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diff --git a/ssh-keygen.1 b/ssh-keygen.1 new file mode 100644 index 0000000..8b1f617 --- /dev/null +++ b/ssh-keygen.1 @@ -0,0 +1,1334 @@ +.\" $OpenBSD: ssh-keygen.1,v 1.226 2022/09/10 08:50:53 jsg Exp $ +.\" +.\" Author: Tatu Ylonen <ylo@cs.hut.fi> +.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland +.\" All rights reserved +.\" +.\" As far as I am concerned, the code I have written for this software +.\" can be used freely for any purpose. Any derived versions of this +.\" software must be clearly marked as such, and if the derived work is +.\" incompatible with the protocol description in the RFC file, it must be +.\" called by a name other than "ssh" or "Secure Shell". +.\" +.\" +.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. +.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. +.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR +.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES +.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. +.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, +.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT +.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, +.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY +.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT +.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF +.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. +.\" +.Dd $Mdocdate: September 10 2022 $ +.Dt SSH-KEYGEN 1 +.Os +.Sh NAME +.Nm ssh-keygen +.Nd OpenSSH authentication key utility +.Sh SYNOPSIS +.Nm ssh-keygen +.Op Fl q +.Op Fl a Ar rounds +.Op Fl b Ar bits +.Op Fl C Ar comment +.Op Fl f Ar output_keyfile +.Op Fl m Ar format +.Op Fl N Ar new_passphrase +.Op Fl O Ar option +.Op Fl t Cm dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa +.Op Fl w Ar provider +.Op Fl Z Ar cipher +.Nm ssh-keygen +.Fl p +.Op Fl a Ar rounds +.Op Fl f Ar keyfile +.Op Fl m Ar format +.Op Fl N Ar new_passphrase +.Op Fl P Ar old_passphrase +.Op Fl Z Ar cipher +.Nm ssh-keygen +.Fl i +.Op Fl f Ar input_keyfile +.Op Fl m Ar key_format +.Nm ssh-keygen +.Fl e +.Op Fl f Ar input_keyfile +.Op Fl m Ar key_format +.Nm ssh-keygen +.Fl y +.Op Fl f Ar input_keyfile +.Nm ssh-keygen +.Fl c +.Op Fl a Ar rounds +.Op Fl C Ar comment +.Op Fl f Ar keyfile +.Op Fl P Ar passphrase +.Nm ssh-keygen +.Fl l +.Op Fl v +.Op Fl E Ar fingerprint_hash +.Op Fl f Ar input_keyfile +.Nm ssh-keygen +.Fl B +.Op Fl f Ar input_keyfile +.Nm ssh-keygen +.Fl D Ar pkcs11 +.Nm ssh-keygen +.Fl F Ar hostname +.Op Fl lv +.Op Fl f Ar known_hosts_file +.Nm ssh-keygen +.Fl H +.Op Fl f Ar known_hosts_file +.Nm ssh-keygen +.Fl K +.Op Fl a Ar rounds +.Op Fl w Ar provider +.Nm ssh-keygen +.Fl R Ar hostname +.Op Fl f Ar known_hosts_file +.Nm ssh-keygen +.Fl r Ar hostname +.Op Fl g +.Op Fl f Ar input_keyfile +.Nm ssh-keygen +.Fl M Cm generate +.Op Fl O Ar option +.Ar output_file +.Nm ssh-keygen +.Fl M Cm screen +.Op Fl f Ar input_file +.Op Fl O Ar option +.Ar output_file +.Nm ssh-keygen +.Fl I Ar certificate_identity +.Fl s Ar ca_key +.Op Fl hU +.Op Fl D Ar pkcs11_provider +.Op Fl n Ar principals +.Op Fl O Ar option +.Op Fl V Ar validity_interval +.Op Fl z Ar serial_number +.Ar +.Nm ssh-keygen +.Fl L +.Op Fl f Ar input_keyfile +.Nm ssh-keygen +.Fl A +.Op Fl a Ar rounds +.Op Fl f Ar prefix_path +.Nm ssh-keygen +.Fl k +.Fl f Ar krl_file +.Op Fl u +.Op Fl s Ar ca_public +.Op Fl z Ar version_number +.Ar +.Nm ssh-keygen +.Fl Q +.Op Fl l +.Fl f Ar krl_file +.Ar +.Nm ssh-keygen +.Fl Y Cm find-principals +.Op Fl O Ar option +.Fl s Ar signature_file +.Fl f Ar allowed_signers_file +.Nm ssh-keygen +.Fl Y Cm match-principals +.Fl I Ar signer_identity +.Fl f Ar allowed_signers_file +.Nm ssh-keygen +.Fl Y Cm check-novalidate +.Op Fl O Ar option +.Fl n Ar namespace +.Fl s Ar signature_file +.Nm ssh-keygen +.Fl Y Cm sign +.Op Fl O Ar option +.Fl f Ar key_file +.Fl n Ar namespace +.Ar +.Nm ssh-keygen +.Fl Y Cm verify +.Op Fl O Ar option +.Fl f Ar allowed_signers_file +.Fl I Ar signer_identity +.Fl n Ar namespace +.Fl s Ar signature_file +.Op Fl r Ar revocation_file +.Sh DESCRIPTION +.Nm +generates, manages and converts authentication keys for +.Xr ssh 1 . +.Nm +can create keys for use by SSH protocol version 2. +.Pp +The type of key to be generated is specified with the +.Fl t +option. +If invoked without any arguments, +.Nm +will generate an RSA key. +.Pp +.Nm +is also used to generate groups for use in Diffie-Hellman group +exchange (DH-GEX). +See the +.Sx MODULI GENERATION +section for details. +.Pp +Finally, +.Nm +can be used to generate and update Key Revocation Lists, and to test whether +given keys have been revoked by one. +See the +.Sx KEY REVOCATION LISTS +section for details. +.Pp +Normally each user wishing to use SSH +with public key authentication runs this once to create the authentication +key in +.Pa ~/.ssh/id_dsa , +.Pa ~/.ssh/id_ecdsa , +.Pa ~/.ssh/id_ecdsa_sk , +.Pa ~/.ssh/id_ed25519 , +.Pa ~/.ssh/id_ed25519_sk +or +.Pa ~/.ssh/id_rsa . +Additionally, the system administrator may use this to generate host keys, +as seen in +.Pa /etc/rc . +.Pp +Normally this program generates the key and asks for a file in which +to store the private key. +The public key is stored in a file with the same name but +.Dq .pub +appended. +The program also asks for a passphrase. +The passphrase may be empty to indicate no passphrase +(host keys must have an empty passphrase), or it may be a string of +arbitrary length. +A passphrase is similar to a password, except it can be a phrase with a +series of words, punctuation, numbers, whitespace, or any string of +characters you want. +Good passphrases are 10-30 characters long, are +not simple sentences or otherwise easily guessable (English +prose has only 1-2 bits of entropy per character, and provides very bad +passphrases), and contain a mix of upper and lowercase letters, +numbers, and non-alphanumeric characters. +The passphrase can be changed later by using the +.Fl p +option. +.Pp +There is no way to recover a lost passphrase. +If the passphrase is lost or forgotten, a new key must be generated +and the corresponding public key copied to other machines. +.Pp +.Nm +will by default write keys in an OpenSSH-specific format. +This format is preferred as it offers better protection for +keys at rest as well as allowing storage of key comments within +the private key file itself. +The key comment may be useful to help identify the key. +The comment is initialized to +.Dq user@host +when the key is created, but can be changed using the +.Fl c +option. +.Pp +It is still possible for +.Nm +to write the previously-used PEM format private keys using the +.Fl m +flag. +This may be used when generating new keys, and existing new-format +keys may be converted using this option in conjunction with the +.Fl p +(change passphrase) flag. +.Pp +After a key is generated, +.Nm +will ask where the keys +should be placed to be activated. +.Pp +The options are as follows: +.Bl -tag -width Ds +.It Fl A +Generate host keys of all default key types (rsa, ecdsa, and +ed25519) if they do not already exist. +The host keys are generated with the default key file path, +an empty passphrase, default bits for the key type, and default comment. +If +.Fl f +has also been specified, its argument is used as a prefix to the +default path for the resulting host key files. +This is used by +.Pa /etc/rc +to generate new host keys. +.It Fl a Ar rounds +When saving a private key, this option specifies the number of KDF +(key derivation function, currently +.Xr bcrypt_pbkdf 3 ) +rounds used. +Higher numbers result in slower passphrase verification and increased +resistance to brute-force password cracking (should the keys be stolen). +The default is 16 rounds. +.It Fl B +Show the bubblebabble digest of specified private or public key file. +.It Fl b Ar bits +Specifies the number of bits in the key to create. +For RSA keys, the minimum size is 1024 bits and the default is 3072 bits. +Generally, 3072 bits is considered sufficient. +DSA keys must be exactly 1024 bits as specified by FIPS 186-2. +For ECDSA keys, the +.Fl b +flag determines the key length by selecting from one of three elliptic +curve sizes: 256, 384 or 521 bits. +Attempting to use bit lengths other than these three values for ECDSA keys +will fail. +ECDSA-SK, Ed25519 and Ed25519-SK keys have a fixed length and the +.Fl b +flag will be ignored. +.It Fl C Ar comment +Provides a new comment. +.It Fl c +Requests changing the comment in the private and public key files. +The program will prompt for the file containing the private keys, for +the passphrase if the key has one, and for the new comment. +.It Fl D Ar pkcs11 +Download the public keys provided by the PKCS#11 shared library +.Ar pkcs11 . +When used in combination with +.Fl s , +this option indicates that a CA key resides in a PKCS#11 token (see the +.Sx CERTIFICATES +section for details). +.It Fl E Ar fingerprint_hash +Specifies the hash algorithm used when displaying key fingerprints. +Valid options are: +.Dq md5 +and +.Dq sha256 . +The default is +.Dq sha256 . +.It Fl e +This option will read a private or public OpenSSH key file and +print to stdout a public key in one of the formats specified by the +.Fl m +option. +The default export format is +.Dq RFC4716 . +This option allows exporting OpenSSH keys for use by other programs, including +several commercial SSH implementations. +.It Fl F Ar hostname | [hostname]:port +Search for the specified +.Ar hostname +(with optional port number) +in a +.Pa known_hosts +file, listing any occurrences found. +This option is useful to find hashed host names or addresses and may also be +used in conjunction with the +.Fl H +option to print found keys in a hashed format. +.It Fl f Ar filename +Specifies the filename of the key file. +.It Fl g +Use generic DNS format when printing fingerprint resource records using the +.Fl r +command. +.It Fl H +Hash a +.Pa known_hosts +file. +This replaces all hostnames and addresses with hashed representations +within the specified file; the original content is moved to a file with +a .old suffix. +These hashes may be used normally by +.Nm ssh +and +.Nm sshd , +but they do not reveal identifying information should the file's contents +be disclosed. +This option will not modify existing hashed hostnames and is therefore safe +to use on files that mix hashed and non-hashed names. +.It Fl h +When signing a key, create a host certificate instead of a user +certificate. +See the +.Sx CERTIFICATES +section for details. +.It Fl I Ar certificate_identity +Specify the key identity when signing a public key. +See the +.Sx CERTIFICATES +section for details. +.It Fl i +This option will read an unencrypted private (or public) key file +in the format specified by the +.Fl m +option and print an OpenSSH compatible private +(or public) key to stdout. +This option allows importing keys from other software, including several +commercial SSH implementations. +The default import format is +.Dq RFC4716 . +.It Fl K +Download resident keys from a FIDO authenticator. +Public and private key files will be written to the current directory for +each downloaded key. +If multiple FIDO authenticators are attached, keys will be downloaded from +the first touched authenticator. +See the +.Sx FIDO AUTHENTICATOR +section for more information. +.It Fl k +Generate a KRL file. +In this mode, +.Nm +will generate a KRL file at the location specified via the +.Fl f +flag that revokes every key or certificate presented on the command line. +Keys/certificates to be revoked may be specified by public key file or +using the format described in the +.Sx KEY REVOCATION LISTS +section. +.It Fl L +Prints the contents of one or more certificates. +.It Fl l +Show fingerprint of specified public key file. +For RSA and DSA keys +.Nm +tries to find the matching public key file and prints its fingerprint. +If combined with +.Fl v , +a visual ASCII art representation of the key is supplied with the +fingerprint. +.It Fl M Cm generate +Generate candidate Diffie-Hellman Group Exchange (DH-GEX) parameters for +eventual use by the +.Sq diffie-hellman-group-exchange-* +key exchange methods. +The numbers generated by this operation must be further screened before +use. +See the +.Sx MODULI GENERATION +section for more information. +.It Fl M Cm screen +Screen candidate parameters for Diffie-Hellman Group Exchange. +This will accept a list of candidate numbers and test that they are +safe (Sophie Germain) primes with acceptable group generators. +The results of this operation may be added to the +.Pa /etc/moduli +file. +See the +.Sx MODULI GENERATION +section for more information. +.It Fl m Ar key_format +Specify a key format for key generation, the +.Fl i +(import), +.Fl e +(export) conversion options, and the +.Fl p +change passphrase operation. +The latter may be used to convert between OpenSSH private key and PEM +private key formats. +The supported key formats are: +.Dq RFC4716 +(RFC 4716/SSH2 public or private key), +.Dq PKCS8 +(PKCS8 public or private key) +or +.Dq PEM +(PEM public key). +By default OpenSSH will write newly-generated private keys in its own +format, but when converting public keys for export the default format is +.Dq RFC4716 . +Setting a format of +.Dq PEM +when generating or updating a supported private key type will cause the +key to be stored in the legacy PEM private key format. +.It Fl N Ar new_passphrase +Provides the new passphrase. +.It Fl n Ar principals +Specify one or more principals (user or host names) to be included in +a certificate when signing a key. +Multiple principals may be specified, separated by commas. +See the +.Sx CERTIFICATES +section for details. +.It Fl O Ar option +Specify a key/value option. +These are specific to the operation that +.Nm +has been requested to perform. +.Pp +When signing certificates, one of the options listed in the +.Sx CERTIFICATES +section may be specified here. +.Pp +When performing moduli generation or screening, one of the options +listed in the +.Sx MODULI GENERATION +section may be specified. +.Pp +When generating FIDO authenticator-backed keys, the options listed in the +.Sx FIDO AUTHENTICATOR +section may be specified. +.Pp +When performing signature-related options using the +.Fl Y +flag, the following options are accepted: +.Bl -tag -width Ds +.It Cm hashalg Ns = Ns Ar algorithm +Selects the hash algorithm to use for hashing the message to be signed. +Valid algorithms are +.Dq sha256 +and +.Dq sha512. +The default is +.Dq sha512. +.It Cm print-pubkey +Print the full public key to standard output after signature verification. +.It Cm verify-time Ns = Ns Ar timestamp +Specifies a time to use when validating signatures instead of the current +time. +The time may be specified as a date or time in the YYYYMMDD[Z] or +in YYYYMMDDHHMM[SS][Z] formats. +Dates and times will be interpreted in the current system time zone unless +suffixed with a Z character, which causes them to be interpreted in the +UTC time zone. +.El +.Pp +The +.Fl O +option may be specified multiple times. +.It Fl P Ar passphrase +Provides the (old) passphrase. +.It Fl p +Requests changing the passphrase of a private key file instead of +creating a new private key. +The program will prompt for the file +containing the private key, for the old passphrase, and twice for the +new passphrase. +.It Fl Q +Test whether keys have been revoked in a KRL. +If the +.Fl l +option is also specified then the contents of the KRL will be printed. +.It Fl q +Silence +.Nm ssh-keygen . +.It Fl R Ar hostname | [hostname]:port +Removes all keys belonging to the specified +.Ar hostname +(with optional port number) +from a +.Pa known_hosts +file. +This option is useful to delete hashed hosts (see the +.Fl H +option above). +.It Fl r Ar hostname +Print the SSHFP fingerprint resource record named +.Ar hostname +for the specified public key file. +.It Fl s Ar ca_key +Certify (sign) a public key using the specified CA key. +See the +.Sx CERTIFICATES +section for details. +.Pp +When generating a KRL, +.Fl s +specifies a path to a CA public key file used to revoke certificates directly +by key ID or serial number. +See the +.Sx KEY REVOCATION LISTS +section for details. +.It Fl t Cm dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa +Specifies the type of key to create. +The possible values are +.Dq dsa , +.Dq ecdsa , +.Dq ecdsa-sk , +.Dq ed25519 , +.Dq ed25519-sk , +or +.Dq rsa . +.Pp +This flag may also be used to specify the desired signature type when +signing certificates using an RSA CA key. +The available RSA signature variants are +.Dq ssh-rsa +(SHA1 signatures, not recommended), +.Dq rsa-sha2-256 , +and +.Dq rsa-sha2-512 +(the default). +.It Fl U +When used in combination with +.Fl s +or +.Fl Y Cm sign , +this option indicates that a CA key resides in a +.Xr ssh-agent 1 . +See the +.Sx CERTIFICATES +section for more information. +.It Fl u +Update a KRL. +When specified with +.Fl k , +keys listed via the command line are added to the existing KRL rather than +a new KRL being created. +.It Fl V Ar validity_interval +Specify a validity interval when signing a certificate. +A validity interval may consist of a single time, indicating that the +certificate is valid beginning now and expiring at that time, or may consist +of two times separated by a colon to indicate an explicit time interval. +.Pp +The start time may be specified as: +.Bl -bullet -compact +.It +The string +.Dq always +to indicate the certificate has no specified start time. +.It +A date or time in the system time zone formatted as YYYYMMDD or +YYYYMMDDHHMM[SS]. +.It +A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z. +.It +A relative time before the current system time consisting of a minus sign +followed by an interval in the format described in the +TIME FORMATS section of +.Xr sshd_config 5 . +.It +A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a hexadecimal +number beginning with +.Dq 0x . +.El +.Pp +The end time may be specified similarly to the start time: +.Bl -bullet -compact +.It +The string +.Dq forever +to indicate the certificate has no specified end time. +.It +A date or time in the system time zone formatted as YYYYMMDD or +YYYYMMDDHHMM[SS]. +.It +A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z. +.It +A relative time after the current system time consisting of a plus sign +followed by an interval in the format described in the +TIME FORMATS section of +.Xr sshd_config 5 . +.It +A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a hexadecimal +number beginning with +.Dq 0x . +.El +.Pp +For example: +.Bl -tag -width Ds +.It +52w1d +Valid from now to 52 weeks and one day from now. +.It -4w:+4w +Valid from four weeks ago to four weeks from now. +.It 20100101123000:20110101123000 +Valid from 12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011. +.It 20100101123000Z:20110101123000Z +Similar, but interpreted in the UTC time zone rather than the system time zone. +.It -1d:20110101 +Valid from yesterday to midnight, January 1st, 2011. +.It 0x1:0x2000000000 +Valid from roughly early 1970 to May 2033. +.It -1m:forever +Valid from one minute ago and never expiring. +.El +.It Fl v +Verbose mode. +Causes +.Nm +to print debugging messages about its progress. +This is helpful for debugging moduli generation. +Multiple +.Fl v +options increase the verbosity. +The maximum is 3. +.It Fl w Ar provider +Specifies a path to a library that will be used when creating +FIDO authenticator-hosted keys, overriding the default of using +the internal USB HID support. +.It Fl Y Cm find-principals +Find the principal(s) associated with the public key of a signature, +provided using the +.Fl s +flag in an authorized signers file provided using the +.Fl f +flag. +The format of the allowed signers file is documented in the +.Sx ALLOWED SIGNERS +section below. +If one or more matching principals are found, they are returned on +standard output. +.It Fl Y Cm match-principals +Find principal matching the principal name provided using the +.Fl I +flag in the authorized signers file specified using the +.Fl f +flag. +If one or more matching principals are found, they are returned on +standard output. +.It Fl Y Cm check-novalidate +Checks that a signature generated using +.Nm +.Fl Y Cm sign +has a valid structure. +This does not validate if a signature comes from an authorized signer. +When testing a signature, +.Nm +accepts a message on standard input and a signature namespace using +.Fl n . +A file containing the corresponding signature must also be supplied using the +.Fl s +flag. +Successful testing of the signature is signalled by +.Nm +returning a zero exit status. +.It Fl Y Cm sign +Cryptographically sign a file or some data using a SSH key. +When signing, +.Nm +accepts zero or more files to sign on the command-line - if no files +are specified then +.Nm +will sign data presented on standard input. +Signatures are written to the path of the input file with +.Dq .sig +appended, or to standard output if the message to be signed was read from +standard input. +.Pp +The key used for signing is specified using the +.Fl f +option and may refer to either a private key, or a public key with the private +half available via +.Xr ssh-agent 1 . +An additional signature namespace, used to prevent signature confusion across +different domains of use (e.g. file signing vs email signing) must be provided +via the +.Fl n +flag. +Namespaces are arbitrary strings, and may include: +.Dq file +for file signing, +.Dq email +for email signing. +For custom uses, it is recommended to use names following a +NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces. +.It Fl Y Cm verify +Request to verify a signature generated using +.Nm +.Fl Y Cm sign +as described above. +When verifying a signature, +.Nm +accepts a message on standard input and a signature namespace using +.Fl n . +A file containing the corresponding signature must also be supplied using the +.Fl s +flag, along with the identity of the signer using +.Fl I +and a list of allowed signers via the +.Fl f +flag. +The format of the allowed signers file is documented in the +.Sx ALLOWED SIGNERS +section below. +A file containing revoked keys can be passed using the +.Fl r +flag. +The revocation file may be a KRL or a one-per-line list of public keys. +Successful verification by an authorized signer is signalled by +.Nm +returning a zero exit status. +.It Fl y +This option will read a private +OpenSSH format file and print an OpenSSH public key to stdout. +.It Fl Z Ar cipher +Specifies the cipher to use for encryption when writing an OpenSSH-format +private key file. +The list of available ciphers may be obtained using +.Qq ssh -Q cipher . +The default is +.Dq aes256-ctr . +.It Fl z Ar serial_number +Specifies a serial number to be embedded in the certificate to distinguish +this certificate from others from the same CA. +If the +.Ar serial_number +is prefixed with a +.Sq + +character, then the serial number will be incremented for each certificate +signed on a single command-line. +The default serial number is zero. +.Pp +When generating a KRL, the +.Fl z +flag is used to specify a KRL version number. +.El +.Sh MODULI GENERATION +.Nm +may be used to generate groups for the Diffie-Hellman Group Exchange +(DH-GEX) protocol. +Generating these groups is a two-step process: first, candidate +primes are generated using a fast, but memory intensive process. +These candidate primes are then tested for suitability (a CPU-intensive +process). +.Pp +Generation of primes is performed using the +.Fl M Cm generate +option. +The desired length of the primes may be specified by the +.Fl O Cm bits +option. +For example: +.Pp +.Dl # ssh-keygen -M generate -O bits=2048 moduli-2048.candidates +.Pp +By default, the search for primes begins at a random point in the +desired length range. +This may be overridden using the +.Fl O Cm start +option, which specifies a different start point (in hex). +.Pp +Once a set of candidates have been generated, they must be screened for +suitability. +This may be performed using the +.Fl M Cm screen +option. +In this mode +.Nm +will read candidates from standard input (or a file specified using the +.Fl f +option). +For example: +.Pp +.Dl # ssh-keygen -M screen -f moduli-2048.candidates moduli-2048 +.Pp +By default, each candidate will be subjected to 100 primality tests. +This may be overridden using the +.Fl O Cm prime-tests +option. +The DH generator value will be chosen automatically for the +prime under consideration. +If a specific generator is desired, it may be requested using the +.Fl O Cm generator +option. +Valid generator values are 2, 3, and 5. +.Pp +Screened DH groups may be installed in +.Pa /etc/moduli . +It is important that this file contains moduli of a range of bit lengths. +.Pp +A number of options are available for moduli generation and screening via the +.Fl O +flag: +.Bl -tag -width Ds +.It Ic lines Ns = Ns Ar number +Exit after screening the specified number of lines while performing DH +candidate screening. +.It Ic start-line Ns = Ns Ar line-number +Start screening at the specified line number while performing DH candidate +screening. +.It Ic checkpoint Ns = Ns Ar filename +Write the last line processed to the specified file while performing DH +candidate screening. +This will be used to skip lines in the input file that have already been +processed if the job is restarted. +.It Ic memory Ns = Ns Ar mbytes +Specify the amount of memory to use (in megabytes) when generating +candidate moduli for DH-GEX. +.It Ic start Ns = Ns Ar hex-value +Specify start point (in hex) when generating candidate moduli for DH-GEX. +.It Ic generator Ns = Ns Ar value +Specify desired generator (in decimal) when testing candidate moduli for DH-GEX. +.El +.Sh CERTIFICATES +.Nm +supports signing of keys to produce certificates that may be used for +user or host authentication. +Certificates consist of a public key, some identity information, zero or +more principal (user or host) names and a set of options that +are signed by a Certification Authority (CA) key. +Clients or servers may then trust only the CA key and verify its signature +on a certificate rather than trusting many user/host keys. +Note that OpenSSH certificates are a different, and much simpler, format to +the X.509 certificates used in +.Xr ssl 8 . +.Pp +.Nm +supports two types of certificates: user and host. +User certificates authenticate users to servers, whereas host certificates +authenticate server hosts to users. +To generate a user certificate: +.Pp +.Dl $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub +.Pp +The resultant certificate will be placed in +.Pa /path/to/user_key-cert.pub . +A host certificate requires the +.Fl h +option: +.Pp +.Dl $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub +.Pp +The host certificate will be output to +.Pa /path/to/host_key-cert.pub . +.Pp +It is possible to sign using a CA key stored in a PKCS#11 token by +providing the token library using +.Fl D +and identifying the CA key by providing its public half as an argument +to +.Fl s : +.Pp +.Dl $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub +.Pp +Similarly, it is possible for the CA key to be hosted in a +.Xr ssh-agent 1 . +This is indicated by the +.Fl U +flag and, again, the CA key must be identified by its public half. +.Pp +.Dl $ ssh-keygen -Us ca_key.pub -I key_id user_key.pub +.Pp +In all cases, +.Ar key_id +is a "key identifier" that is logged by the server when the certificate +is used for authentication. +.Pp +Certificates may be limited to be valid for a set of principal (user/host) +names. +By default, generated certificates are valid for all users or hosts. +To generate a certificate for a specified set of principals: +.Pp +.Dl $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub +.Dl "$ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub" +.Pp +Additional limitations on the validity and use of user certificates may +be specified through certificate options. +A certificate option may disable features of the SSH session, may be +valid only when presented from particular source addresses or may +force the use of a specific command. +.Pp +The options that are valid for user certificates are: +.Pp +.Bl -tag -width Ds -compact +.It Ic clear +Clear all enabled permissions. +This is useful for clearing the default set of permissions so permissions may +be added individually. +.Pp +.It Ic critical : Ns Ar name Ns Op Ns = Ns Ar contents +.It Ic extension : Ns Ar name Ns Op Ns = Ns Ar contents +Includes an arbitrary certificate critical option or extension. +The specified +.Ar name +should include a domain suffix, e.g.\& +.Dq name@example.com . +If +.Ar contents +is specified then it is included as the contents of the extension/option +encoded as a string, otherwise the extension/option is created with no +contents (usually indicating a flag). +Extensions may be ignored by a client or server that does not recognise them, +whereas unknown critical options will cause the certificate to be refused. +.Pp +.It Ic force-command Ns = Ns Ar command +Forces the execution of +.Ar command +instead of any shell or command specified by the user when +the certificate is used for authentication. +.Pp +.It Ic no-agent-forwarding +Disable +.Xr ssh-agent 1 +forwarding (permitted by default). +.Pp +.It Ic no-port-forwarding +Disable port forwarding (permitted by default). +.Pp +.It Ic no-pty +Disable PTY allocation (permitted by default). +.Pp +.It Ic no-user-rc +Disable execution of +.Pa ~/.ssh/rc +by +.Xr sshd 8 +(permitted by default). +.Pp +.It Ic no-x11-forwarding +Disable X11 forwarding (permitted by default). +.Pp +.It Ic permit-agent-forwarding +Allows +.Xr ssh-agent 1 +forwarding. +.Pp +.It Ic permit-port-forwarding +Allows port forwarding. +.Pp +.It Ic permit-pty +Allows PTY allocation. +.Pp +.It Ic permit-user-rc +Allows execution of +.Pa ~/.ssh/rc +by +.Xr sshd 8 . +.Pp +.It Ic permit-X11-forwarding +Allows X11 forwarding. +.Pp +.It Ic no-touch-required +Do not require signatures made using this key include demonstration +of user presence (e.g. by having the user touch the authenticator). +This option only makes sense for the FIDO authenticator algorithms +.Cm ecdsa-sk +and +.Cm ed25519-sk . +.Pp +.It Ic source-address Ns = Ns Ar address_list +Restrict the source addresses from which the certificate is considered valid. +The +.Ar address_list +is a comma-separated list of one or more address/netmask pairs in CIDR +format. +.Pp +.It Ic verify-required +Require signatures made using this key indicate that the user was first +verified. +This option only makes sense for the FIDO authenticator algorithms +.Cm ecdsa-sk +and +.Cm ed25519-sk . +Currently PIN authentication is the only supported verification method, +but other methods may be supported in the future. +.El +.Pp +At present, no standard options are valid for host keys. +.Pp +Finally, certificates may be defined with a validity lifetime. +The +.Fl V +option allows specification of certificate start and end times. +A certificate that is presented at a time outside this range will not be +considered valid. +By default, certificates are valid from the +.Ux +Epoch to the distant future. +.Pp +For certificates to be used for user or host authentication, the CA +public key must be trusted by +.Xr sshd 8 +or +.Xr ssh 1 . +Refer to those manual pages for details. +.Sh FIDO AUTHENTICATOR +.Nm +is able to generate FIDO authenticator-backed keys, after which +they may be used much like any other key type supported by OpenSSH, so +long as the hardware authenticator is attached when the keys are used. +FIDO authenticators generally require the user to explicitly authorise +operations by touching or tapping them. +FIDO keys consist of two parts: a key handle part stored in the +private key file on disk, and a per-device private key that is unique +to each FIDO authenticator and that cannot be exported from the +authenticator hardware. +These are combined by the hardware at authentication time to derive +the real key that is used to sign authentication challenges. +Supported key types are +.Cm ecdsa-sk +and +.Cm ed25519-sk . +.Pp +The options that are valid for FIDO keys are: +.Bl -tag -width Ds +.It Cm application +Override the default FIDO application/origin string of +.Dq ssh: . +This may be useful when generating host or domain-specific resident keys. +The specified application string must begin with +.Dq ssh: . +.It Cm challenge Ns = Ns Ar path +Specifies a path to a challenge string that will be passed to the +FIDO authenticator during key generation. +The challenge string may be used as part of an out-of-band +protocol for key enrollment +(a random challenge is used by default). +.It Cm device +Explicitly specify a +.Xr fido 4 +device to use, rather than letting the authenticator middleware select one. +.It Cm no-touch-required +Indicate that the generated private key should not require touch +events (user presence) when making signatures. +Note that +.Xr sshd 8 +will refuse such signatures by default, unless overridden via +an authorized_keys option. +.It Cm resident +Indicate that the key handle should be stored on the FIDO +authenticator itself. +This makes it easier to use the authenticator on multiple computers. +Resident keys may be supported on FIDO2 authenticators and typically +require that a PIN be set on the authenticator prior to generation. +Resident keys may be loaded off the authenticator using +.Xr ssh-add 1 . +Storing both parts of a key on a FIDO authenticator increases the likelihood +of an attacker being able to use a stolen authenticator device. +.It Cm user +A username to be associated with a resident key, +overriding the empty default username. +Specifying a username may be useful when generating multiple resident keys +for the same application name. +.It Cm verify-required +Indicate that this private key should require user verification for +each signature. +Not all FIDO authenticators support this option. +Currently PIN authentication is the only supported verification method, +but other methods may be supported in the future. +.It Cm write-attestation Ns = Ns Ar path +May be used at key generation time to record the attestation data +returned from FIDO authenticators during key generation. +This information is potentially sensitive. +By default, this information is discarded. +.El +.Sh KEY REVOCATION LISTS +.Nm +is able to manage OpenSSH format Key Revocation Lists (KRLs). +These binary files specify keys or certificates to be revoked using a +compact format, taking as little as one bit per certificate if they are being +revoked by serial number. +.Pp +KRLs may be generated using the +.Fl k +flag. +This option reads one or more files from the command line and generates a new +KRL. +The files may either contain a KRL specification (see below) or public keys, +listed one per line. +Plain public keys are revoked by listing their hash or contents in the KRL and +certificates revoked by serial number or key ID (if the serial is zero or +not available). +.Pp +Revoking keys using a KRL specification offers explicit control over the +types of record used to revoke keys and may be used to directly revoke +certificates by serial number or key ID without having the complete original +certificate on hand. +A KRL specification consists of lines containing one of the following directives +followed by a colon and some directive-specific information. +.Bl -tag -width Ds +.It Cm serial : Ar serial_number Ns Op - Ns Ar serial_number +Revokes a certificate with the specified serial number. +Serial numbers are 64-bit values, not including zero and may be expressed +in decimal, hex or octal. +If two serial numbers are specified separated by a hyphen, then the range +of serial numbers including and between each is revoked. +The CA key must have been specified on the +.Nm +command line using the +.Fl s +option. +.It Cm id : Ar key_id +Revokes a certificate with the specified key ID string. +The CA key must have been specified on the +.Nm +command line using the +.Fl s +option. +.It Cm key : Ar public_key +Revokes the specified key. +If a certificate is listed, then it is revoked as a plain public key. +.It Cm sha1 : Ar public_key +Revokes the specified key by including its SHA1 hash in the KRL. +.It Cm sha256 : Ar public_key +Revokes the specified key by including its SHA256 hash in the KRL. +KRLs that revoke keys by SHA256 hash are not supported by OpenSSH versions +prior to 7.9. +.It Cm hash : Ar fingerprint +Revokes a key using a fingerprint hash, as obtained from a +.Xr sshd 8 +authentication log message or the +.Nm +.Fl l +flag. +Only SHA256 fingerprints are supported here and resultant KRLs are +not supported by OpenSSH versions prior to 7.9. +.El +.Pp +KRLs may be updated using the +.Fl u +flag in addition to +.Fl k . +When this option is specified, keys listed via the command line are merged into +the KRL, adding to those already there. +.Pp +It is also possible, given a KRL, to test whether it revokes a particular key +(or keys). +The +.Fl Q +flag will query an existing KRL, testing each key specified on the command line. +If any key listed on the command line has been revoked (or an error encountered) +then +.Nm +will exit with a non-zero exit status. +A zero exit status will only be returned if no key was revoked. +.Sh ALLOWED SIGNERS +When verifying signatures, +.Nm +uses a simple list of identities and keys to determine whether a signature +comes from an authorized source. +This "allowed signers" file uses a format patterned after the +AUTHORIZED_KEYS FILE FORMAT described in +.Xr sshd 8 . +Each line of the file contains the following space-separated fields: +principals, options, keytype, base64-encoded key. +Empty lines and lines starting with a +.Ql # +are ignored as comments. +.Pp +The principals field is a pattern-list (see PATTERNS in +.Xr ssh_config 5 ) +consisting of one or more comma-separated USER@DOMAIN identity patterns +that are accepted for signing. +When verifying, the identity presented via the +.Fl I +option must match a principals pattern in order for the corresponding key to be +considered acceptable for verification. +.Pp +The options (if present) consist of comma-separated option specifications. +No spaces are permitted, except within double quotes. +The following option specifications are supported (note that option keywords +are case-insensitive): +.Bl -tag -width Ds +.It Cm cert-authority +Indicates that this key is accepted as a certificate authority (CA) and +that certificates signed by this CA may be accepted for verification. +.It Cm namespaces Ns = Ns "namespace-list" +Specifies a pattern-list of namespaces that are accepted for this key. +If this option is present, the signature namespace embedded in the +signature object and presented on the verification command-line must +match the specified list before the key will be considered acceptable. +.It Cm valid-after Ns = Ns "timestamp" +Indicates that the key is valid for use at or after the specified timestamp, +which may be a date or time in the YYYYMMDD[Z] or YYYYMMDDHHMM[SS][Z] formats. +Dates and times will be interpreted in the current system time zone unless +suffixed with a Z character, which causes them to be interpreted in the UTC +time zone. +.It Cm valid-before Ns = Ns "timestamp" +Indicates that the key is valid for use at or before the specified timestamp. +.El +.Pp +When verifying signatures made by certificates, the expected principal +name must match both the principals pattern in the allowed signers file and +the principals embedded in the certificate itself. +.Pp +An example allowed signers file: +.Bd -literal -offset 3n +# Comments allowed at start of line +user1@example.com,user2@example.com ssh-rsa AAAAX1... +# A certificate authority, trusted for all principals in a domain. +*@example.com cert-authority ssh-ed25519 AAAB4... +# A key that is accepted only for file signing. +user2@example.com namespaces="file" ssh-ed25519 AAA41... +.Ed +.Sh ENVIRONMENT +.Bl -tag -width Ds +.It Ev SSH_SK_PROVIDER +Specifies a path to a library that will be used when loading any +FIDO authenticator-hosted keys, overriding the default of using +the built-in USB HID support. +.El +.Sh FILES +.Bl -tag -width Ds -compact +.It Pa ~/.ssh/id_dsa +.It Pa ~/.ssh/id_ecdsa +.It Pa ~/.ssh/id_ecdsa_sk +.It Pa ~/.ssh/id_ed25519 +.It Pa ~/.ssh/id_ed25519_sk +.It Pa ~/.ssh/id_rsa +Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, +authenticator-hosted Ed25519 or RSA authentication identity of the user. +This file should not be readable by anyone but the user. +It is possible to +specify a passphrase when generating the key; that passphrase will be +used to encrypt the private part of this file using 128-bit AES. +This file is not automatically accessed by +.Nm +but it is offered as the default file for the private key. +.Xr ssh 1 +will read this file when a login attempt is made. +.Pp +.It Pa ~/.ssh/id_dsa.pub +.It Pa ~/.ssh/id_ecdsa.pub +.It Pa ~/.ssh/id_ecdsa_sk.pub +.It Pa ~/.ssh/id_ed25519.pub +.It Pa ~/.ssh/id_ed25519_sk.pub +.It Pa ~/.ssh/id_rsa.pub +Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, +authenticator-hosted Ed25519 or RSA public key for authentication. +The contents of this file should be added to +.Pa ~/.ssh/authorized_keys +on all machines +where the user wishes to log in using public key authentication. +There is no need to keep the contents of this file secret. +.Pp +.It Pa /etc/moduli +Contains Diffie-Hellman groups used for DH-GEX. +The file format is described in +.Xr moduli 5 . +.El +.Sh SEE ALSO +.Xr ssh 1 , +.Xr ssh-add 1 , +.Xr ssh-agent 1 , +.Xr moduli 5 , +.Xr sshd 8 +.Rs +.%R RFC 4716 +.%T "The Secure Shell (SSH) Public Key File Format" +.%D 2006 +.Re +.Sh AUTHORS +OpenSSH is a derivative of the original and free +ssh 1.2.12 release by Tatu Ylonen. +Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, +Theo de Raadt and Dug Song +removed many bugs, re-added newer features and +created OpenSSH. +Markus Friedl contributed the support for SSH +protocol versions 1.5 and 2.0. 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