which the certificate is considered valid.
1043 is a comma-separated list of one or more address/netmask pairs in CIDR
1046 .It Ic verify-required
1047 Require signatures made using this key indicate that the user was first
1049 This option only makes sense for the FIDO authenticator algorithms
1053 Currently PIN authentication is the only supported verification method,
1054 but other methods may be supported in the future.
1057 At present, no standard options are valid for host keys.
1059 Finally, certificates may be defined with a validity lifetime.
1062 option allows specification of certificate start and end times.
1063 A certificate that is presented at a time outside this range will not be
1065 By default, certificates are valid from the
1067 Epoch to the distant future.
1069 For certificates to be used for user or host authentication, the CA
1070 public key must be trusted by
1074 Refer to those manual pages for details.
1075 .Sh FIDO AUTHENTICATOR
1077 is able to generate FIDO authenticator-backed keys, after which
1078 they may be used much like any other key type supported by OpenSSH, so
1079 long as the hardware authenticator is attached when the keys are used.
1080 FIDO authenticators generally require the user to explicitly authorise
1081 operations by touching or tapping them.
1082 FIDO keys consist of two parts: a key handle part stored in the
1083 private key file on disk, and a per-device private key that is unique
1084 to each FIDO authenticator and that cannot be exported from the
1085 authenticator hardware.
1086 These are combined by the hardware at authentication time to derive
1087 the real key that is used to sign authentication challenges.
1088 Supported key types are
1093 The options that are valid for FIDO keys are:
1096 Override the default FIDO application/origin string of
1098 This may be useful when generating host or domain-specific resident keys.
1099 The specified application string must begin with
1101 .It Cm challenge Ns = Ns Ar path
1102 Specifies a path to a challenge string that will be passed to the
1103 FIDO authenticator during key generation.
1104 The challenge string may be used as part of an out-of-band
1105 protocol for key enrollment
1106 (a random challenge is used by default).
1108 Explicitly specify a
1110 device to use, rather than letting the authenticator middleware select one.
1111 .It Cm no-touch-required
1112 Indicate that the generated private key should not require touch
1113 events (user presence) when making signatures.
1116 will refuse such signatures by default, unless overridden via
1117 an authorized_keys option.
1119 Indicate that the key handle should be stored on the FIDO
1120 authenticator itself.
1121 This makes it easier to use the authenticator on multiple computers.
1122 Resident keys may be supported on FIDO2 authenticators and typically
1123 require that a PIN be set on the authenticator prior to generation.
1124 Resident keys may be loaded off the authenticator using
1126 Storing both parts of a key on a FIDO authenticator increases the likelihood
1127 of an attacker being able to use a stolen authenticator device.
1129 A username to be associated with a resident key,
1130 overriding the empty default username.
1131 Specifying a username may be useful when generating multiple resident keys
1132 for the same application name.
1133 .It Cm verify-required
1134 Indicate that this private key should require user verification for
1136 Not all FIDO authenticators support this option.
1137 Currently PIN authentication is the only supported verification method,
1138 but other methods may be supported in the future.
1139 .It Cm write-attestation Ns = Ns Ar path
1140 May be used at key generation time to record the attestation data
1141 returned from FIDO authenticators during key generation.
1142 This information is potentially sensitive.
1143 By default, this information is discarded.
1145 .Sh KEY REVOCATION LISTS
1147 is able to manage OpenSSH format Key Revocation Lists (KRLs).
1148 These binary files specify keys or certificates to be revoked using a
1149 compact format, taking as little as one bit per certificate if they are being
1150 revoked by serial number.
1152 KRLs may be generated using the
1155 This option reads one or more files from the command line and generates a new
1157 The files may either contain a KRL specification (see below) or public keys,
1158 listed one per line.
1159 Plain public keys are revoked by listing their hash or contents in the KRL and
1160 certificates revoked by serial number or key ID (if the serial is zero or
1163 Revoking keys using a KRL specification offers explicit control over the
1164 types of record used to revoke keys and may be used to directly revoke
1165 certificates by serial number or key ID without having the complete original
1166 certificate on hand.
1167 A KRL specification consists of lines containing one of the following directives
1168 followed by a colon and some directive-specific information.
1170 .It Cm serial : Ar serial_number Ns Op - Ns Ar serial_number
1171 Revokes a certificate with the specified serial number.
1172 Serial numbers are 64-bit values, not including zero and may be expressed
1173 in decimal, hex or octal.
1174 If two serial numbers are specified separated by a hyphen, then the range
1175 of serial numbers including and between each is revoked.
1176 The CA key must have been specified on the
1178 command line using the
1181 .It Cm id : Ar key_id
1182 Revokes a certificate with the specified key ID string.
1183 The CA key must have been specified on the
1185 command line using the
1188 .It Cm key : Ar public_key
1189 Revokes the specified key.
1190 If a certificate is listed, then it is revoked as a plain public key.
1191 .It Cm sha1 : Ar public_key
1192 Revokes the specified key by including its SHA1 hash in the KRL.
1193 .It Cm sha256 : Ar public_key
1194 Revokes the specified key by including its SHA256 hash in the KRL.
1195 KRLs that revoke keys by SHA256 hash are not supported by OpenSSH versions
1197 .It Cm hash : Ar fingerprint
1198 Revokes a key using a fingerprint hash, as obtained from a
1200 authentication log message or the
1204 Only SHA256 fingerprints are supported here and resultant KRLs are
1205 not supported by OpenSSH versions prior to 7.9.
1208 KRLs may be updated using the
1212 When this option is specified, keys listed via the command line are merged into
1213 the KRL, adding to those already there.
1215 It is also possible, given a KRL, to test whether it revokes a particular key
1219 flag will query an existing KRL, testing each key specified on the command line.
1220 If any key listed on the command line has been revoked (or an error encountered)
1223 will exit with a non-zero exit status.
1224 A zero exit status will only be returned if no key was revoked.
1226 When verifying signatures,
1228 uses a simple list of identities and keys to determine whether a signature
1229 comes from an authorized source.
1230 This "allowed signers" file uses a format patterned after the
1231 AUTHORIZED_KEYS FILE FORMAT described in
1233 Each line of the file contains the following space-separated fields:
1234 principals, options, keytype, base64-encoded key.
1235 Empty lines and lines starting with a
1237 are ignored as comments.
1239 The principals field is a pattern-list (see PATTERNS in
1241 consisting of one or more comma-separated USER@DOMAIN identity patterns
1242 that are accepted for signing.
1243 When verifying, the identity presented via the
1245 option must match a principals pattern in order for the corresponding key to be
1246 considered acceptable for verification.
1248 The options (if present) consist of comma-separated option specifications.
1249 No spaces are permitted, except within double quotes.
1250 The following option specifications are supported (note that option keywords
1251 are case-insensitive):
1253 .It Cm cert-authority
1254 Indicates that this key is accepted as a certificate authority (CA) and
1255 that certificates signed by this CA may be accepted for verification.
1256 .It Cm namespaces Ns = Ns "namespace-list"
1257 Specifies a pattern-list of namespaces that are accepted for this key.
1258 If this option is present, the signature namespace embedded in the
1259 signature object and presented on the verification command-line must
1260 match the specified list before the key will be considered acceptable.
1261 .It Cm valid-after Ns = Ns "timestamp"
1262 Indicates that the key is valid for use at or after the specified timestamp,
1263 which may be a date or time in the YYYYMMDD[Z] or YYYYMMDDHHMM[SS][Z] formats.
1264 Dates and times will be interpreted in the current system time zone unless
1265 suffixed with a Z character, which causes them to be interpreted in the UTC
1267 .It Cm valid-before Ns = Ns "timestamp"
1268 Indicates that the key is valid for use at or before the specified timestamp.
1271 When verifying signatures made by certificates, the expected principal
1272 name must match both the principals pattern in the allowed signers file and
1273 the principals embedded in the certificate itself.
1275 An example allowed signers file:
1276 .Bd -literal -offset 3n
1277 # Comments allowed at start of line
1278 user1@example.com,user2@example.com ssh-rsa AAAAX1...
1279 # A certificate authority, trusted for all principals in a domain.
1280 *@example.com cert-authority ssh-ed25519 AAAB4...
1281 # A key that is accepted only for file signing.
1282 user2@example.com namespaces="file" ssh-ed25519 AAA41...
1286 .It Ev SSH_SK_PROVIDER
1287 Specifies a path to a library that will be used when loading any
1288 FIDO authenticator-hosted keys, overriding the default of using
1289 the built-in USB HID support.
1292 .Bl -tag -width Ds -compact
1293 .It Pa ~/.ssh/id_dsa
1294 .It Pa ~/.ssh/id_ecdsa
1295 .It Pa ~/.ssh/id_ecdsa_sk
1296 .It Pa ~/.ssh/id_ed25519
1297 .It Pa ~/.ssh/id_ed25519_sk
1298 .It Pa ~/.ssh/id_rsa
1299 Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519,
1300 authenticator-hosted Ed25519 or RSA authentication identity of the user.
1301 This file should not be readable by anyone but the user.
1303 specify a passphrase when generating the key; that passphrase will be
1304 used to encrypt the private part of this file using 128-bit AES.
1305 This file is not automatically accessed by
1307 but it is offered as the default file for the private key.
1309 will read this file when a login attempt is made.
1311 .It Pa ~/.ssh/id_dsa.pub
1312 .It Pa ~/.ssh/id_ecdsa.pub
1313 .It Pa ~/.ssh/id_ecdsa_sk.pub
1314 .It Pa ~/.ssh/id_ed25519.pub
1315 .It Pa ~/.ssh/id_ed25519_sk.pub
1316 .It Pa ~/.ssh/id_rsa.pub
1317 Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519,
1318 authenticator-hosted Ed25519 or RSA public key for authentication.
1319 The contents of this file should be added to
1320 .Pa ~/.ssh/authorized_keys
1322 where the user wishes to log in using public key authentication.
1323 There is no need to keep the contents of this file secret.
1326 Contains Diffie-Hellman groups used for DH-GEX.
1327 The file format is described in
1338 .%T "The Secure Shell (SSH) Public Key File Format"
1342 OpenSSH is a derivative of the original and free
1343 ssh 1.2.12 release by Tatu Ylonen.
1344 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1345 Theo de Raadt and Dug Song
1346 removed many bugs, re-added newer features and
1348 Markus Friedl contributed the support for SSH
1349 protocol versions 1.5 and 2.0.
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