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1SSH-KEYGEN(1) General Commands Manual SSH-KEYGEN(1)
2
3NAME
4 ssh-keygen M-bM-^@M-^S authentication key generation, management and conversion
5
6SYNOPSIS
7 ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa]
8 [-N new_passphrase] [-C comment] [-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 [-v] [-E fingerprint_hash] [-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] [-U]
25 [-D pkcs11_provider] [-n principals] [-O option]
26 [-V validity_interval] [-z serial_number] file ...
27 ssh-keygen -L [-f input_keyfile]
28 ssh-keygen -A [-f prefix_path]
29 ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]
30 file ...
31 ssh-keygen -Q -f krl_file file ...
32
33DESCRIPTION
34 ssh-keygen generates, manages and converts authentication keys for
35 ssh(1). ssh-keygen can create keys for use by SSH protocol version 2.
36
37 The type of key to be generated is specified with the -t option. If
38 invoked without any arguments, ssh-keygen will generate an RSA key.
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44 Lists, and to test whether given keys have been revoked by one.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/id_dsa,
49 ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 or ~/.ssh/id_rsa. Additionally, the
50 system administrator may use this to generate host keys, as seen in
51 /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 M-bM-^@M-^\.pubM-bM-^@M-^] 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 keys stored in the newer OpenSSH format, there is also a comment
72 field in the key file that is only for convenience to the user to help
73 identify the key. The comment can tell what the key is for, or whatever
74 is useful. The comment is initialized to M-bM-^@M-^\user@hostM-bM-^@M-^] when the key is
75 created, but can be changed 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 (rsa, dsa, ecdsa and ed25519) for which
83 host keys do not exist, generate the host keys with the default
84 key file path, an empty passphrase, default bits for the key
85 type, and default comment. If -f has also been specified, its
86 argument is used as a prefix to the default path for the
87 resulting host key files. This is used by /etc/rc to generate
88 new host keys.
89
90 -a rounds
91 When saving a private key this option specifies the number of KDF
92 (key derivation function) rounds used. Higher numbers result in
93 slower passphrase verification and increased resistance to brute-
94 force password cracking (should 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 1024 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. The program will prompt for the file containing the
118 private keys, for the passphrase if the key has one, and for the
119 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 fingerprint_hash
128 Specifies the hash algorithm used when displaying key
129 fingerprints. Valid options are: M-bM-^@M-^\md5M-bM-^@M-^] and M-bM-^@M-^\sha256M-bM-^@M-^]. The
130 default is M-bM-^@M-^\sha256M-bM-^@M-^].
131
132 -e This option will read a private or public OpenSSH key file and
133 print to stdout the key in one of the formats specified by the -m
134 option. The default export format is M-bM-^@M-^\RFC4716M-bM-^@M-^]. This option
135 allows exporting OpenSSH keys for use by other programs,
136 including several commercial SSH implementations.
137
138 -F hostname
139 Search for the specified hostname in a known_hosts file, listing
140 any occurrences found. This option is useful to find hashed host
141 names or addresses and may also be used in conjunction with the
142 -H option to print found keys in a hashed format.
143
144 -f filename
145 Specifies the filename of the key file.
146
147 -G output_file
148 Generate candidate primes for DH-GEX. These primes must be
149 screened for safety (using the -T option) before use.
150
151 -g Use generic DNS format when printing fingerprint resource records
152 using the -r command.
153
154 -H Hash a known_hosts file. This replaces all hostnames and
155 addresses with hashed representations within the specified file;
156 the original content is moved to a file with a .old suffix.
157 These hashes may be used normally by ssh and sshd, but they do
158 not reveal identifying information should the file's contents be
159 disclosed. This option will not modify existing hashed hostnames
160 and is therefore safe to use on files that mix hashed and non-
161 hashed names.
162
163 -h When signing a key, create a host certificate instead of a user
164 certificate. Please see the CERTIFICATES section for details.
165
166 -I certificate_identity
167 Specify the key identity when signing a public key. Please see
168 the CERTIFICATES section for details.
169
170 -i This option will read an unencrypted private (or public) key file
171 in the format specified by the -m option and print an OpenSSH
172 compatible private (or public) key to stdout. This option allows
173 importing keys from other software, including several commercial
174 SSH implementations. The default import format is M-bM-^@M-^\RFC4716M-bM-^@M-^].
175
176 -J num_lines
177 Exit after screening the specified number of lines while
178 performing DH candidate screening using the -T option.
179
180 -j start_line
181 Start screening at the specified line number while performing DH
182 candidate screening using the -T option.
183
184 -K checkpt
185 Write the last line processed to the file checkpt while
186 performing DH candidate screening using the -T option. This will
187 be used to skip lines in the input file that have already been
188 processed if the job is restarted.
189
190 -k Generate a KRL file. In this mode, ssh-keygen will generate a
191 KRL file at the location specified via the -f flag that revokes
192 every key or certificate presented on the command line.
193 Keys/certificates to be revoked may be specified by public key
194 file or using the format described in the KEY REVOCATION LISTS
195 section.
196
197 -L Prints the contents of one or more certificates.
198
199 -l Show fingerprint of specified public key file. For RSA and DSA
200 keys ssh-keygen tries to find the matching public key file and
201 prints its fingerprint. If combined with -v, a visual ASCII art
202 representation of the key is supplied with the fingerprint.
203
204 -M memory
205 Specify the amount of memory to use (in megabytes) when
206 generating candidate moduli for DH-GEX.
207
208 -m key_format
209 Specify a key format for the -i (import) or -e (export)
210 conversion options. The supported key formats are: M-bM-^@M-^\RFC4716M-bM-^@M-^]
211 (RFC 4716/SSH2 public or private key), M-bM-^@M-^\PKCS8M-bM-^@M-^] (PEM PKCS8 public
212 key) or M-bM-^@M-^\PEMM-bM-^@M-^] (PEM public key). The default conversion format is
213 M-bM-^@M-^\RFC4716M-bM-^@M-^]. Setting a format of M-bM-^@M-^\PEMM-bM-^@M-^] when generating or updating
214 a supported private key type will cause the key to be stored in
215 the legacy PEM private key format.
216
217 -N new_passphrase
218 Provides the new passphrase.
219
220 -n principals
221 Specify one or more principals (user or host names) to be
222 included in a certificate when signing a key. Multiple
223 principals may be specified, separated by commas. Please see the
224 CERTIFICATES section for details.
225
226 -O option
227 Specify a certificate option when signing a key. This option may
228 be specified multiple times. See also the CERTIFICATES section
229 for further details.
230
231 At present, no standard options are valid for host keys. The
232 options that are valid for user certificates are:
233
234 clear Clear all enabled permissions. This is useful for
235 clearing the default set of permissions so permissions
236 may be added individually.
237
238 critical:name[=contents]
239 extension:name[=contents]
240 Includes an arbitrary certificate critical option or
241 extension. The specified name should include a domain
242 suffix, e.g. M-bM-^@M-^\name@example.comM-bM-^@M-^]. If contents is
243 specified then it is included as the contents of the
244 extension/option encoded as a string, otherwise the
245 extension/option is created with no contents (usually
246 indicating a flag). Extensions may be ignored by a
247 client or server that does not recognise them, whereas
248 unknown critical options will cause the certificate to be
249 refused.
250
251 force-command=command
252 Forces the execution of command instead of any shell or
253 command specified by the user when the certificate is
254 used for authentication.
255
256 no-agent-forwarding
257 Disable ssh-agent(1) forwarding (permitted by default).
258
259 no-port-forwarding
260 Disable port forwarding (permitted by default).
261
262 no-pty Disable PTY allocation (permitted by default).
263
264 no-user-rc
265 Disable execution of ~/.ssh/rc by sshd(8) (permitted by
266 default).
267
268 no-x11-forwarding
269 Disable X11 forwarding (permitted by default).
270
271 permit-agent-forwarding
272 Allows ssh-agent(1) forwarding.
273
274 permit-port-forwarding
275 Allows port forwarding.
276
277 permit-pty
278 Allows PTY allocation.
279
280 permit-user-rc
281 Allows execution of ~/.ssh/rc by sshd(8).
282
283 permit-X11-forwarding
284 Allows X11 forwarding.
285
286 source-address=address_list
287 Restrict the source addresses from which the certificate
288 is considered valid. The address_list is a comma-
289 separated list of one or more address/netmask pairs in
290 CIDR format.
291
292 -P passphrase
293 Provides the (old) passphrase.
294
295 -p Requests changing the passphrase of a private key file instead of
296 creating a new private key. The program will prompt for the file
297 containing the private key, for the old passphrase, and twice for
298 the new passphrase.
299
300 -Q Test whether keys have been revoked in a KRL.
301
302 -q Silence ssh-keygen.
303
304 -R hostname
305 Removes all keys belonging to hostname from a known_hosts file.
306 This option is useful to delete hashed hosts (see the -H option
307 above).
308
309 -r hostname
310 Print the SSHFP fingerprint resource record named hostname for
311 the specified public key file.
312
313 -S start
314 Specify start point (in hex) when generating candidate moduli for
315 DH-GEX.
316
317 -s ca_key
318 Certify (sign) a public key using the specified CA key. Please
319 see the CERTIFICATES section for details.
320
321 When generating a KRL, -s specifies a path to a CA public key
322 file used to revoke certificates directly by key ID or serial
323 number. See the KEY REVOCATION LISTS section for details.
324
325 -T output_file
326 Test DH group exchange candidate primes (generated using the -G
327 option) for safety.
328
329 -t dsa | ecdsa | ed25519 | rsa
330 Specifies the type of key to create. The possible values are
331 M-bM-^@M-^\dsaM-bM-^@M-^], M-bM-^@M-^\ecdsaM-bM-^@M-^], M-bM-^@M-^\ed25519M-bM-^@M-^], or M-bM-^@M-^\rsaM-bM-^@M-^].
332
333 -U When used in combination with -s, this option indicates that a CA
334 key resides in a ssh-agent(1). See the CERTIFICATES section for
335 more information.
336
337 -u Update a KRL. When specified with -k, keys listed via the
338 command line are added to the existing KRL rather than a new KRL
339 being created.
340
341 -V validity_interval
342 Specify a validity interval when signing a certificate. A
343 validity interval may consist of a single time, indicating that
344 the certificate is valid beginning now and expiring at that time,
345 or may consist of two times separated by a colon to indicate an
346 explicit time interval.
347
348 The start time may be specified as the string M-bM-^@M-^\alwaysM-bM-^@M-^] to
349 indicate the certificate has no specified start time, a date in
350 YYYYMMDD format, a time in YYYYMMDDHHMM[SS] format, a relative
351 time (to the current time) consisting of a minus sign followed by
352 an interval in the format described in the TIME FORMATS section
353 of sshd_config(5).
354
355 The end time may be specified as a YYYYMMDD date, a
356 YYYYMMDDHHMM[SS] time, a relative time starting with a plus
357 character or the string M-bM-^@M-^\foreverM-bM-^@M-^] to indicate that the
358 certificate has no expirty date.
359
360 For example: M-bM-^@M-^\+52w1dM-bM-^@M-^] (valid from now to 52 weeks and one day
361 from now), M-bM-^@M-^\-4w:+4wM-bM-^@M-^] (valid from four weeks ago to four weeks
362 from now), M-bM-^@M-^\20100101123000:20110101123000M-bM-^@M-^] (valid from 12:30 PM,
363 January 1st, 2010 to 12:30 PM, January 1st, 2011), M-bM-^@M-^\-1d:20110101M-bM-^@M-^]
364 (valid from yesterday to midnight, January 1st, 2011).
365 M-bM-^@M-^\-1m:foreverM-bM-^@M-^] (valid from one minute ago and never expiring).
366
367 -v Verbose mode. Causes ssh-keygen to print debugging messages
368 about its progress. This is helpful for debugging moduli
369 generation. Multiple -v options increase the verbosity. The
370 maximum is 3.
371
372 -W generator
373 Specify desired generator when testing candidate moduli for DH-
374 GEX.
375
376 -y This option will read a private OpenSSH format file and print an
377 OpenSSH public key to stdout.
378
379 -z serial_number
380 Specifies a serial number to be embedded in the certificate to
381 distinguish this certificate from others from the same CA. The
382 default serial number is zero.
383
384 When generating a KRL, the -z flag is used to specify a KRL
385 version number.
386
387MODULI GENERATION
388 ssh-keygen may be used to generate groups for the Diffie-Hellman Group
389 Exchange (DH-GEX) protocol. Generating these groups is a two-step
390 process: first, candidate primes are generated using a fast, but memory
391 intensive process. These candidate primes are then tested for
392 suitability (a CPU-intensive process).
393
394 Generation of primes is performed using the -G option. The desired
395 length of the primes may be specified by the -b option. For example:
396
397 # ssh-keygen -G moduli-2048.candidates -b 2048
398
399 By default, the search for primes begins at a random point in the desired
400 length range. This may be overridden using the -S option, which
401 specifies a different start point (in hex).
402
403 Once a set of candidates have been generated, they must be screened for
404 suitability. This may be performed using the -T option. In this mode
405 ssh-keygen will read candidates from standard input (or a file specified
406 using the -f option). For example:
407
408 # ssh-keygen -T moduli-2048 -f moduli-2048.candidates
409
410 By default, each candidate will be subjected to 100 primality tests.
411 This may be overridden using the -a option. The DH generator value will
412 be chosen automatically for the prime under consideration. If a specific
413 generator is desired, it may be requested using the -W option. Valid
414 generator values are 2, 3, and 5.
415
416 Screened DH groups may be installed in /etc/moduli. It is important that
417 this file contains moduli of a range of bit lengths and that both ends of
418 a connection share common moduli.
419
420CERTIFICATES
421 ssh-keygen supports signing of keys to produce certificates that may be
422 used for user or host authentication. Certificates consist of a public
423 key, some identity information, zero or more principal (user or host)
424 names and a set of options that are signed by a Certification Authority
425 (CA) key. Clients or servers may then trust only the CA key and verify
426 its signature on a certificate rather than trusting many user/host keys.
427 Note that OpenSSH certificates are a different, and much simpler, format
428 to the X.509 certificates used in ssl(8).
429
430 ssh-keygen supports two types of certificates: user and host. User
431 certificates authenticate users to servers, whereas host certificates
432 authenticate server hosts to users. To generate a user certificate:
433
434 $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub
435
436 The resultant certificate will be placed in /path/to/user_key-cert.pub.
437 A host certificate requires the -h option:
438
439 $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub
440
441 The host certificate will be output to /path/to/host_key-cert.pub.
442
443 It is possible to sign using a CA key stored in a PKCS#11 token by
444 providing the token library using -D and identifying the CA key by
445 providing its public half as an argument to -s:
446
447 $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub
448
449 Similarly, it is possible for the CA key to be hosted in a ssh-agent(1).
450 This is indicated by the -U flag and, again, the CA key must be
451 identified by its public half.
452
453 $ ssh-keygen -Us ca_key.pub -I key_id user_key.pub
454
455 In all cases, key_id is a "key identifier" that is logged by the server
456 when the certificate is used for authentication.
457
458 Certificates may be limited to be valid for a set of principal
459 (user/host) names. By default, generated certificates are valid for all
460 users or hosts. To generate a certificate for a specified set of
461 principals:
462
463 $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
464 $ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub
465
466 Additional limitations on the validity and use of user certificates may
467 be specified through certificate options. A certificate option may
468 disable features of the SSH session, may be valid only when presented
469 from particular source addresses or may force the use of a specific
470 command. For a list of valid certificate options, see the documentation
471 for the -O option above.
472
473 Finally, certificates may be defined with a validity lifetime. The -V
474 option allows specification of certificate start and end times. A
475 certificate that is presented at a time outside this range will not be
476 considered valid. By default, certificates are valid from UNIX Epoch to
477 the distant future.
478
479 For certificates to be used for user or host authentication, the CA
480 public key must be trusted by sshd(8) or ssh(1). Please refer to those
481 manual pages for details.
482
483KEY REVOCATION LISTS
484 ssh-keygen is able to manage OpenSSH format Key Revocation Lists (KRLs).
485 These binary files specify keys or certificates to be revoked using a
486 compact format, taking as little as one bit per certificate if they are
487 being revoked by serial number.
488
489 KRLs may be generated using the -k flag. This option reads one or more
490 files from the command line and generates a new KRL. The files may
491 either contain a KRL specification (see below) or public keys, listed one
492 per line. Plain public keys are revoked by listing their hash or
493 contents in the KRL and certificates revoked by serial number or key ID
494 (if the serial is zero or not available).
495
496 Revoking keys using a KRL specification offers explicit control over the
497 types of record used to revoke keys and may be used to directly revoke
498 certificates by serial number or key ID without having the complete
499 original certificate on hand. A KRL specification consists of lines
500 containing one of the following directives followed by a colon and some
501 directive-specific information.
502
503 serial: serial_number[-serial_number]
504 Revokes a certificate with the specified serial number. Serial
505 numbers are 64-bit values, not including zero and may be
506 expressed in decimal, hex or octal. If two serial numbers are
507 specified separated by a hyphen, then the range of serial numbers
508 including and between each is revoked. The CA key must have been
509 specified on the ssh-keygen command line using the -s option.
510
511 id: key_id
512 Revokes a certificate with the specified key ID string. The CA
513 key must have been specified on the ssh-keygen command line using
514 the -s option.
515
516 key: public_key
517 Revokes the specified key. If a certificate is listed, then it
518 is revoked as a plain public key.
519
520 sha1: public_key
521 Revokes the specified key by including its SHA1 hash in the KRL.
522
523 sha256: public_key
524 Revokes the specified key by including its SHA256 hash in the
525 KRL. KRLs that revoke keys by SHA256 hash are not supported by
526 OpenSSH versions prior to 7.9.
527
528 hash: fingerprint
529 Revokes a key using a fingerprint hash, as obtained from a
530 sshd(8) authentication log message or the ssh-keygen -l flag.
531 Only SHA256 fingerprints are supported here and resultant KRLs
532 are not supported by OpenSSH versions prior to 7.9.
533
534 KRLs may be updated using the -u flag in addition to -k. When this
535 option is specified, keys listed via the command line are merged into the
536 KRL, adding to those already there.
537
538 It is also possible, given a KRL, to test whether it revokes a particular
539 key (or keys). The -Q flag will query an existing KRL, testing each key
540 specified on the command line. If any key listed on the command line has
541 been revoked (or an error encountered) then ssh-keygen will exit with a
542 non-zero exit status. A zero exit status will only be returned if no key
543 was revoked.
544
545FILES
546 ~/.ssh/id_dsa
547 ~/.ssh/id_ecdsa
548 ~/.ssh/id_ed25519
549 ~/.ssh/id_rsa
550 Contains the DSA, ECDSA, Ed25519 or RSA authentication identity
551 of the user. This file should not be readable by anyone but the
552 user. It is possible to specify a passphrase when generating the
553 key; that passphrase will be used to encrypt the private part of
554 this file using 128-bit AES. This file is not automatically
555 accessed by ssh-keygen but it is offered as the default file for
556 the private key. ssh(1) will read this file when a login attempt
557 is made.
558
559 ~/.ssh/id_dsa.pub
560 ~/.ssh/id_ecdsa.pub
561 ~/.ssh/id_ed25519.pub
562 ~/.ssh/id_rsa.pub
563 Contains the DSA, ECDSA, Ed25519 or RSA public key for
564 authentication. The contents of this file should be added to
565 ~/.ssh/authorized_keys on all machines where the user wishes to
566 log in using public key authentication. There is no need to keep
567 the contents of this file secret.
568
569 /etc/moduli
570 Contains Diffie-Hellman groups used for DH-GEX. The file format
571 is described in moduli(5).
572
573SEE ALSO
574 ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)
575
576 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
577
578AUTHORS
579 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
580 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
581 de Raadt and Dug Song removed many bugs, re-added newer features and
582 created OpenSSH. Markus Friedl contributed the support for SSH protocol
583 versions 1.5 and 2.0.
584
585OpenBSD 6.4 September 12, 2018 OpenBSD 6.4