1SSH-KEYGEN(1) BSD General Commands Manual SSH-KEYGEN(1)
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4 ssh-keygen — authentication key generation, management and conversion
5
7 ssh-keygen [-q] [-o] [-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
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 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 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 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 char‐
51 acters you want. Good passphrases are 10-30 characters long, are not
52 simple sentences or otherwise easily guessable (English prose has only
53 1-2 bits of entropy per character, and provides very bad passphrases),
54 and 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 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 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 the
66 -c option.
67 After a key is generated, instructions below detail where the keys should
69 be placed to be activated.
70 The options are as follows:
72 -a trials
74 Specifies the number of primality tests to perform when screening
75 DH-GEX candidates using the -T command.
76 -B Show the bubblebabble digest of specified private or public key
78 file.
79 -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
84 exactly 1024 bits as specified by FIPS 186-2.
85 -C comment
87 Provides a new comment.
88 -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 -D pkcs11
95 Download the RSA public keys provided by the PKCS#11 shared
96 library pkcs11.
97 -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 -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 -f filename
110 Specifies the filename of the key file.
111 -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 -g Use generic DNS format when printing fingerprint resource records
117 using the -r command.
118 -H Hash a known_hosts file. This replaces all hostnames and
120 addresses 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 -h When signing a key, create a host certificate instead of a user
129 certificate. Please see the CERTIFICATES section for details.
130 -I certificate_identity
132 Specify the key identity when signing a public key. Please see
133 the CERTIFICATES section for details.
134 -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 -L Prints the contents of a certificate.
142 -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 -M memory
150 Specify the amount of memory to use (in megabytes) when generat‐
151 ing candidate moduli for DH-GEX.
152 -N new_passphrase
154 Provides the new passphrase.
155 -n principals
157 Specify one or more principals (user or host names) to be
158 included in a certificate when signing a key. Multiple princi‐
159 pals may be specified, separated by commas. Please see the
160 CERTIFICATES section for details.
161 -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 clear Clear all enabled permissions. This is useful for clear‐
169 ing the default set of permissions so permissions may be
170 added individually.
171 force-command=command
173 Forces the execution of command instead of any shell or
174 command specified by the user when the certificate is
175 used for authentication.
176 no-agent-forwarding
178 Disable ssh-agent(1) forwarding (permitted by default).
179 no-port-forwarding
181 Disable port forwarding (permitted by default).
182 no-pty Disable PTY allocation (permitted by default).
184 no-user-rc
186 Disable execution of ~/.ssh/rc by sshd(8) (permitted by
187 default).
188 no-x11-forwarding
190 Disable X11 forwarding (permitted by default).
191 permit-agent-forwarding
193 Allows ssh-agent(1) forwarding.
194 permit-port-forwarding
196 Allows port forwarding.
197 permit-pty
199 Allows PTY allocation.
200 permit-user-rc
202 Allows execution of ~/.ssh/rc by sshd(8).
203 permit-x11-forwarding
205 Allows X11 forwarding.
206 source-address=address_list
208 Restrict the source addresses from which the certificate
209 is considered valid. The address_list is a comma-sepa‐
210 rated list of one or more address/netmask pairs in CIDR
211 format.
212 At present, no constraints are valid for host keys.
214 -P passphrase
216 Provides the (old) passphrase.
217 -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 -q Silence ssh-keygen. Used by /etc/rc when creating a new key.
224 -o Overwrite the key without prompting user.
226 -R hostname
228 Removes all keys belonging to hostname from a known_hosts file.
229 This option is useful to delete hashed hosts (see the -H option
230 above).
231 -r hostname
233 Print the SSHFP fingerprint resource record named hostname for
234 the specified public key file.
235 -S start
237 Specify start point (in hex) when generating candidate moduli for
238 DH-GEX.
239 -s ca_key
241 Certify (sign) a public key using the specified CA key. Please
242 see the CERTIFICATES section for details.
243 -T output_file
245 Test DH group exchange candidate primes (generated using the -G
246 option) for safety.
247 -t type
249 Specifies the type of key to create. The possible values are
250 “rsa1” for protocol version 1 and “rsa” or “dsa” for protocol
251 version 2.
252 -V validity_interval
254 Specify a validity interval when signing a certificate. A valid‐
255 ity interval may consist of a single time, indicating that the
256 certificate is valid beginning now and expiring at that time, or
257 may consist of two times separated by a colon to indicate an
258 explicit time interval. The start time may be specified as a
259 date in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a
260 relative time (to the current time) consisting of a minus sign
261 followed by a relative time in the format described in the TIME
262 FORMATS section of sshd_config(5). The end time may be specified
263 as a YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time
264 starting with a plus character.
265 For example: “+52w1d” (valid from now to 52 weeks and one day
267 from now), “-4w:+4w” (valid from four weeks ago to four weeks
268 from now), “20100101123000:20110101123000” (valid from 12:30 PM,
269 January 1st, 2010 to 12:30 PM, January 1st, 2011), “-1d:20110101”
270 (valid from yesterday to midnight, January 1st, 2011).
271 -v Verbose mode. Causes ssh-keygen to print debugging messages
273 about its progress. This is helpful for debugging moduli genera‐
274 tion. Multiple -v options increase the verbosity. The maximum
275 is 3.
276 -W generator
278 Specify desired generator when testing candidate moduli for DH-
279 GEX.
280 -y This option will read a private OpenSSH format file and print an
282 OpenSSH public key to stdout.
283
285 ssh-keygen may be used to generate groups for the Diffie-Hellman Group
286 Exchange (DH-GEX) protocol. Generating these groups is a two-step
287 process: first, candidate primes are generated using a fast, but memory
288 intensive process. These candidate primes are then tested for suitabil‐
289 ity (a CPU-intensive process).
290 Generation of primes is performed using the -G option. The desired
292 length of the primes may be specified by the -b option. For example:
293 # ssh-keygen -G moduli-2048.candidates -b 2048
295 By default, the search for primes begins at a random point in the desired
297 length range. This may be overridden using the -S option, which speci‐
298 fies a different start point (in hex).
299 Once a set of candidates have been generated, they must be tested for
301 suitability. This may be performed using the -T option. In this mode
302 ssh-keygen will read candidates from standard input (or a file specified
303 using the -f option). For example:
304 # ssh-keygen -T moduli-2048 -f moduli-2048.candidates
306 By default, each candidate will be subjected to 100 primality tests.
308 This may be overridden using the -a option. The DH generator value will
309 be chosen automatically for the prime under consideration. If a specific
310 generator is desired, it may be requested using the -W option. Valid
311 generator values are 2, 3, and 5.
312 Screened DH groups may be installed in /etc/ssh/moduli. It is important
314 that this file contains moduli of a range of bit lengths and that both
315 ends of a connection share common moduli.
316
318 ssh-keygen supports signing of keys to produce certificates that may be
319 used for user or host authentication. Certificates consist of a public
320 key, some identity information, zero or more principal (user or host)
321 names and an optional set of constraints that are signed by a Certifica‐
322 tion Authority (CA) key. Clients or servers may then trust only the CA
323 key and verify its signature on a certificate rather than trusting many
324 user/host keys. Note that OpenSSH certificates are a different, and much
325 simpler, format to the X.509 certificates used in ssl(8).
326 ssh-keygen supports two types of certificates: user and host. User cer‐
328 tificates authenticate users to servers, whereas host certificates
329 authenticate server hosts to users. To generate a user certificate:
330 $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
332 The resultant certificate will be placed in /path/to/user_key-cert.pub.
334 A host certificate requires the -h option:
335 $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
337 The host certificate will be output to /path/to/host_key-cert.pub. In
339 both cases, key_id is a "key identifier" that is logged by the server
340 when the certificate is used for authentication.
341 Certificates may be limited to be valid for a set of principal
343 (user/host) names. By default, generated certificates are valid for all
344 users or hosts. To generate a certificate for a specified set of princi‐
345 pals:
346 $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
348 $ ssh-keygen -s ca_key -I key_id -h -n host.domain user_key.pub
349 Additional limitations on the validity and use of user certificates may
351 be specified through certificate constraints. A constrained certificate
352 may disable features of the SSH session, may be valid only when presented
353 from particular source addresses or may force the use of a specific com‐
354 mand. For a list of valid certificate constraints, see the documentation
355 for the -O option above.
356 Finally, certificates may be defined with a validity lifetime. The -V
358 option allows specification of certificate start and end times. A cer‐
359 tificate that is presented at a time outside this range will not be con‐
360 sidered valid. By default, certificates have a maximum validity inter‐
361 val.
362 For certificates to be used for user or host authentication, the CA pub‐
364 lic key must be trusted by sshd(8) or ssh(1). Please refer to those man‐
365 ual pages for details.
366
368 ~/.ssh/identity
369 Contains the protocol version 1 RSA authentication identity of
370 the user. This file should not be readable by anyone but the
371 user. It is possible to specify a passphrase when generating the
372 key; that passphrase will be used to encrypt the private part of
373 this file using 128-bit AES. This file is not automatically
374 accessed by ssh-keygen but it is offered as the default file for
375 the private key. ssh(1) will read this file when a login attempt
376 is made.
377 ~/.ssh/identity.pub
379 Contains the protocol version 1 RSA public key for authentica‐
380 tion. The contents of this file should be added to
381 ~/.ssh/authorized_keys on all machines where the user wishes to
382 log in using RSA authentication. There is no need to keep the
383 contents of this file secret.
384 ~/.ssh/id_dsa
386 Contains the protocol version 2 DSA authentication identity of
387 the user. This file should not be readable by anyone but the
388 user. It is possible to specify a passphrase when generating the
389 key; that passphrase will be used to encrypt the private part of
390 this file using 128-bit AES. This file is not automatically
391 accessed by ssh-keygen but it is offered as the default file for
392 the private key. ssh(1) will read this file when a login attempt
393 is made.
394 ~/.ssh/id_dsa.pub
396 Contains the protocol version 2 DSA public key for authentica‐
397 tion. The contents of this file should be added to
398 ~/.ssh/authorized_keys on all machines where the user wishes to
399 log in using public key authentication. There is no need to keep
400 the contents of this file secret.
401 ~/.ssh/id_rsa
403 Contains the protocol version 2 RSA authentication identity of
404 the user. This file should not be readable by anyone but the
405 user. It is possible to specify a passphrase when generating the
406 key; that passphrase will be used to encrypt the private part of
407 this file using 128-bit AES. This file is not automatically
408 accessed by ssh-keygen but it is offered as the default file for
409 the private key. ssh(1) will read this file when a login attempt
410 is made.
411 ~/.ssh/id_rsa.pub
413 Contains the protocol version 2 RSA public key for authentica‐
414 tion. The contents of this file should be added to
415 ~/.ssh/authorized_keys on all machines where the user wishes to
416 log in using public key authentication. There is no need to keep
417 the contents of this file secret.
418 /etc/ssh/moduli
420 Contains Diffie-Hellman groups used for DH-GEX. The file format
421 is described in moduli(5).
422
424 ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)
425 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
427
429 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
430 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
431 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
432 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
433 versions 1.5 and 2.0.
434BSD June 22, 2019 BSD