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authorColin Watson <cjwatson@debian.org>2010-08-23 22:50:14 +0100
committerColin Watson <cjwatson@debian.org>2010-08-23 22:50:14 +0100
commit43094ebf14c9b16f1ea398bc5b65a7335e947288 (patch)
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parentff0095389ba9a9e4599e6051c8d5bae6777c4d64 (diff)
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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 num_trials]
23 [-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
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
50 series of words, punctuation, numbers, whitespace, or any string of
51 characters you want. Good passphrases are 10-30 characters long, are not
52 simple sentences or otherwise easily guessable (English prose has only 1-
53 2 bits of entropy per character, and provides very bad passphrases), and
54 contain a mix of upper and lowercase letters, numbers, and non-
55 alphanumeric characters. The passphrase can be changed later by using
56 the -p 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
65 initialized to ``user@host'' when the key is created, but can be changed
66 using 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.
83 Generally, 2048 bits is considered sufficient. DSA keys must be
84 exactly 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
91 program 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
96 library pkcs11. When used in combination with -s, this option
97 indicates that a CA key resides in a PKCS#11 token (see the
98 CERTIFICATES section for details).
99
100 -e This option will read a private or public OpenSSH key file and
101 print to stdout the key in one of the formats specified by the -m
102 option. The default export format is ``RFC4716''. This option
103 allows exporting OpenSSH keys for use by other programs,
104 including several commercial SSH implementations.
105
106 -F hostname
107 Search for the specified hostname in a known_hosts file, listing
108 any occurrences found. This option is useful to find hashed host
109 names or addresses and may also be used in conjunction with the
110 -H option to print found keys in a hashed format.
111
112 -f filename
113 Specifies the filename of the key file.
114
115 -G output_file
116 Generate candidate primes for DH-GEX. These primes must be
117 screened for safety (using the -T option) before use.
118
119 -g Use generic DNS format when printing fingerprint resource records
120 using the -r command.
121
122 -H Hash a known_hosts file. This replaces all hostnames and
123 addresses with hashed representations within the specified file;
124 the original content is moved to a file with a .old suffix.
125 These hashes may be used normally by ssh and sshd, but they do
126 not reveal identifying information should the file's contents be
127 disclosed. This option will not modify existing hashed hostnames
128 and is therefore safe to use on files that mix hashed and non-
129 hashed names.
130
131 -h When signing a key, create a host certificate instead of a user
132 certificate. Please see the CERTIFICATES section for details.
133
134 -I certificate_identity
135 Specify the key identity when signing a public key. Please see
136 the CERTIFICATES section for details.
137
138 -i This option will read an unencrypted private (or public) key file
139 in the format specified by the -m option and print an OpenSSH
140 compatible private (or public) key to stdout. This option allows
141 importing keys from other software, including several commercial
142 SSH implementations. The default import format is ``RFC4716''.
143
144 -L Prints the contents of a certificate.
145
146 -l Show fingerprint of specified public key file. Private RSA1 keys
147 are also supported. For RSA and DSA keys ssh-keygen tries to
148 find the matching public key file and prints its fingerprint. If
149 combined with -v, an ASCII art representation of the key is
150 supplied with the fingerprint.
151
152 -M memory
153 Specify the amount of memory to use (in megabytes) when
154 generating candidate moduli for DH-GEX.
155
156 -m key_format
157 Specify a key format for the -i (import) or -e (export)
158 conversion options. The supported key formats are: ``RFC4716''
159 (RFC 4716/SSH2 public or private key), ``PKCS8'' (PEM PKCS8
160 public key) or ``PEM'' (PEM public key). The default conversion
161 format is ``RFC4716''.
162
163 -N new_passphrase
164 Provides the new passphrase.
165
166 -n principals
167 Specify one or more principals (user or host names) to be
168 included in a certificate when signing a key. Multiple
169 principals may be specified, separated by commas. Please see the
170 CERTIFICATES section for details.
171
172 -O option
173 Specify a certificate option when signing a key. This option may
174 be specified multiple times. Please see the CERTIFICATES section
175 for details. The options that are valid for user certificates
176 are:
177
178 clear Clear all enabled permissions. This is useful for
179 clearing the default set of permissions so permissions
180 may be added individually.
181
182 force-command=command
183 Forces the execution of command instead of any shell or
184 command specified by the user when the certificate is
185 used for authentication.
186
187 no-agent-forwarding
188 Disable ssh-agent(1) forwarding (permitted by default).
189
190 no-port-forwarding
191 Disable port forwarding (permitted by default).
192
193 no-pty Disable PTY allocation (permitted by default).
194
195 no-user-rc
196 Disable execution of ~/.ssh/rc by sshd(8) (permitted by
197 default).
198
199 no-x11-forwarding
200 Disable X11 forwarding (permitted by default).
201
202 permit-agent-forwarding
203 Allows ssh-agent(1) forwarding.
204
205 permit-port-forwarding
206 Allows port forwarding.
207
208 permit-pty
209 Allows PTY allocation.
210
211 permit-user-rc
212 Allows execution of ~/.ssh/rc by sshd(8).
213
214 permit-x11-forwarding
215 Allows X11 forwarding.
216
217 source-address=address_list
218 Restrict the source addresses from which the certificate
219 is considered valid. The address_list is a comma-
220 separated list of one or more address/netmask pairs in
221 CIDR format.
222
223 At present, no options are valid for host keys.
224
225 -P passphrase
226 Provides the (old) passphrase.
227
228 -p Requests changing the passphrase of a private key file instead of
229 creating a new private key. The program will prompt for the file
230 containing the private key, for the old passphrase, and twice for
231 the new passphrase.
232
233 -q Silence ssh-keygen. Used by /etc/rc when creating a new key.
234
235 -R hostname
236 Removes all keys belonging to hostname from a known_hosts file.
237 This option is useful to delete hashed hosts (see the -H option
238 above).
239
240 -r hostname
241 Print the SSHFP fingerprint resource record named hostname for
242 the specified public key file.
243
244 -S start
245 Specify start point (in hex) when generating candidate moduli for
246 DH-GEX.
247
248 -s ca_key
249 Certify (sign) a public key using the specified CA key. Please
250 see the CERTIFICATES section for details.
251
252 -T output_file
253 Test DH group exchange candidate primes (generated using the -G
254 option) for safety.
255
256 -t type
257 Specifies the type of key to create. The possible values are
258 ``rsa1'' for protocol version 1 and ``rsa'' or ``dsa'' for
259 protocol version 2.
260
261 -V validity_interval
262 Specify a validity interval when signing a certificate. A
263 validity interval may consist of a single time, indicating that
264 the certificate is valid beginning now and expiring at that time,
265 or may consist of two times separated by a colon to indicate an
266 explicit time interval. The start time may be specified as a
267 date in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a
268 relative time (to the current time) consisting of a minus sign
269 followed by a relative time in the format described in the TIME
270 FORMATS section of sshd_config(5). The end time may be specified
271 as a YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time
272 starting with a plus character.
273
274 For example: ``+52w1d'' (valid from now to 52 weeks and one day
275 from now), ``-4w:+4w'' (valid from four weeks ago to four weeks
276 from now), ``20100101123000:20110101123000'' (valid from 12:30
277 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011),
278 ``-1d:20110101'' (valid from yesterday to midnight, January 1st,
279 2011).
280
281 -v Verbose mode. Causes ssh-keygen to print debugging messages
282 about its progress. This is helpful for debugging moduli
283 generation. Multiple -v options increase the verbosity. The
284 maximum is 3.
285
286 -W generator
287 Specify desired generator when testing candidate moduli for DH-
288 GEX.
289
290 -y This option will read a private OpenSSH format file and print an
291 OpenSSH public key to stdout.
292
293 -z serial_number
294 Specifies a serial number to be embedded in the certificate to
295 distinguish this certificate from others from the same CA. The
296 default serial number is zero.
297
298MODULI GENERATION
299 ssh-keygen may be used to generate groups for the Diffie-Hellman Group
300 Exchange (DH-GEX) protocol. Generating these groups is a two-step
301 process: first, candidate primes are generated using a fast, but memory
302 intensive process. These candidate primes are then tested for
303 suitability (a CPU-intensive process).
304
305 Generation of primes is performed using the -G option. The desired
306 length of the primes may be specified by the -b option. For example:
307
308 # ssh-keygen -G moduli-2048.candidates -b 2048
309
310 By default, the search for primes begins at a random point in the desired
311 length range. This may be overridden using the -S option, which
312 specifies a different start point (in hex).
313
314 Once a set of candidates have been generated, they must be tested for
315 suitability. This may be performed using the -T option. In this mode
316 ssh-keygen will read candidates from standard input (or a file specified
317 using the -f option). For example:
318
319 # ssh-keygen -T moduli-2048 -f moduli-2048.candidates
320
321 By default, each candidate will be subjected to 100 primality tests.
322 This may be overridden using the -a option. The DH generator value will
323 be chosen automatically for the prime under consideration. If a specific
324 generator is desired, it may be requested using the -W option. Valid
325 generator values are 2, 3, and 5.
326
327 Screened DH groups may be installed in /etc/moduli. It is important that
328 this file contains moduli of a range of bit lengths and that both ends of
329 a connection share common moduli.
330
331CERTIFICATES
332 ssh-keygen supports signing of keys to produce certificates that may be
333 used for user or host authentication. Certificates consist of a public
334 key, some identity information, zero or more principal (user or host)
335 names and a set of options that are signed by a Certification Authority
336 (CA) key. Clients or servers may then trust only the CA key and verify
337 its signature on a certificate rather than trusting many user/host keys.
338 Note that OpenSSH certificates are a different, and much simpler, format
339 to the X.509 certificates used in ssl(8).
340
341 ssh-keygen supports two types of certificates: user and host. User
342 certificates authenticate users to servers, whereas host certificates
343 authenticate server hosts to users. To generate a user certificate:
344
345 $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
346
347 The resultant certificate will be placed in /path/to/user_key-cert.pub.
348 A host certificate requires the -h option:
349
350 $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
351
352 The host certificate will be output to /path/to/host_key-cert.pub.
353
354 It is possible to sign using a CA key stored in a PKCS#11 token by
355 providing the token library using -D and identifying the CA key by
356 providing its public half as an argument to -s:
357
358 $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id host_key.pub
359
360 In all cases, key_id is a "key identifier" that is logged by the server
361 when the certificate is used for authentication.
362
363 Certificates may be limited to be valid for a set of principal
364 (user/host) names. By default, generated certificates are valid for all
365 users or hosts. To generate a certificate for a specified set of
366 principals:
367
368 $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
369 $ ssh-keygen -s ca_key -I key_id -h -n host.domain user_key.pub
370
371 Additional limitations on the validity and use of user certificates may
372 be specified through certificate options. A certificate option may
373 disable features of the SSH session, may be valid only when presented
374 from particular source addresses or may force the use of a specific
375 command. For a list of valid certificate options, see the documentation
376 for the -O option above.
377
378 Finally, certificates may be defined with a validity lifetime. The -V
379 option allows specification of certificate start and end times. A
380 certificate that is presented at a time outside this range will not be
381 considered valid. By default, certificates have a maximum validity
382 interval.
383
384 For certificates to be used for user or host authentication, the CA
385 public key must be trusted by sshd(8) or ssh(1). Please refer to those
386 manual pages for details.
387
388FILES
389 ~/.ssh/identity
390 Contains the protocol version 1 RSA authentication identity of
391 the user. This file should not be readable by anyone but the
392 user. It is possible to specify a passphrase when generating the
393 key; that passphrase will be used to encrypt the private part of
394 this file using 128-bit AES. This file is not automatically
395 accessed by ssh-keygen but it is offered as the default file for
396 the private key. ssh(1) will read this file when a login attempt
397 is made.
398
399 ~/.ssh/identity.pub
400 Contains the protocol version 1 RSA public key for
401 authentication. The contents of this file should be added to
402 ~/.ssh/authorized_keys on all machines where the user wishes to
403 log in using RSA authentication. There is no need to keep the
404 contents of this file secret.
405
406 ~/.ssh/id_dsa
407 Contains the protocol version 2 DSA authentication identity of
408 the user. This file should not be readable by anyone but the
409 user. It is possible to specify a passphrase when generating the
410 key; that passphrase will be used to encrypt the private part of
411 this file using 128-bit AES. This file is not automatically
412 accessed by ssh-keygen but it is offered as the default file for
413 the private key. ssh(1) will read this file when a login attempt
414 is made.
415
416 ~/.ssh/id_dsa.pub
417 Contains the protocol version 2 DSA public key for
418 authentication. The contents of this file should be added to
419 ~/.ssh/authorized_keys on all machines where the user wishes to
420 log in using public key authentication. There is no need to keep
421 the contents of this file secret.
422
423 ~/.ssh/id_rsa
424 Contains the protocol version 2 RSA authentication identity of
425 the user. This file should not be readable by anyone but the
426 user. It is possible to specify a passphrase when generating the
427 key; that passphrase will be used to encrypt the private part of
428 this file using 128-bit AES. This file is not automatically
429 accessed by ssh-keygen but it is offered as the default file for
430 the private key. ssh(1) will read this file when a login attempt
431 is made.
432
433 ~/.ssh/id_rsa.pub
434 Contains the protocol version 2 RSA public key for
435 authentication. The contents of this file should be added to
436 ~/.ssh/authorized_keys on all machines where the user wishes to
437 log in using public key authentication. There is no need to keep
438 the contents of this file secret.
439
440 /etc/moduli
441 Contains Diffie-Hellman groups used for DH-GEX. The file format
442 is described in moduli(5).
443
444SEE ALSO
445 ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)
446
447 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
448
449AUTHORS
450 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
451 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
452 de Raadt and Dug Song removed many bugs, re-added newer features and
453 created OpenSSH. Markus Friedl contributed the support for SSH protocol
454 versions 1.5 and 2.0.
455
456OpenBSD 4.8 August 4, 2010 OpenBSD 4.8