1 SSH-KEYGEN(1) OpenBSD Reference Manual SSH-KEYGEN(1)
4 ssh-keygen - authentication key generation, management and conversion
7 ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C comment]
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]
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 [-J num_lines] [-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]
30 ssh-keygen generates, manages and converts authentication keys for
31 ssh(1). ssh-keygen can create RSA keys for use by SSH protocol version 1
32 and DSA, ECDSA or RSA keys for use by SSH protocol version 2. The type
33 of key to be generated is specified with the -t option. If invoked
34 without any arguments, ssh-keygen will generate an RSA key for use in SSH
35 protocol 2 connections.
37 ssh-keygen is also used to generate groups for use in Diffie-Hellman
38 group exchange (DH-GEX). See the MODULI GENERATION section for details.
40 Normally each user wishing to use SSH with public key authentication runs
41 this once to create the authentication key in ~/.ssh/identity,
42 ~/.ssh/id_ecdsa, ~/.ssh/id_dsa or ~/.ssh/id_rsa. Additionally, the
43 system administrator may use this to generate host keys, as seen in
46 Normally this program generates the key and asks for a file in which to
47 store the private key. The public key is stored in a file with the same
48 name but ``.pub'' appended. The program also asks for a passphrase. The
49 passphrase may be empty to indicate no passphrase (host keys must have an
50 empty passphrase), or it may be a string of arbitrary length. A
51 passphrase is similar to a password, except it can be a phrase with a
52 series of words, punctuation, numbers, whitespace, or any string of
53 characters you want. Good passphrases are 10-30 characters long, are not
54 simple sentences or otherwise easily guessable (English prose has only
55 1-2 bits of entropy per character, and provides very bad passphrases),
56 and contain a mix of upper and lowercase letters, numbers, and non-
57 alphanumeric characters. The passphrase can be changed later by using
60 There is no way to recover a lost passphrase. If the passphrase is lost
61 or forgotten, a new key must be generated and the corresponding public
62 key copied to other machines.
64 For RSA1 keys, there is also a comment field in the key file that is only
65 for convenience to the user to help identify the key. The comment can
66 tell what the key is for, or whatever is useful. The comment is
67 initialized to ``user@host'' when the key is created, but can be changed
70 After a key is generated, instructions below detail where the keys should
71 be placed to be activated.
73 The options are as follows:
75 -A For each of the key types (rsa1, rsa, dsa and ecdsa) for which
76 host keys do not exist, generate the host keys with the default
77 key file path, an empty passphrase, default bits for the key
78 type, and default comment. This is used by /etc/rc to generate
82 Specifies the number of primality tests to perform when screening
83 DH-GEX candidates using the -T command.
85 -B Show the bubblebabble digest of specified private or public key
89 Specifies the number of bits in the key to create. For RSA keys,
90 the minimum size is 768 bits and the default is 2048 bits.
91 Generally, 2048 bits is considered sufficient. DSA keys must be
92 exactly 1024 bits as specified by FIPS 186-2. For ECDSA keys,
93 the -b flag determines the key length by selecting from one of
94 three elliptic curve sizes: 256, 384 or 521 bits. Attempting to
95 use bit lengths other than these three values for ECDSA keys will
99 Provides a new comment.
101 -c Requests changing the comment in the private and public key
102 files. This operation is only supported for RSA1 keys. The
103 program will prompt for the file containing the private keys, for
104 the passphrase if the key has one, and for the new comment.
107 Download the RSA public keys provided by the PKCS#11 shared
108 library pkcs11. When used in combination with -s, this option
109 indicates that a CA key resides in a PKCS#11 token (see the
110 CERTIFICATES section for details).
112 -e This option will read a private or public OpenSSH key file and
113 print to stdout the key in one of the formats specified by the -m
114 option. The default export format is ``RFC4716''. This option
115 allows exporting OpenSSH keys for use by other programs,
116 including several commercial SSH implementations.
119 Search for the specified hostname in a known_hosts file, listing
120 any occurrences found. This option is useful to find hashed host
121 names or addresses and may also be used in conjunction with the
122 -H option to print found keys in a hashed format.
125 Specifies the filename of the key file.
128 Generate candidate primes for DH-GEX. These primes must be
129 screened for safety (using the -T option) before use.
131 -g Use generic DNS format when printing fingerprint resource records
132 using the -r command.
134 -H Hash a known_hosts file. This replaces all hostnames and
135 addresses with hashed representations within the specified file;
136 the original content is moved to a file with a .old suffix.
137 These hashes may be used normally by ssh and sshd, but they do
138 not reveal identifying information should the file's contents be
139 disclosed. This option will not modify existing hashed hostnames
140 and is therefore safe to use on files that mix hashed and non-
143 -h When signing a key, create a host certificate instead of a user
144 certificate. Please see the CERTIFICATES section for details.
146 -I certificate_identity
147 Specify the key identity when signing a public key. Please see
148 the CERTIFICATES section for details.
150 -i This option will read an unencrypted private (or public) key file
151 in the format specified by the -m option and print an OpenSSH
152 compatible private (or public) key to stdout.
155 Exit after screening the specified number of lines while
156 performing DH candidate screening using the -T option.
159 Start screening at the specified line number while performing DH
160 candidate screening using the -T option.
163 Write the last line processed to the file checkpt while
164 performing DH candidate screening using the -T option. This will
165 be used to skip lines in the input file that have already been
166 processed if the job is restarted. This option allows importing
167 keys from other software, including several commercial SSH
168 implementations. The default import format is ``RFC4716''.
170 -L Prints the contents of a certificate.
172 -l Show fingerprint of specified public key file. Private RSA1 keys
173 are also supported. For RSA and DSA keys ssh-keygen tries to
174 find the matching public key file and prints its fingerprint. If
175 combined with -v, an ASCII art representation of the key is
176 supplied with the fingerprint.
179 Specify the amount of memory to use (in megabytes) when
180 generating candidate moduli for DH-GEX.
183 Specify a key format for the -i (import) or -e (export)
184 conversion options. The supported key formats are: ``RFC4716''
185 (RFC 4716/SSH2 public or private key), ``PKCS8'' (PEM PKCS8
186 public key) or ``PEM'' (PEM public key). The default conversion
187 format is ``RFC4716''.
190 Provides the new passphrase.
193 Specify one or more principals (user or host names) to be
194 included in a certificate when signing a key. Multiple
195 principals may be specified, separated by commas. Please see the
196 CERTIFICATES section for details.
199 Specify a certificate option when signing a key. This option may
200 be specified multiple times. Please see the CERTIFICATES section
201 for details. The options that are valid for user certificates
204 clear Clear all enabled permissions. This is useful for
205 clearing the default set of permissions so permissions
206 may be added individually.
208 force-command=command
209 Forces the execution of command instead of any shell or
210 command specified by the user when the certificate is
211 used for authentication.
214 Disable ssh-agent(1) forwarding (permitted by default).
217 Disable port forwarding (permitted by default).
219 no-pty Disable PTY allocation (permitted by default).
222 Disable execution of ~/.ssh/rc by sshd(8) (permitted by
226 Disable X11 forwarding (permitted by default).
228 permit-agent-forwarding
229 Allows ssh-agent(1) forwarding.
231 permit-port-forwarding
232 Allows port forwarding.
235 Allows PTY allocation.
238 Allows execution of ~/.ssh/rc by sshd(8).
240 permit-x11-forwarding
241 Allows X11 forwarding.
243 source-address=address_list
244 Restrict the source addresses from which the certificate
245 is considered valid. The address_list is a comma-
246 separated list of one or more address/netmask pairs in
249 At present, no options are valid for host keys.
252 Provides the (old) passphrase.
254 -p Requests changing the passphrase of a private key file instead of
255 creating a new private key. The program will prompt for the file
256 containing the private key, for the old passphrase, and twice for
259 -q Silence ssh-keygen.
262 Removes all keys belonging to hostname from a known_hosts file.
263 This option is useful to delete hashed hosts (see the -H option
267 Print the SSHFP fingerprint resource record named hostname for
268 the specified public key file.
271 Specify start point (in hex) when generating candidate moduli for
275 Certify (sign) a public key using the specified CA key. Please
276 see the CERTIFICATES section for details.
279 Test DH group exchange candidate primes (generated using the -G
283 Specifies the type of key to create. The possible values are
284 ``rsa1'' for protocol version 1 and ``dsa'', ``ecdsa'' or ``rsa''
285 for protocol version 2.
288 Specify a validity interval when signing a certificate. A
289 validity interval may consist of a single time, indicating that
290 the certificate is valid beginning now and expiring at that time,
291 or may consist of two times separated by a colon to indicate an
292 explicit time interval. The start time may be specified as a
293 date in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a
294 relative time (to the current time) consisting of a minus sign
295 followed by a relative time in the format described in the TIME
296 FORMATS section of sshd_config(5). The end time may be specified
297 as a YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time
298 starting with a plus character.
300 For example: ``+52w1d'' (valid from now to 52 weeks and one day
301 from now), ``-4w:+4w'' (valid from four weeks ago to four weeks
302 from now), ``20100101123000:20110101123000'' (valid from 12:30
303 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011),
304 ``-1d:20110101'' (valid from yesterday to midnight, January 1st,
307 -v Verbose mode. Causes ssh-keygen to print debugging messages
308 about its progress. This is helpful for debugging moduli
309 generation. Multiple -v options increase the verbosity. The
313 Specify desired generator when testing candidate moduli for DH-
316 -y This option will read a private OpenSSH format file and print an
317 OpenSSH public key to stdout.
320 Specifies a serial number to be embedded in the certificate to
321 distinguish this certificate from others from the same CA. The
322 default serial number is zero.
325 ssh-keygen may be used to generate groups for the Diffie-Hellman Group
326 Exchange (DH-GEX) protocol. Generating these groups is a two-step
327 process: first, candidate primes are generated using a fast, but memory
328 intensive process. These candidate primes are then tested for
329 suitability (a CPU-intensive process).
331 Generation of primes is performed using the -G option. The desired
332 length of the primes may be specified by the -b option. For example:
334 # ssh-keygen -G moduli-2048.candidates -b 2048
336 By default, the search for primes begins at a random point in the desired
337 length range. This may be overridden using the -S option, which
338 specifies a different start point (in hex).
340 Once a set of candidates have been generated, they must be screened for
341 suitability. This may be performed using the -T option. In this mode
342 ssh-keygen will read candidates from standard input (or a file specified
343 using the -f option). For example:
345 # ssh-keygen -T moduli-2048 -f moduli-2048.candidates
347 By default, each candidate will be subjected to 100 primality tests.
348 This may be overridden using the -a option. The DH generator value will
349 be chosen automatically for the prime under consideration. If a specific
350 generator is desired, it may be requested using the -W option. Valid
351 generator values are 2, 3, and 5.
353 Screened DH groups may be installed in /etc/moduli. It is important that
354 this file contains moduli of a range of bit lengths and that both ends of
355 a connection share common moduli.
358 ssh-keygen supports signing of keys to produce certificates that may be
359 used for user or host authentication. Certificates consist of a public
360 key, some identity information, zero or more principal (user or host)
361 names and a set of options that are signed by a Certification Authority
362 (CA) key. Clients or servers may then trust only the CA key and verify
363 its signature on a certificate rather than trusting many user/host keys.
364 Note that OpenSSH certificates are a different, and much simpler, format
365 to the X.509 certificates used in ssl(8).
367 ssh-keygen supports two types of certificates: user and host. User
368 certificates authenticate users to servers, whereas host certificates
369 authenticate server hosts to users. To generate a user certificate:
371 $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
373 The resultant certificate will be placed in /path/to/user_key-cert.pub.
374 A host certificate requires the -h option:
376 $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
378 The host certificate will be output to /path/to/host_key-cert.pub.
380 It is possible to sign using a CA key stored in a PKCS#11 token by
381 providing the token library using -D and identifying the CA key by
382 providing its public half as an argument to -s:
384 $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id host_key.pub
386 In all cases, key_id is a "key identifier" that is logged by the server
387 when the certificate is used for authentication.
389 Certificates may be limited to be valid for a set of principal
390 (user/host) names. By default, generated certificates are valid for all
391 users or hosts. To generate a certificate for a specified set of
394 $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
395 $ ssh-keygen -s ca_key -I key_id -h -n host.domain user_key.pub
397 Additional limitations on the validity and use of user certificates may
398 be specified through certificate options. A certificate option may
399 disable features of the SSH session, may be valid only when presented
400 from particular source addresses or may force the use of a specific
401 command. For a list of valid certificate options, see the documentation
402 for the -O option above.
404 Finally, certificates may be defined with a validity lifetime. The -V
405 option allows specification of certificate start and end times. A
406 certificate that is presented at a time outside this range will not be
407 considered valid. By default, certificates have a maximum validity
410 For certificates to be used for user or host authentication, the CA
411 public key must be trusted by sshd(8) or ssh(1). Please refer to those
412 manual pages for details.
416 Contains the protocol version 1 RSA authentication identity of
417 the user. This file should not be readable by anyone but the
418 user. It is possible to specify a passphrase when generating the
419 key; that passphrase will be used to encrypt the private part of
420 this file using 3DES. This file is not automatically accessed by
421 ssh-keygen but it is offered as the default file for the private
422 key. ssh(1) will read this file when a login attempt is made.
425 Contains the protocol version 1 RSA public key for
426 authentication. The contents of this file should be added to
427 ~/.ssh/authorized_keys on all machines where the user wishes to
428 log in using RSA authentication. There is no need to keep the
429 contents of this file secret.
434 Contains the protocol version 2 DSA, ECDSA or RSA authentication
435 identity of the user. This file should not be readable by anyone
436 but the user. It is possible to specify a passphrase when
437 generating the key; that passphrase will be used to encrypt the
438 private part of this file using 128-bit AES. This file is not
439 automatically accessed by ssh-keygen but it is offered as the
440 default file for the private key. ssh(1) will read this file
441 when a login attempt is made.
446 Contains the protocol version 2 DSA, ECDSA or RSA public key for
447 authentication. The contents of this file should be added to
448 ~/.ssh/authorized_keys on all machines where the user wishes to
449 log in using public key authentication. There is no need to keep
450 the contents of this file secret.
453 Contains Diffie-Hellman groups used for DH-GEX. The file format
454 is described in moduli(5).
457 ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)
459 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
462 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
463 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
464 de Raadt and Dug Song removed many bugs, re-added newer features and
465 created OpenSSH. Markus Friedl contributed the support for SSH protocol
466 versions 1.5 and 2.0.
468 OpenBSD 5.2 July 6, 2012 OpenBSD 5.2