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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 [-f input_keyfile]
11 ssh-keygen -e [-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 num_trials]
23 [-W generator]
24 ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals]
25 [-O constraint] [-V validity_interval] file ...
26 ssh-keygen -L [-f input_keyfile]
27
28DESCRIPTION
29 ssh-keygen generates, manages and converts authentication keys for
30 ssh(1). ssh-keygen can create RSA keys for use by SSH protocol version 1
31 and RSA or DSA keys for use by SSH protocol version 2. The type of key
32 to be generated is specified with the -t option. If invoked without any
33 arguments, ssh-keygen will generate an RSA key for use in SSH protocol 2
34 connections.
35
36 ssh-keygen is also used to generate groups for use in Diffie-Hellman
37 group exchange (DH-GEX). See the MODULI GENERATION section for details.
38
39 Normally each user wishing to use SSH with RSA or DSA authentication runs
40 this once to create the authentication key in ~/.ssh/identity,
41 ~/.ssh/id_dsa or ~/.ssh/id_rsa. Additionally, the system administrator
42 may use this to generate host keys, as seen in /etc/rc.
43
44 Normally this program generates the key and asks for a file in which to
45 store the private key. The public key is stored in a file with the same
46 name but ``.pub'' appended. The program also asks for a passphrase. The
47 passphrase may be empty to indicate no passphrase (host keys must have an
48 empty passphrase), or it may be a string of arbitrary length. A
49 passphrase is similar to a password, except it can be a phrase with a se-
50 ries of words, punctuation, numbers, whitespace, or any string of charac-
51 ters you want. Good passphrases are 10-30 characters long, are not sim-
52 ple sentences or otherwise easily guessable (English prose has only 1-2
53 bits of entropy per character, and provides very bad passphrases), and
54 contain a mix of upper and lowercase letters, numbers, and non-alphanu-
55 meric characters. The passphrase can be changed later by using the -p
56 option.
57
58 There is no way to recover a lost passphrase. If the passphrase is lost
59 or forgotten, a new key must be generated and copied to the corresponding
60 public key to other machines.
61
62 For RSA1 keys, there is also a comment field in the key file that is only
63 for convenience to the user to help identify the key. The comment can
64 tell what the key is for, or whatever is useful. The comment is initial-
65 ized to ``user@host'' when the key is created, but can be changed using
66 the -c option.
67
68 After a key is generated, instructions below detail where the keys should
69 be placed to be activated.
70
71 The options are as follows:
72
73 -a trials
74 Specifies the number of primality tests to perform when screening
75 DH-GEX candidates using the -T command.
76
77 -B Show the bubblebabble digest of specified private or public key
78 file.
79
80 -b bits
81 Specifies the number of bits in the key to create. For RSA keys,
82 the minimum size is 768 bits and the default is 2048 bits. Gen-
83 erally, 2048 bits is considered sufficient. DSA keys must be ex-
84 actly 1024 bits as specified by FIPS 186-2.
85
86 -C comment
87 Provides a new comment.
88
89 -c Requests changing the comment in the private and public key
90 files. This operation is only supported for RSA1 keys. The pro-
91 gram will prompt for the file containing the private keys, for
92 the passphrase if the key has one, and for the new comment.
93
94 -D pkcs11
95 Download the RSA public keys provided by the PKCS#11 shared li-
96 brary pkcs11.
97
98 -e This option will read a private or public OpenSSH key file and
99 print the key in RFC 4716 SSH Public Key File Format to stdout.
100 This option allows exporting keys for use by several commercial
101 SSH implementations.
102
103 -F hostname
104 Search for the specified hostname in a known_hosts file, listing
105 any occurrences found. This option is useful to find hashed host
106 names or addresses and may also be used in conjunction with the
107 -H option to print found keys in a hashed format.
108
109 -f filename
110 Specifies the filename of the key file.
111
112 -G output_file
113 Generate candidate primes for DH-GEX. These primes must be
114 screened for safety (using the -T option) before use.
115
116 -g Use generic DNS format when printing fingerprint resource records
117 using the -r command.
118
119 -H Hash a known_hosts file. This replaces all hostnames and ad-
120 dresses with hashed representations within the specified file;
121 the original content is moved to a file with a .old suffix.
122 These hashes may be used normally by ssh and sshd, but they do
123 not reveal identifying information should the file's contents be
124 disclosed. This option will not modify existing hashed hostnames
125 and is therefore safe to use on files that mix hashed and non-
126 hashed names.
127
128 -h When signing a key, create a host certificate instead of a user
129 certificate. Please see the CERTIFICATES section for details.
130
131 -I certificate_identity
132 Specify the key identity when signing a public key. Please see
133 the CERTIFICATES section for details.
134
135 -i This option will read an unencrypted private (or public) key file
136 in SSH2-compatible format and print an OpenSSH compatible private
137 (or public) key to stdout. ssh-keygen also reads the RFC 4716
138 SSH Public Key File Format. This option allows importing keys
139 from several commercial SSH implementations.
140
141 -L Prints the contents of a certificate.
142
143 -l Show fingerprint of specified public key file. Private RSA1 keys
144 are also supported. For RSA and DSA keys ssh-keygen tries to
145 find the matching public key file and prints its fingerprint. If
146 combined with -v, an ASCII art representation of the key is sup-
147 plied with the fingerprint.
148
149 -M memory
150 Specify the amount of memory to use (in megabytes) when generat-
151 ing candidate moduli for DH-GEX.
152
153 -N new_passphrase
154 Provides the new passphrase.
155
156 -n principals
157 Specify one or more principals (user or host names) to be includ-
158 ed in a certificate when signing a key. Multiple principals may
159 be specified, separated by commas. Please see the CERTIFICATES
160 section for details.
161
162 -O constraint
163 Specify a certificate constraint when signing a key. This option
164 may be specified multiple times. Please see the CERTIFICATES
165 section for details. The constraints that are valid for user
166 certificates are:
167
168 no-x11-forwarding
169 Disable X11 forwarding (permitted by default).
170
171 no-agent-forwarding
172 Disable ssh-agent(1) forwarding (permitted by default).
173
174 no-port-forwarding
175 Disable port forwarding (permitted by default).
176
177 no-pty Disable PTY allocation (permitted by default).
178
179 no-user-rc
180 Disable execution of ~/.ssh/rc by sshd(8) (permitted by
181 default).
182
183 clear Clear all enabled permissions. This is useful for clear-
184 ing the default set of permissions so permissions may be
185 added individually.
186
187 permit-x11-forwarding
188 Allows X11 forwarding.
189
190 permit-agent-forwarding
191 Allows ssh-agent(1) forwarding.
192
193 permit-port-forwarding
194 Allows port forwarding.
195
196 permit-pty
197 Allows PTY allocation.
198
199 permit-user-rc
200 Allows execution of ~/.ssh/rc by sshd(8).
201
202 force-command=command
203 Forces the execution of command instead of any shell or
204 command specified by the user when the certificate is
205 used for authentication.
206
207 source-address=address_list
208 Restrict the source addresses from which the certificate
209 is considered valid from. The address_list is a comma-
210 separated list of one or more address/netmask pairs in
211 CIDR format.
212
213 At present, no constraints are valid for host keys.
214
215 -P passphrase
216 Provides the (old) passphrase.
217
218 -p Requests changing the passphrase of a private key file instead of
219 creating a new private key. The program will prompt for the file
220 containing the private key, for the old passphrase, and twice for
221 the new passphrase.
222
223 -q Silence ssh-keygen. Used by /etc/rc when creating a new key.
224
225 -R hostname
226 Removes all keys belonging to hostname from a known_hosts file.
227 This option is useful to delete hashed hosts (see the -H option
228 above).
229
230 -r hostname
231 Print the SSHFP fingerprint resource record named hostname for
232 the specified public key file.
233
234 -S start
235 Specify start point (in hex) when generating candidate moduli for
236 DH-GEX.
237
238 -s ca_key
239 Certify (sign) a public key using the specified CA key. Please
240 see the CERTIFICATES section for details.
241
242 -T output_file
243 Test DH group exchange candidate primes (generated using the -G
244 option) for safety.
245
246 -t type
247 Specifies the type of key to create. The possible values are
248 ``rsa1'' for protocol version 1 and ``rsa'' or ``dsa'' for proto-
249 col version 2.
250
251 -V validity_interval
252 Specify a validity interval when signing a certificate. A valid-
253 ity interval may consist of a single time, indicating that the
254 certificate is valid beginning now and expiring at that time, or
255 may consist of two times separated by a colon to indicate an ex-
256 plicit time interval. The start time may be specified as a date
257 in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a relative
258 time (to the current time) consisting of a minus sign followed by
259 a relative time in the format described in the TIME FORMATS sec-
260 tion of ssh_config(5). The end time may be specified as a YYYYM-
261 MDD date, a YYYYMMDDHHMMSS time or a relative time starting with
262 a plus character.
263
264 For example: ``+52w1d'' (valid from now to 52 weeks and one day
265 from now), ``-4w:+4w'' (valid from four weeks ago to four weeks
266 from now), ``20100101123000:20110101123000'' (valid from 12:30
267 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011),
268 ``-1d:20110101'' (valid from yesterday to midnight, January 1st,
269 2011).
270
271 -v Verbose mode. Causes ssh-keygen to print debugging messages
272 about its progress. This is helpful for debugging moduli genera-
273 tion. Multiple -v options increase the verbosity. The maximum
274 is 3.
275
276 -W generator
277 Specify desired generator when testing candidate moduli for DH-
278 GEX.
279
280 -y This option will read a private OpenSSH format file and print an
281 OpenSSH public key to stdout.
282
283MODULI GENERATION
284 ssh-keygen may be used to generate groups for the Diffie-Hellman Group
285 Exchange (DH-GEX) protocol. Generating these groups is a two-step pro-
286 cess: first, candidate primes are generated using a fast, but memory in-
287 tensive process. These candidate primes are then tested for suitability
288 (a CPU-intensive process).
289
290 Generation of primes is performed using the -G option. The desired
291 length of the primes may be specified by the -b option. For example:
292
293 # ssh-keygen -G moduli-2048.candidates -b 2048
294
295 By default, the search for primes begins at a random point in the desired
296 length range. This may be overridden using the -S option, which speci-
297 fies a different start point (in hex).
298
299 Once a set of candidates have been generated, they must be tested for
300 suitability. This may be performed using the -T option. In this mode
301 ssh-keygen will read candidates from standard input (or a file specified
302 using the -f option). For example:
303
304 # ssh-keygen -T moduli-2048 -f moduli-2048.candidates
305
306 By default, each candidate will be subjected to 100 primality tests.
307 This may be overridden using the -a option. The DH generator value will
308 be chosen automatically for the prime under consideration. If a specific
309 generator is desired, it may be requested using the -W option. Valid
310 generator values are 2, 3, and 5.
311
312 Screened DH groups may be installed in /etc/moduli. It is important that
313 this file contains moduli of a range of bit lengths and that both ends of
314 a connection share common moduli.
315
316CERTIFICATES
317 ssh-keygen supports signing of keys to produce certificates that may be
318 used for user or host authentication. Certificates consist of a public
319 key, some identity information, zero or more principal (user or host)
320 names and an optional set of constraints that are signed by a Certifica-
321 tion Authority (CA) key. Clients or servers may then trust only the CA
322 key and verify its signature on a certificate rather than trusting many
323 user/host keys. Note that OpenSSH certificates are a different, and much
324 simpler, format to the X.509 certificates used in ssl(8).
325
326 ssh-keygen supports two types of certificates: user and host. User cer-
327 tificates authenticate users to servers, whereas host certificates au-
328 thenticate server hosts to users. To generate a user certificate:
329
330 $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
331
332 The resultant certificate will be placed in /path/to/user_key_cert.pub.
333 A host certificate requires the -h option:
334
335 $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
336
337 The host certificate will be output to /path/to/host_key_cert.pub. In
338 both cases, key_id is a "key identifier" that is logged by the server
339 when the certificate is used for authentication.
340
341 Certificates may be limited to be valid for a set of principal (us-
342 er/host) names. By default, generated certificates are valid for all
343 users or hosts. To generate a certificate for a specified set of princi-
344 pals:
345
346 $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
347 $ ssh-keygen -s ca_key -I key_id -h -n host.domain $0
348
349 Additional limitations on the validity and use of user certificates may
350 be specified through certificate constraints. A constrained certificate
351 may disable features of the SSH session, may be valid only when presented
352 from particular source addresses or may force the use of a specific com-
353 mand. For a list of valid certificate constraints, see the documentation
354 for the -O option above.
355
356 Finally, certificates may be defined with a validity lifetime. The -V
357 option allows specification of certificate start and end times. A cer-
358 tificate that is presented at a time outside this range will not be con-
359 sidered valid. By default, certificates have a maximum validity inter-
360 val.
361
362 For certificates to be used for user or host authentication, the CA pub-
363 lic key must be trusted by sshd(8) or ssh(1). Please refer to those man-
364 ual pages for details.
365
366FILES
367 ~/.ssh/identity
368 Contains the protocol version 1 RSA authentication identity of
369 the user. This file should not be readable by anyone but the us-
370 er. It is possible to specify a passphrase when generating the
371 key; that passphrase will be used to encrypt the private part of
372 this file using 128-bit AES. This file is not automatically ac-
373 cessed by ssh-keygen but it is offered as the default file for
374 the private key. ssh(1) will read this file when a login attempt
375 is made.
376
377 ~/.ssh/identity.pub
378 Contains the protocol version 1 RSA public key for authentica-
379 tion. The contents of this file should be added to
380 ~/.ssh/authorized_keys on all machines where the user wishes to
381 log in using RSA authentication. There is no need to keep the
382 contents of this file secret.
383
384 ~/.ssh/id_dsa
385 Contains the protocol version 2 DSA authentication identity of
386 the user. This file should not be readable by anyone but the us-
387 er. It is possible to specify a passphrase when generating the
388 key; that passphrase will be used to encrypt the private part of
389 this file using 128-bit AES. This file is not automatically ac-
390 cessed by ssh-keygen but it is offered as the default file for
391 the private key. ssh(1) will read this file when a login attempt
392 is made.
393
394 ~/.ssh/id_dsa.pub
395 Contains the protocol version 2 DSA public key for authentica-
396 tion. The contents of this file should be added to
397 ~/.ssh/authorized_keys on all machines where the user wishes to
398 log in using public key authentication. There is no need to keep
399 the contents of this file secret.
400
401 ~/.ssh/id_rsa
402 Contains the protocol version 2 RSA authentication identity of
403 the user. This file should not be readable by anyone but the us-
404 er. It is possible to specify a passphrase when generating the
405 key; that passphrase will be used to encrypt the private part of
406 this file using 128-bit AES. This file is not automatically ac-
407 cessed by ssh-keygen but it is offered as the default file for
408 the private key. ssh(1) will read this file when a login attempt
409 is made.
410
411 ~/.ssh/id_rsa.pub
412 Contains the protocol version 2 RSA public key for authentica-
413 tion. The contents of this file should be added to
414 ~/.ssh/authorized_keys on all machines where the user wishes to
415 log in using public key authentication. There is no need to keep
416 the contents of this file secret.
417
418 /etc/moduli
419 Contains Diffie-Hellman groups used for DH-GEX. The file format
420 is described in moduli(5).
421
422SEE ALSO
423 ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)
424
425 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
426
427AUTHORS
428 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
429 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
430 de Raadt and Dug Song removed many bugs, re-added newer features and
431 created OpenSSH. Markus Friedl contributed the support for SSH protocol
432 versions 1.5 and 2.0.
433
434OpenBSD 4.6 March 8, 2010 7