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1SSH-KEYGEN(1) OpenBSD Reference Manual SSH-KEYGEN(1)
2
3NAME
4 ssh-keygen - authentication key generation, management and conversion
5
6SYNOPSIS
7 ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C comment]
8 [-f output_keyfile]
9 ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
10 ssh-keygen -i [-m key_format] [-f input_keyfile]
11 ssh-keygen -e [-m key_format] [-f input_keyfile]
12 ssh-keygen -y [-f input_keyfile]
13 ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile]
14 ssh-keygen -l [-f input_keyfile]
15 ssh-keygen -B [-f input_keyfile]
16 ssh-keygen -D pkcs11
17 ssh-keygen -F hostname [-f known_hosts_file] [-l]
18 ssh-keygen -H [-f known_hosts_file]
19 ssh-keygen -R hostname [-f known_hosts_file]
20 ssh-keygen -r hostname [-f input_keyfile] [-g]
21 ssh-keygen -G output_file [-v] [-b bits] [-M memory] [-S start_point]
22 ssh-keygen -T output_file -f input_file [-v] [-a rounds] [-J num_lines]
23 [-j start_line] [-K checkpt] [-W generator]
24 ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals]
25 [-O option] [-V validity_interval] [-z serial_number] file ...
26 ssh-keygen -L [-f input_keyfile]
27 ssh-keygen -A
28 ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]
29 file ...
30 ssh-keygen -Q -f krl_file file ...
31
32DESCRIPTION
33 ssh-keygen generates, manages and converts authentication keys for
34 ssh(1). ssh-keygen can create RSA keys for use by SSH protocol version 1
35 and DSA, ECDSA, ED25519 or RSA keys for use by SSH protocol version 2.
36 The type of key to be generated is specified with the -t option. If
37 invoked without any arguments, ssh-keygen will generate an RSA key for
38 use in SSH protocol 2 connections.
39
40 ssh-keygen is also used to generate groups for use in Diffie-Hellman
41 group exchange (DH-GEX). See the MODULI GENERATION section for details.
42
43 Finally, ssh-keygen can be used to generate and update Key Revocation
44 Lists, and to test whether given keys have been revoked by one. See the
45 KEY REVOCATION LISTS section for details.
46
47 Normally each user wishing to use SSH with public key authentication runs
48 this once to create the authentication key in ~/.ssh/identity,
49 ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 or ~/.ssh/id_rsa.
50 Additionally, the system administrator may use this to generate host
51 keys, as seen in /etc/rc.
52
53 Normally this program generates the key and asks for a file in which to
54 store the private key. The public key is stored in a file with the same
55 name but ``.pub'' appended. The program also asks for a passphrase. The
56 passphrase may be empty to indicate no passphrase (host keys must have an
57 empty passphrase), or it may be a string of arbitrary length. A
58 passphrase is similar to a password, except it can be a phrase with a
59 series of words, punctuation, numbers, whitespace, or any string of
60 characters you want. Good passphrases are 10-30 characters long, are not
61 simple sentences or otherwise easily guessable (English prose has only
62 1-2 bits of entropy per character, and provides very bad passphrases),
63 and contain a mix of upper and lowercase letters, numbers, and non-
64 alphanumeric characters. The passphrase can be changed later by using
65 the -p option.
66
67 There is no way to recover a lost passphrase. If the passphrase is lost
68 or forgotten, a new key must be generated and the corresponding public
69 key copied to other machines.
70
71 For RSA1 keys, there is also a comment field in the key file that is only
72 for convenience to the user to help identify the key. The comment can
73 tell what the key is for, or whatever is useful. The comment is
74 initialized to ``user@host'' when the key is created, but can be changed
75 using the -c option.
76
77 After a key is generated, instructions below detail where the keys should
78 be placed to be activated.
79
80 The options are as follows:
81
82 -A For each of the key types (rsa1, rsa, dsa, ecdsa and ed25519) for
83 which host keys do not exist, generate the host keys with the
84 default key file path, an empty passphrase, default bits for the
85 key type, and default comment. This is used by /etc/rc to
86 generate new host keys.
87
88 -a rounds
89 When saving a new-format private key (i.e. an ed25519 key or any
90 SSH protocol 2 key when the -o flag is set), this option
91 specifies the number of KDF (key derivation function) rounds
92 used. Higher numbers result in slower passphrase verification
93 and increased resistance to brute-force password cracking (should
94 the keys be stolen).
95
96 When screening DH-GEX candidates ( using the -T command). This
97 option specifies the number of primality tests to perform.
98
99 -B Show the bubblebabble digest of specified private or public key
100 file.
101
102 -b bits
103 Specifies the number of bits in the key to create. For RSA keys,
104 the minimum size is 768 bits and the default is 2048 bits.
105 Generally, 2048 bits is considered sufficient. DSA keys must be
106 exactly 1024 bits as specified by FIPS 186-2. For ECDSA keys,
107 the -b flag determines the key length by selecting from one of
108 three elliptic curve sizes: 256, 384 or 521 bits. Attempting to
109 use bit lengths other than these three values for ECDSA keys will
110 fail. ED25519 keys have a fixed length and the -b flag will be
111 ignored.
112
113 -C comment
114 Provides a new comment.
115
116 -c Requests changing the comment in the private and public key
117 files. This operation is only supported for RSA1 keys. The
118 program will prompt for the file containing the private keys, for
119 the passphrase if the key has one, and for the new comment.
120
121 -D pkcs11
122 Download the RSA public keys provided by the PKCS#11 shared
123 library pkcs11. When used in combination with -s, this option
124 indicates that a CA key resides in a PKCS#11 token (see the
125 CERTIFICATES section for details).
126
127 -e This option will read a private or public OpenSSH key file and
128 print to stdout the key in one of the formats specified by the -m
129 option. The default export format is ``RFC4716''. This option
130 allows exporting OpenSSH keys for use by other programs,
131 including several commercial SSH implementations.
132
133 -F hostname
134 Search for the specified hostname in a known_hosts file, listing
135 any occurrences found. This option is useful to find hashed host
136 names or addresses and may also be used in conjunction with the
137 -H option to print found keys in a hashed format.
138
139 -f filename
140 Specifies the filename of the key file.
141
142 -G output_file
143 Generate candidate primes for DH-GEX. These primes must be
144 screened for safety (using the -T option) before use.
145
146 -g Use generic DNS format when printing fingerprint resource records
147 using the -r command.
148
149 -H Hash a known_hosts file. This replaces all hostnames and
150 addresses with hashed representations within the specified file;
151 the original content is moved to a file with a .old suffix.
152 These hashes may be used normally by ssh and sshd, but they do
153 not reveal identifying information should the file's contents be
154 disclosed. This option will not modify existing hashed hostnames
155 and is therefore safe to use on files that mix hashed and non-
156 hashed names.
157
158 -h When signing a key, create a host certificate instead of a user
159 certificate. Please see the CERTIFICATES section for details.
160
161 -I certificate_identity
162 Specify the key identity when signing a public key. Please see
163 the CERTIFICATES section for details.
164
165 -i This option will read an unencrypted private (or public) key file
166 in the format specified by the -m option and print an OpenSSH
167 compatible private (or public) key to stdout.
168
169 -J num_lines
170 Exit after screening the specified number of lines while
171 performing DH candidate screening using the -T option.
172
173 -j start_line
174 Start screening at the specified line number while performing DH
175 candidate screening using the -T option.
176
177 -K checkpt
178 Write the last line processed to the file checkpt while
179 performing DH candidate screening using the -T option. This will
180 be used to skip lines in the input file that have already been
181 processed if the job is restarted. This option allows importing
182 keys from other software, including several commercial SSH
183 implementations. The default import format is ``RFC4716''.
184
185 -k Generate a KRL file. In this mode, ssh-keygen will generate a
186 KRL file at the location specified via the -f flag that revokes
187 every key or certificate presented on the command line.
188 Keys/certificates to be revoked may be specified by public key
189 file or using the format described in the KEY REVOCATION LISTS
190 section.
191
192 -L Prints the contents of a certificate.
193
194 -l Show fingerprint of specified public key file. Private RSA1 keys
195 are also supported. For RSA and DSA keys ssh-keygen tries to
196 find the matching public key file and prints its fingerprint. If
197 combined with -v, an ASCII art representation of the key is
198 supplied with the fingerprint.
199
200 -M memory
201 Specify the amount of memory to use (in megabytes) when
202 generating candidate moduli for DH-GEX.
203
204 -m key_format
205 Specify a key format for the -i (import) or -e (export)
206 conversion options. The supported key formats are: ``RFC4716''
207 (RFC 4716/SSH2 public or private key), ``PKCS8'' (PEM PKCS8
208 public key) or ``PEM'' (PEM public key). The default conversion
209 format is ``RFC4716''.
210
211 -N new_passphrase
212 Provides the new passphrase.
213
214 -n principals
215 Specify one or more principals (user or host names) to be
216 included in a certificate when signing a key. Multiple
217 principals may be specified, separated by commas. Please see the
218 CERTIFICATES section for details.
219
220 -O option
221 Specify a certificate option when signing a key. This option may
222 be specified multiple times. Please see the CERTIFICATES section
223 for details. The options that are valid for user certificates
224 are:
225
226 clear Clear all enabled permissions. This is useful for
227 clearing the default set of permissions so permissions
228 may be added individually.
229
230 force-command=command
231 Forces the execution of command instead of any shell or
232 command specified by the user when the certificate is
233 used for authentication.
234
235 no-agent-forwarding
236 Disable ssh-agent(1) forwarding (permitted by default).
237
238 no-port-forwarding
239 Disable port forwarding (permitted by default).
240
241 no-pty Disable PTY allocation (permitted by default).
242
243 no-user-rc
244 Disable execution of ~/.ssh/rc by sshd(8) (permitted by
245 default).
246
247 no-x11-forwarding
248 Disable X11 forwarding (permitted by default).
249
250 permit-agent-forwarding
251 Allows ssh-agent(1) forwarding.
252
253 permit-port-forwarding
254 Allows port forwarding.
255
256 permit-pty
257 Allows PTY allocation.
258
259 permit-user-rc
260 Allows execution of ~/.ssh/rc by sshd(8).
261
262 permit-x11-forwarding
263 Allows X11 forwarding.
264
265 source-address=address_list
266 Restrict the source addresses from which the certificate
267 is considered valid. The address_list is a comma-
268 separated list of one or more address/netmask pairs in
269 CIDR format.
270
271 At present, no options are valid for host keys.
272
273 -o Causes ssh-keygen to save SSH protocol 2 private keys using the
274 new OpenSSH format rather than the more compatible PEM format.
275 The new format has increased resistance to brute-force password
276 cracking but is not supported by versions of OpenSSH prior to
277 6.5. Ed25519 keys always use the new private key format.
278
279 -P passphrase
280 Provides the (old) passphrase.
281
282 -p Requests changing the passphrase of a private key file instead of
283 creating a new private key. The program will prompt for the file
284 containing the private key, for the old passphrase, and twice for
285 the new passphrase.
286
287 -Q Test whether keys have been revoked in a KRL.
288
289 -q Silence ssh-keygen.
290
291 -R hostname
292 Removes all keys belonging to hostname from a known_hosts file.
293 This option is useful to delete hashed hosts (see the -H option
294 above).
295
296 -r hostname
297 Print the SSHFP fingerprint resource record named hostname for
298 the specified public key file.
299
300 -S start
301 Specify start point (in hex) when generating candidate moduli for
302 DH-GEX.
303
304 -s ca_key
305 Certify (sign) a public key using the specified CA key. Please
306 see the CERTIFICATES section for details.
307
308 When generating a KRL, -s specifies a path to a CA public key
309 file used to revoke certificates directly by key ID or serial
310 number. See the KEY REVOCATION LISTS section for details.
311
312 -T output_file
313 Test DH group exchange candidate primes (generated using the -G
314 option) for safety.
315
316 -t type
317 Specifies the type of key to create. The possible values are
318 ``rsa1'' for protocol version 1 and ``dsa'', ``ecdsa'',
319 ``ed25519'', or ``rsa'' for protocol version 2.
320
321 -u Update a KRL. When specified with -k, keys listed via the
322 command line are added to the existing KRL rather than a new KRL
323 being created.
324
325 -V validity_interval
326 Specify a validity interval when signing a certificate. A
327 validity interval may consist of a single time, indicating that
328 the certificate is valid beginning now and expiring at that time,
329 or may consist of two times separated by a colon to indicate an
330 explicit time interval. The start time may be specified as a
331 date in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a
332 relative time (to the current time) consisting of a minus sign
333 followed by a relative time in the format described in the TIME
334 FORMATS section of sshd_config(5). The end time may be specified
335 as a YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time
336 starting with a plus character.
337
338 For example: ``+52w1d'' (valid from now to 52 weeks and one day
339 from now), ``-4w:+4w'' (valid from four weeks ago to four weeks
340 from now), ``20100101123000:20110101123000'' (valid from 12:30
341 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011),
342 ``-1d:20110101'' (valid from yesterday to midnight, January 1st,
343 2011).
344
345 -v Verbose mode. Causes ssh-keygen to print debugging messages
346 about its progress. This is helpful for debugging moduli
347 generation. Multiple -v options increase the verbosity. The
348 maximum is 3.
349
350 -W generator
351 Specify desired generator when testing candidate moduli for DH-
352 GEX.
353
354 -y This option will read a private OpenSSH format file and print an
355 OpenSSH public key to stdout.
356
357 -z serial_number
358 Specifies a serial number to be embedded in the certificate to
359 distinguish this certificate from others from the same CA. The
360 default serial number is zero.
361
362 When generating a KRL, the -z flag is used to specify a KRL
363 version number.
364
365MODULI GENERATION
366 ssh-keygen may be used to generate groups for the Diffie-Hellman Group
367 Exchange (DH-GEX) protocol. Generating these groups is a two-step
368 process: first, candidate primes are generated using a fast, but memory
369 intensive process. These candidate primes are then tested for
370 suitability (a CPU-intensive process).
371
372 Generation of primes is performed using the -G option. The desired
373 length of the primes may be specified by the -b option. For example:
374
375 # ssh-keygen -G moduli-2048.candidates -b 2048
376
377 By default, the search for primes begins at a random point in the desired
378 length range. This may be overridden using the -S option, which
379 specifies a different start point (in hex).
380
381 Once a set of candidates have been generated, they must be screened for
382 suitability. This may be performed using the -T option. In this mode
383 ssh-keygen will read candidates from standard input (or a file specified
384 using the -f option). For example:
385
386 # ssh-keygen -T moduli-2048 -f moduli-2048.candidates
387
388 By default, each candidate will be subjected to 100 primality tests.
389 This may be overridden using the -a option. The DH generator value will
390 be chosen automatically for the prime under consideration. If a specific
391 generator is desired, it may be requested using the -W option. Valid
392 generator values are 2, 3, and 5.
393
394 Screened DH groups may be installed in /etc/moduli. It is important that
395 this file contains moduli of a range of bit lengths and that both ends of
396 a connection share common moduli.
397
398CERTIFICATES
399 ssh-keygen supports signing of keys to produce certificates that may be
400 used for user or host authentication. Certificates consist of a public
401 key, some identity information, zero or more principal (user or host)
402 names and a set of options that are signed by a Certification Authority
403 (CA) key. Clients or servers may then trust only the CA key and verify
404 its signature on a certificate rather than trusting many user/host keys.
405 Note that OpenSSH certificates are a different, and much simpler, format
406 to the X.509 certificates used in ssl(8).
407
408 ssh-keygen supports two types of certificates: user and host. User
409 certificates authenticate users to servers, whereas host certificates
410 authenticate server hosts to users. To generate a user certificate:
411
412 $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
413
414 The resultant certificate will be placed in /path/to/user_key-cert.pub.
415 A host certificate requires the -h option:
416
417 $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
418
419 The host certificate will be output to /path/to/host_key-cert.pub.
420
421 It is possible to sign using a CA key stored in a PKCS#11 token by
422 providing the token library using -D and identifying the CA key by
423 providing its public half as an argument to -s:
424
425 $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id host_key.pub
426
427 In all cases, key_id is a "key identifier" that is logged by the server
428 when the certificate is used for authentication.
429
430 Certificates may be limited to be valid for a set of principal
431 (user/host) names. By default, generated certificates are valid for all
432 users or hosts. To generate a certificate for a specified set of
433 principals:
434
435 $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
436 $ ssh-keygen -s ca_key -I key_id -h -n host.domain user_key.pub
437
438 Additional limitations on the validity and use of user certificates may
439 be specified through certificate options. A certificate option may
440 disable features of the SSH session, may be valid only when presented
441 from particular source addresses or may force the use of a specific
442 command. For a list of valid certificate options, see the documentation
443 for the -O option above.
444
445 Finally, certificates may be defined with a validity lifetime. The -V
446 option allows specification of certificate start and end times. A
447 certificate that is presented at a time outside this range will not be
448 considered valid. By default, certificates are valid from UNIX Epoch to
449 the distant future.
450
451 For certificates to be used for user or host authentication, the CA
452 public key must be trusted by sshd(8) or ssh(1). Please refer to those
453 manual pages for details.
454
455KEY REVOCATION LISTS
456 ssh-keygen is able to manage OpenSSH format Key Revocation Lists (KRLs).
457 These binary files specify keys or certificates to be revoked using a
458 compact format, taking as little as one bit per certificate if they are
459 being revoked by serial number.
460
461 KRLs may be generated using the -k flag. This option reads one or more
462 files from the command line and generates a new KRL. The files may
463 either contain a KRL specification (see below) or public keys, listed one
464 per line. Plain public keys are revoked by listing their hash or
465 contents in the KRL and certificates revoked by serial number or key ID
466 (if the serial is zero or not available).
467
468 Revoking keys using a KRL specification offers explicit control over the
469 types of record used to revoke keys and may be used to directly revoke
470 certificates by serial number or key ID without having the complete
471 original certificate on hand. A KRL specification consists of lines
472 containing one of the following directives followed by a colon and some
473 directive-specific information.
474
475 serial: serial_number[-serial_number]
476 Revokes a certificate with the specified serial number. Serial
477 numbers are 64-bit values, not including zero and may be
478 expressed in decimal, hex or octal. If two serial numbers are
479 specified separated by a hyphen, then the range of serial numbers
480 including and between each is revoked. The CA key must have been
481 specified on the ssh-keygen command line using the -s option.
482
483 id: key_id
484 Revokes a certificate with the specified key ID string. The CA
485 key must have been specified on the ssh-keygen command line using
486 the -s option.
487
488 key: public_key
489 Revokes the specified key. If a certificate is listed, then it
490 is revoked as a plain public key.
491
492 sha1: public_key
493 Revokes the specified key by its SHA1 hash.
494
495 KRLs may be updated using the -u flag in addition to -k. When this
496 option is specified, keys listed via the command line are merged into the
497 KRL, adding to those already there.
498
499 It is also possible, given a KRL, to test whether it revokes a particular
500 key (or keys). The -Q flag will query an existing KRL, testing each key
501 specified on the commandline. If any key listed on the command line has
502 been revoked (or an error encountered) then ssh-keygen will exit with a
503 non-zero exit status. A zero exit status will only be returned if no key
504 was revoked.
505
506FILES
507 ~/.ssh/identity
508 Contains the protocol version 1 RSA authentication identity of
509 the user. This file should not be readable by anyone but the
510 user. It is possible to specify a passphrase when generating the
511 key; that passphrase will be used to encrypt the private part of
512 this file using 3DES. This file is not automatically accessed by
513 ssh-keygen but it is offered as the default file for the private
514 key. ssh(1) will read this file when a login attempt is made.
515
516 ~/.ssh/identity.pub
517 Contains the protocol version 1 RSA public key for
518 authentication. The contents of this file should be added to
519 ~/.ssh/authorized_keys on all machines where the user wishes to
520 log in using RSA authentication. There is no need to keep the
521 contents of this file secret.
522
523 ~/.ssh/id_dsa
524 ~/.ssh/id_ecdsa
525 ~/.ssh/id_ed25519
526 ~/.ssh/id_rsa
527 Contains the protocol version 2 DSA, ECDSA, ED25519 or RSA
528 authentication identity of the user. This file should not be
529 readable by anyone but the user. It is possible to specify a
530 passphrase when generating the key; that passphrase will be used
531 to encrypt the private part of this file using 128-bit AES. This
532 file is not automatically accessed by ssh-keygen but it is
533 offered as the default file for the private key. ssh(1) will
534 read this file when a login attempt is made.
535
536 ~/.ssh/id_dsa.pub
537 ~/.ssh/id_ecdsa.pub
538 ~/.ssh/id_ed25519.pub
539 ~/.ssh/id_rsa.pub
540 Contains the protocol version 2 DSA, ECDSA, ED25519 or RSA public
541 key for authentication. The contents of this file should be
542 added to ~/.ssh/authorized_keys on all machines where the user
543 wishes to log in using public key authentication. There is no
544 need to keep the contents of this file secret.
545
546 /etc/moduli
547 Contains Diffie-Hellman groups used for DH-GEX. The file format
548 is described in moduli(5).
549
550SEE ALSO
551 ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)
552
553 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
554
555AUTHORS
556 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
557 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
558 de Raadt and Dug Song removed many bugs, re-added newer features and
559 created OpenSSH. Markus Friedl contributed the support for SSH protocol
560 versions 1.5 and 2.0.
561
562OpenBSD 5.4 December 21, 2013 OpenBSD 5.4