summaryrefslogtreecommitdiffstats
path: root/ssh-keygen.1
blob: 74b3124f57c53237f62a61b1434bfce70571d7ac (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
.\"	$OpenBSD: ssh-keygen.1,v 1.129 2015/11/13 04:34:15 djm 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:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: November 13 2015 $
.Dt SSH-KEYGEN 1
.Os
.Sh NAME
.Nm ssh-keygen
.Nd authentication key generation, management and conversion
.Sh SYNOPSIS
.Bk -words
.Nm ssh-keygen
.Op Fl q
.Op Fl b Ar bits
.Op Fl t Cm dsa | ecdsa | ed25519 | rsa | rsa1
.Op Fl N Ar new_passphrase
.Op Fl C Ar comment
.Op Fl f Ar output_keyfile
.Nm ssh-keygen
.Fl p
.Op Fl P Ar old_passphrase
.Op Fl N Ar new_passphrase
.Op Fl f Ar keyfile
.Nm ssh-keygen
.Fl i
.Op Fl m Ar key_format
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl e
.Op Fl m Ar key_format
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl y
.Op Fl f Ar input_keyfile
.Nm ssh-keygen
.Fl c
.Op Fl P Ar passphrase
.Op Fl C Ar comment
.Op Fl f Ar keyfile
.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 f Ar known_hosts_file
.Op Fl l
.Nm ssh-keygen
.Fl H
.Op Fl f Ar known_hosts_file
.Nm ssh-keygen
.Fl R Ar hostname
.Op Fl f Ar known_hosts_file
.Nm ssh-keygen
.Fl r Ar hostname
.Op Fl f Ar input_keyfile
.Op Fl g
.Nm ssh-keygen
.Fl G Ar output_file
.Op Fl v
.Op Fl b Ar bits
.Op Fl M Ar memory
.Op Fl S Ar start_point
.Nm ssh-keygen
.Fl T Ar output_file
.Fl f Ar input_file
.Op Fl v
.Op Fl a Ar rounds
.Op Fl J Ar num_lines
.Op Fl j Ar start_line
.Op Fl K Ar checkpt
.Op Fl W Ar generator
.Nm ssh-keygen
.Fl s Ar ca_key
.Fl I Ar certificate_identity
.Op Fl h
.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
.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
.Ek
.Sh DESCRIPTION
.Nm
generates, manages and converts authentication keys for
.Xr ssh 1 .
.Nm
can create RSA keys for use by SSH protocol version 1 and
DSA, ECDSA, Ed25519 or RSA keys for use by SSH protocol version 2.
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 for use in SSH protocol 2 connections.
.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/identity ,
.Pa ~/.ssh/id_dsa ,
.Pa ~/.ssh/id_ecdsa ,
.Pa ~/.ssh/id_ed25519
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
For RSA1 keys,
there is also a comment field in the key file that is only for
convenience to the user to help identify the key.
The comment can tell what the key is for, or whatever is useful.
The comment is initialized to
.Dq user@host
when the key is created, but can be changed using the
.Fl c
option.
.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 (rsa1, 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.
This is used by
.Pa /etc/rc
to generate new host keys.
.It Fl a Ar rounds
When saving a new-format private key (i.e. an ed25519 key or any SSH protocol
2 key when the
.Fl o
flag is set), this option specifies the number of KDF (key derivation function)
rounds used.
Higher numbers result in slower passphrase verification and increased
resistance to brute-force password cracking (should the keys be stolen).
.Pp
When screening DH-GEX candidates (
using the
.Fl T
command).
This option specifies the number of primality tests to perform.
.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 2048 bits.
Generally, 2048 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.
Ed25519 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.
This operation is only supported for RSA1 keys.
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 RSA 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 the 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
Search for the specified
.Ar hostname
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 Ar output_file
Generate candidate primes for DH-GEX.
These primes must be screened for
safety (using the
.Fl T
option) before use.
.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 J Ar num_lines
Exit after screening the specified number of lines
while performing DH candidate screening using the
.Fl T
option.
.It Fl j Ar start_line
Start screening at the specified line number
while performing DH candidate screening using the
.Fl T
option.
.It Fl K Ar checkpt
Write the last line processed to the file
.Ar checkpt
while performing DH candidate screening using the
.Fl T
option.
This will be used to skip lines in the input file that have already been
processed if the job is restarted.
.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.
Private RSA1 keys are also supported.
For RSA and DSA keys
.Nm
tries to find the matching public key file and prints its fingerprint.
If combined with
.Fl v ,
an ASCII art representation of the key is supplied with the fingerprint.
.It Fl M Ar memory
Specify the amount of memory to use (in megabytes) when generating
candidate moduli for DH-GEX.
.It Fl m Ar key_format
Specify a key format for the
.Fl i
(import) or
.Fl e
(export) conversion options.
The supported key formats are:
.Dq RFC4716
(RFC 4716/SSH2 public or private key),
.Dq PKCS8
(PEM PKCS8 public key)
or
.Dq PEM
(PEM public key).
The default conversion format is
.Dq RFC4716 .
.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 certificate option when signing a key.
This option may be specified multiple times.
Please see the
.Sx CERTIFICATES
section for details.
The options that are valid for user certificates are:
.Bl -tag -width Ds
.It Ic clear
Clear all enabled permissions.
This is useful for clearing the default set of permissions so permissions may
be added individually.
.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.
.It Ic no-agent-forwarding
Disable
.Xr ssh-agent 1
forwarding (permitted by default).
.It Ic no-port-forwarding
Disable port forwarding (permitted by default).
.It Ic no-pty
Disable PTY allocation (permitted by default).
.It Ic no-user-rc
Disable execution of
.Pa ~/.ssh/rc
by
.Xr sshd 8
(permitted by default).
.It Ic no-x11-forwarding
Disable X11 forwarding (permitted by default).
.It Ic permit-agent-forwarding
Allows
.Xr ssh-agent 1
forwarding.
.It Ic permit-port-forwarding
Allows port forwarding.
.It Ic permit-pty
Allows PTY allocation.
.It Ic permit-user-rc
Allows execution of
.Pa ~/.ssh/rc
by
.Xr sshd 8 .
.It Ic permit-x11-forwarding
Allows X11 forwarding.
.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 options are valid for host keys.
.It Fl o
Causes
.Nm
to save SSH protocol 2 private keys using the new OpenSSH format rather than
the more compatible PEM format.
The new format has increased resistance to brute-force password cracking
but is not supported by versions of OpenSSH prior to 6.5.
Ed25519 keys always use the new private key format.
.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
Removes all keys belonging to
.Ar hostname
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 start
Specify start point (in hex) when generating candidate moduli for DH-GEX.
.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 Ar output_file
Test DH group exchange candidate primes (generated using the
.Fl G
option) for safety.
.It Fl t Cm dsa | ecdsa | ed25519 | rsa | rsa1
Specifies the type of key to create.
The possible values are
.Dq rsa1
for protocol version 1 and
.Dq dsa ,
.Dq ecdsa ,
.Dq ed25519 ,
or
.Dq rsa
for protocol version 2.
.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.
The start time may be specified as a date in YYYYMMDD format, a time
in YYYYMMDDHHMMSS format or a relative time (to the current time) consisting
of a minus sign followed by a relative time in the format described in the
TIME FORMATS section of
.Xr sshd_config 5 .
The end time may be specified as a YYYYMMDD date, a YYYYMMDDHHMMSS time or
a relative time starting with a plus character.
.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).
.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 generator
Specify desired generator when testing candidate moduli for DH-GEX.
.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.
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 G
option.
The desired length of the primes may be specified by the
.Fl b
option.
For example:
.Pp
.Dl # ssh-keygen -G moduli-2048.candidates -b 2048
.Pp
By default, the search for primes begins at a random point in the
desired length range.
This may be overridden using the
.Fl S
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 T
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 -T moduli-2048 -f moduli-2048.candidates
.Pp
By default, each candidate will be subjected to 100 primality tests.
This may be overridden using the
.Fl a
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 W
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.
.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
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.
For a list of valid certificate options, see the documentation for the
.Fl O
option above.
.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 its SHA1 hash.
.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 FILES
.Bl -tag -width Ds -compact
.It Pa ~/.ssh/identity
Contains the protocol version 1 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 3DES.
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/identity.pub
Contains the protocol version 1 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 RSA authentication.
There is no need to keep the contents of this file secret.
.Pp
.It Pa ~/.ssh/id_dsa
.It Pa ~/.ssh/id_ecdsa
.It Pa ~/.ssh/id_ed25519
.It Pa ~/.ssh/id_rsa
Contains the protocol version 2 DSA, ECDSA, 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_ed25519.pub
.It Pa ~/.ssh/id_rsa.pub
Contains the protocol version 2 DSA, ECDSA, 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.