path: root/ssh-keygen.0

SSH-KEYGEN(1)               General Commands Manual              SSH-KEYGEN(1)

NAME
     ssh-keygen M-bM-^@M-^S OpenSSH authentication key utility

SYNOPSIS
     ssh-keygen [-q] [-a rounds] [-b bits] [-C comment] [-f output_keyfile]
                [-m format] [-N new_passphrase] [-O option]
                [-t ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa]
                [-w provider] [-Z cipher]
     ssh-keygen -p [-a rounds] [-f keyfile] [-m format] [-N new_passphrase]
                [-P old_passphrase] [-Z cipher]
     ssh-keygen -i [-f input_keyfile] [-m key_format]
     ssh-keygen -e [-f input_keyfile] [-m key_format]
     ssh-keygen -y [-f input_keyfile]
     ssh-keygen -c [-a rounds] [-C comment] [-f keyfile] [-P passphrase]
     ssh-keygen -l [-v] [-E fingerprint_hash] [-f input_keyfile]
     ssh-keygen -B [-f input_keyfile]
     ssh-keygen -D pkcs11
     ssh-keygen -F hostname [-lv] [-f known_hosts_file]
     ssh-keygen -H [-f known_hosts_file]
     ssh-keygen -K [-a rounds] [-w provider]
     ssh-keygen -R hostname [-f known_hosts_file]
     ssh-keygen -r hostname [-g] [-f input_keyfile]
     ssh-keygen -M generate [-O option] output_file
     ssh-keygen -M screen [-f input_file] [-O option] output_file
     ssh-keygen -I certificate_identity -s ca_key [-hU] [-D pkcs11_provider]
                [-n principals] [-O option] [-V validity_interval]
                [-z serial_number] file ...
     ssh-keygen -L [-f input_keyfile]
     ssh-keygen -A [-a rounds] [-f prefix_path]
     ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]
                file ...
     ssh-keygen -Q [-l] -f krl_file file ...
     ssh-keygen -Y find-principals [-O option] -s signature_file
                -f allowed_signers_file
     ssh-keygen -Y match-principals -I signer_identity -f allowed_signers_file
     ssh-keygen -Y check-novalidate [-O option] -n namespace -s signature_file
     ssh-keygen -Y sign [-O option] -f key_file -n namespace file ...
     ssh-keygen -Y verify [-O option] -f allowed_signers_file
                -I signer_identity -n namespace -s signature_file
                [-r revocation_file]

DESCRIPTION
     ssh-keygen generates, manages and converts authentication keys for
     ssh(1).  ssh-keygen can create keys for use by SSH protocol version 2.

     The type of key to be generated is specified with the -t option.  If
     invoked without any arguments, ssh-keygen will generate an Ed25519 key.

     ssh-keygen is also used to generate groups for use in Diffie-Hellman
     group exchange (DH-GEX).  See the MODULI GENERATION section for details.

     Finally, ssh-keygen can be used to generate and update Key Revocation
     Lists, and to test whether given keys have been revoked by one.  See the
     KEY REVOCATION LISTS section for details.

     Normally each user wishing to use SSH with public key authentication runs
     this once to create the authentication key in ~/.ssh/id_ecdsa,
     ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk or
     ~/.ssh/id_rsa.  Additionally, the system administrator may use this to
     generate host keys, as seen in /etc/rc.

     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 M-bM-^@M-^\.pubM-bM-^@M-^] 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 -p option.

     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.

     ssh-keygen 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 M-bM-^@M-^\user@hostM-bM-^@M-^] when the key is created, but can be
     changed using the -c option.

     It is still possible for ssh-keygen to write the previously-used PEM
     format private keys using the -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 -p (change passphrase) flag.

     After a key is generated, ssh-keygen will ask where the keys should be
     placed to be activated.

     The options are as follows:

     -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 -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
             /etc/rc to generate new host keys.

     -a rounds
             When saving a private key, this option specifies the number of
             KDF (key derivation function, currently 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.

     -B      Show the bubblebabble digest of specified private or public key
             file.

     -b 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.  For ECDSA keys,
             the -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 -b flag will be ignored.

     -C comment
             Provides a new comment.

     -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.

     -D pkcs11
             Download the public keys provided by the PKCS#11 shared library
             pkcs11.  When used in combination with -s, this option indicates
             that a CA key resides in a PKCS#11 token (see the CERTIFICATES
             section for details).

     -E fingerprint_hash
             Specifies the hash algorithm used when displaying key
             fingerprints.  Valid options are: M-bM-^@M-^\md5M-bM-^@M-^] and M-bM-^@M-^\sha256M-bM-^@M-^].  The
             default is M-bM-^@M-^\sha256M-bM-^@M-^].

     -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 -m option.  The default export format is M-bM-^@M-^\RFC4716M-bM-^@M-^].  This
             option allows exporting OpenSSH keys for use by other programs,
             including several commercial SSH implementations.

     -F hostname | [hostname]:port
             Search for the specified hostname (with optional port number) in
             a 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 -H option to print found keys in a
             hashed format.

     -f filename
             Specifies the filename of the key file.

     -g      Use generic DNS format when printing fingerprint resource records
             using the -r command.

     -H      Hash a 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 ssh and 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.

     -h      When signing a key, create a host certificate instead of a user
             certificate.  See the CERTIFICATES section for details.

     -I certificate_identity
             Specify the key identity when signing a public key.  See the
             CERTIFICATES section for details.

     -i      This option will read an unencrypted private (or public) key file
             in the format specified by the -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 M-bM-^@M-^\RFC4716M-bM-^@M-^].

     -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 FIDO AUTHENTICATOR section for more
             information.

     -k      Generate a KRL file.  In this mode, ssh-keygen will generate a
             KRL file at the location specified via the -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 KEY REVOCATION LISTS
             section.

     -L      Prints the contents of one or more certificates.

     -l      Show fingerprint of specified public key file.  ssh-keygen will
             try to find the matching public key file and prints its
             fingerprint.  If combined with -v, a visual ASCII art
             representation of the key is supplied with the fingerprint.

     -M generate
             Generate candidate Diffie-Hellman Group Exchange (DH-GEX)
             parameters for eventual use by the
             M-bM-^@M-^Xdiffie-hellman-group-exchange-*M-bM-^@M-^Y key exchange methods.  The
             numbers generated by this operation must be further screened
             before use.  See the MODULI GENERATION section for more
             information.

     -M 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
             /etc/moduli file.  See the MODULI GENERATION section for more
             information.

     -m key_format
             Specify a key format for key generation, the -i (import), -e
             (export) conversion options, and the -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: M-bM-^@M-^\RFC4716M-bM-^@M-^] (RFC 4716/SSH2 public or private key),
             M-bM-^@M-^\PKCS8M-bM-^@M-^] (PKCS8 public or private key) or M-bM-^@M-^\PEMM-bM-^@M-^] (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 M-bM-^@M-^\RFC4716M-bM-^@M-^].  Setting a format of M-bM-^@M-^\PEMM-bM-^@M-^] when
             generating or updating a supported private key type will cause
             the key to be stored in the legacy PEM private key format.

     -N new_passphrase
             Provides the new passphrase.

     -n 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
             CERTIFICATES section for details.

     -O option
             Specify a key/value option.  These are specific to the operation
             that ssh-keygen has been requested to perform.

             When signing certificates, one of the options listed in the
             CERTIFICATES section may be specified here.

             When performing moduli generation or screening, one of the
             options listed in the MODULI GENERATION section may be specified.

             When generating FIDO authenticator-backed keys, the options
             listed in the FIDO AUTHENTICATOR section may be specified.

             When performing signature-related options using the -Y flag, the
             following options are accepted:

             hashalg=algorithm
                     Selects the hash algorithm to use for hashing the message
                     to be signed.  Valid algorithms are M-bM-^@M-^\sha256M-bM-^@M-^] and
                     M-bM-^@M-^\sha512.M-bM-^@M-^] The default is M-bM-^@M-^\sha512.M-bM-^@M-^]

             print-pubkey
                     Print the full public key to standard output after
                     signature verification.

             verify-time=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.

             When generating SSHFP DNS records from public keys using the -r
             flag, the following options are accepted:

             hashalg=algorithm
                     Selects a hash algorithm to use when printing SSHFP
                     records using the -D flag.  Valid algorithms are M-bM-^@M-^\sha1M-bM-^@M-^]
                     and M-bM-^@M-^\sha256M-bM-^@M-^].  The default is to print both.

             The -O option may be specified multiple times.

     -P passphrase
             Provides the (old) passphrase.

     -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.

     -Q      Test whether keys have been revoked in a KRL.  If the -l option
             is also specified then the contents of the KRL will be printed.

     -q      Silence ssh-keygen.

     -R hostname | [hostname]:port
             Removes all keys belonging to the specified hostname (with
             optional port number) from a known_hosts file.  This option is
             useful to delete hashed hosts (see the -H option above).

     -r hostname
             Print the SSHFP fingerprint resource record named hostname for
             the specified public key file.

     -s ca_key
             Certify (sign) a public key using the specified CA key.  See the
             CERTIFICATES section for details.

             When generating a KRL, -s specifies a path to a CA public key
             file used to revoke certificates directly by key ID or serial
             number.  See the KEY REVOCATION LISTS section for details.

     -t ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
             Specifies the type of key to create.  The possible values are
             M-bM-^@M-^\ecdsaM-bM-^@M-^], M-bM-^@M-^\ecdsa-skM-bM-^@M-^], M-bM-^@M-^\ed25519 (the default),M-bM-^@M-^] M-bM-^@M-^\ed25519-skM-bM-^@M-^], or
             M-bM-^@M-^\rsaM-bM-^@M-^].

             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 M-bM-^@M-^\ssh-rsaM-bM-^@M-^] (SHA1 signatures, not
             recommended), M-bM-^@M-^\rsa-sha2-256M-bM-^@M-^], and M-bM-^@M-^\rsa-sha2-512M-bM-^@M-^] (the default for
             RSA keys).

     -U      When used in combination with -s or -Y sign, this option
             indicates that a CA key resides in a ssh-agent(1).  See the
             CERTIFICATES section for more information.

     -u      Update a KRL.  When specified with -k, keys listed via the
             command line are added to the existing KRL rather than a new KRL
             being created.

     -V 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.

             The start time may be specified as:
             M-bM-^@M-M-bM-^@M-"   The string M-bM-^@M-^\alwaysM-bM-^@M-^] to indicate the certificate has no
                 specified start time.
             M-bM-^@M-M-bM-^@M-"   A date or time in the system time zone formatted as YYYYMMDD
                 or YYYYMMDDHHMM[SS].
             M-bM-^@M-M-bM-^@M-"   A date or time in the UTC time zone as YYYYMMDDZ or
                 YYYYMMDDHHMM[SS]Z.
             M-bM-^@M-M-bM-^@M-"   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 sshd_config(5).
             M-bM-^@M-M-bM-^@M-"   A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a
                 hexadecimal number beginning with M-bM-^@M-^\0xM-bM-^@M-^].

             The end time may be specified similarly to the start time:
             M-bM-^@M-M-bM-^@M-"   The string M-bM-^@M-^\foreverM-bM-^@M-^] to indicate the certificate has no
                 specified end time.
             M-bM-^@M-M-bM-^@M-"   A date or time in the system time zone formatted as YYYYMMDD
                 or YYYYMMDDHHMM[SS].
             M-bM-^@M-M-bM-^@M-"   A date or time in the UTC time zone as YYYYMMDDZ or
                 YYYYMMDDHHMM[SS]Z.
             M-bM-^@M-M-bM-^@M-"   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 sshd_config(5).
             M-bM-^@M-M-bM-^@M-"   A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a
                 hexadecimal number beginning with M-bM-^@M-^\0xM-bM-^@M-^].

             For example:

             +52w1d  Valid from now to 52 weeks and one day from now.

             -4w:+4w
                     Valid from four weeks ago to four weeks from now.

             20100101123000:20110101123000
                     Valid from 12:30 PM, January 1st, 2010 to 12:30 PM,
                     January 1st, 2011.

             20100101123000Z:20110101123000Z
                     Similar, but interpreted in the UTC time zone rather than
                     the system time zone.

             -1d:20110101
                     Valid from yesterday to midnight, January 1st, 2011.

             0x1:0x2000000000
                     Valid from roughly early 1970 to May 2033.

             -1m:forever
                     Valid from one minute ago and never expiring.

     -v      Verbose mode.  Causes ssh-keygen to print debugging messages
             about its progress.  This is helpful for debugging moduli
             generation.  Multiple -v options increase the verbosity.  The
             maximum is 3.

     -w 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.

     -Y find-principals
             Find the principal(s) associated with the public key of a
             signature, provided using the -s flag in an authorized signers
             file provided using the -f flag.  The format of the allowed
             signers file is documented in the ALLOWED SIGNERS section below.
             If one or more matching principals are found, they are returned
             on standard output.

     -Y match-principals
             Find principal matching the principal name provided using the -I
             flag in the authorized signers file specified using the -f flag.
             If one or more matching principals are found, they are returned
             on standard output.

     -Y check-novalidate
             Checks that a signature generated using ssh-keygen -Y sign has a
             valid structure.  This does not validate if a signature comes
             from an authorized signer.  When testing a signature, ssh-keygen
             accepts a message on standard input and a signature namespace
             using -n.  A file containing the corresponding signature must
             also be supplied using the -s flag.  Successful testing of the
             signature is signalled by ssh-keygen returning a zero exit
             status.

     -Y sign
             Cryptographically sign a file or some data using an SSH key.
             When signing, ssh-keygen accepts zero or more files to sign on
             the command-line - if no files are specified then ssh-keygen will
             sign data presented on standard input.  Signatures are written to
             the path of the input file with M-bM-^@M-^\.sigM-bM-^@M-^] appended, or to standard
             output if the message to be signed was read from standard input.

             The key used for signing is specified using the -f option and may
             refer to either a private key, or a public key with the private
             half available via 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 -n flag.  Namespaces are arbitrary strings, and
             may include: M-bM-^@M-^\fileM-bM-^@M-^] for file signing, M-bM-^@M-^\emailM-bM-^@M-^] for email signing.
             For custom uses, it is recommended to use names following a
             NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces.

     -Y verify
             Request to verify a signature generated using ssh-keygen -Y sign
             as described above.  When verifying a signature, ssh-keygen
             accepts a message on standard input and a signature namespace
             using -n.  A file containing the corresponding signature must
             also be supplied using the -s flag, along with the identity of
             the signer using -I and a list of allowed signers via the -f
             flag.  The format of the allowed signers file is documented in
             the ALLOWED SIGNERS section below.  A file containing revoked
             keys can be passed using the -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 ssh-keygen
             returning a zero exit status.

     -y      This option will read a private OpenSSH format file and print an
             OpenSSH public key to stdout.

     -Z 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 "ssh -Q cipher".  The default is
             M-bM-^@M-^\aes256-ctrM-bM-^@M-^].

     -z serial_number
             Specifies a serial number to be embedded in the certificate to
             distinguish this certificate from others from the same CA.  If
             the serial_number is prefixed with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
             serial number will be incremented for each certificate signed on
             a single command-line.  The default serial number is zero.

             When generating a KRL, the -z flag is used to specify a KRL
             version number.

MODULI GENERATION
     ssh-keygen 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).

     Generation of primes is performed using the -M generate option.  The
     desired length of the primes may be specified by the -O bits option.  For
     example:

           # ssh-keygen -M generate -O bits=2048 moduli-2048.candidates

     By default, the search for primes begins at a random point in the desired
     length range.  This may be overridden using the -O start option, which
     specifies a different start point (in hex).

     Once a set of candidates have been generated, they must be screened for
     suitability.  This may be performed using the -M screen option.  In this
     mode ssh-keygen will read candidates from standard input (or a file
     specified using the -f option).  For example:

           # ssh-keygen -M screen -f moduli-2048.candidates moduli-2048

     By default, each candidate will be subjected to 100 primality tests.
     This may be overridden using the -O 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 -O
     generator option.  Valid generator values are 2, 3, and 5.

     Screened DH groups may be installed in /etc/moduli.  It is important that
     this file contains moduli of a range of bit lengths.

     A number of options are available for moduli generation and screening via
     the -O flag:

     lines=number
             Exit after screening the specified number of lines while
             performing DH candidate screening.

     start-line=line-number
             Start screening at the specified line number while performing DH
             candidate screening.

     checkpoint=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.

     memory=mbytes
             Specify the amount of memory to use (in megabytes) when
             generating candidate moduli for DH-GEX.

     start=hex-value
             Specify start point (in hex) when generating candidate moduli for
             DH-GEX.

     generator=value
             Specify desired generator (in decimal) when testing candidate
             moduli for DH-GEX.

CERTIFICATES
     ssh-keygen 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 ssl(8).

     ssh-keygen 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:

           $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/user_key.pub

     The resultant certificate will be placed in /path/to/user_key-cert.pub.
     A host certificate requires the -h option:

           $ ssh-keygen -s /path/to/ca_key -I key_id -h /path/to/host_key.pub

     The host certificate will be output to /path/to/host_key-cert.pub.

     It is possible to sign using a CA key stored in a PKCS#11 token by
     providing the token library using -D and identifying the CA key by
     providing its public half as an argument to -s:

           $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I key_id user_key.pub

     Similarly, it is possible for the CA key to be hosted in a ssh-agent(1).
     This is indicated by the -U flag and, again, the CA key must be
     identified by its public half.

           $ ssh-keygen -Us ca_key.pub -I key_id user_key.pub

     In all cases, key_id is a "key identifier" that is logged by the server
     when the certificate is used for authentication.

     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:

           $ ssh-keygen -s ca_key -I key_id -n user1,user2 user_key.pub
           $ ssh-keygen -s ca_key -I key_id -h -n host.domain host_key.pub

     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.

     The options that are valid for user certificates are:

     clear   Clear all enabled permissions.  This is useful for clearing the
             default set of permissions so permissions may be added
             individually.

     critical:name[=contents]
     extension:name[=contents]
             Includes an arbitrary certificate critical option or extension.
             The specified name should include a domain suffix, e.g.
             M-bM-^@M-^\name@example.comM-bM-^@M-^].  If 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.

     force-command=command
             Forces the execution of command instead of any shell or command
             specified by the user when the certificate is used for
             authentication.

     no-agent-forwarding
             Disable ssh-agent(1) forwarding (permitted by default).

     no-port-forwarding
             Disable port forwarding (permitted by default).

     no-pty  Disable PTY allocation (permitted by default).

     no-user-rc
             Disable execution of ~/.ssh/rc by sshd(8) (permitted by default).

     no-x11-forwarding
             Disable X11 forwarding (permitted by default).

     permit-agent-forwarding
             Allows ssh-agent(1) forwarding.

     permit-port-forwarding
             Allows port forwarding.

     permit-pty
             Allows PTY allocation.

     permit-user-rc
             Allows execution of ~/.ssh/rc by sshd(8).

     permit-X11-forwarding
             Allows X11 forwarding.

     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 ecdsa-sk and ed25519-sk.

     source-address=address_list
             Restrict the source addresses from which the certificate is
             considered valid.  The address_list is a comma-separated list of
             one or more address/netmask pairs in CIDR format.

     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 ecdsa-sk and ed25519-sk.  Currently PIN
             authentication is the only supported verification method, but
             other methods may be supported in the future.

     At present, no standard options are valid for host keys.

     Finally, certificates may be defined with a validity lifetime.  The -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 UNIX Epoch
     to the distant future.

     For certificates to be used for user or host authentication, the CA
     public key must be trusted by sshd(8) or ssh(1).  Refer to those manual
     pages for details.

FIDO AUTHENTICATOR
     ssh-keygen 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 ecdsa-sk
     and ed25519-sk.

     The options that are valid for FIDO keys are:

     application
             Override the default FIDO application/origin string of M-bM-^@M-^\ssh:M-bM-^@M-^].
             This may be useful when generating host or domain-specific
             resident keys.  The specified application string must begin with
             M-bM-^@M-^\ssh:M-bM-^@M-^].

     challenge=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).

     device  Explicitly specify a fido(4) device to use, rather than letting
             the authenticator middleware select one.

     no-touch-required
             Indicate that the generated private key should not require touch
             events (user presence) when making signatures.  Note that sshd(8)
             will refuse such signatures by default, unless overridden via an
             authorized_keys option.

     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 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.

     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.

     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.

     write-attestation=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.

KEY REVOCATION LISTS
     ssh-keygen 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.

     KRLs may be generated using the -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).

     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.

     serial: serial_number[-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 ssh-keygen command line using the -s option.

     id: key_id
             Revokes a certificate with the specified key ID string.  The CA
             key must have been specified on the ssh-keygen command line using
             the -s option.

     key: public_key
             Revokes the specified key.  If a certificate is listed, then it
             is revoked as a plain public key.

     sha1: public_key
             Revokes the specified key by including its SHA1 hash in the KRL.

     sha256: 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.

     hash: fingerprint
             Revokes a key using a fingerprint hash, as obtained from a
             sshd(8) authentication log message or the ssh-keygen -l flag.
             Only SHA256 fingerprints are supported here and resultant KRLs
             are not supported by OpenSSH versions prior to 7.9.

     KRLs may be updated using the -u flag in addition to -k.  When this
     option is specified, keys listed via the command line are merged into the
     KRL, adding to those already there.

     It is also possible, given a KRL, to test whether it revokes a particular
     key (or keys).  The -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 ssh-keygen will exit with a
     non-zero exit status.  A zero exit status will only be returned if no key
     was revoked.

ALLOWED SIGNERS
     When verifying signatures, ssh-keygen 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 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 M-bM-^@M-^X#M-bM-^@M-^Y
     are ignored as comments.

     The principals field is a pattern-list (see PATTERNS in 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 -I option must match a principals pattern in order for the
     corresponding key to be considered acceptable for verification.

     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):

     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.

     namespaces=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.

     valid-after=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.

     valid-before=timestamp
             Indicates that the key is valid for use at or before the
             specified timestamp.

     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.

     An example allowed signers file:

        # 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...

ENVIRONMENT
     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.

FILES
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ecdsa_sk
     ~/.ssh/id_ed25519
     ~/.ssh/id_ed25519_sk
     ~/.ssh/id_rsa
             Contains the 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 ssh-keygen but it is offered as the default file for
             the private key.  ssh(1) will read this file when a login attempt
             is made.

     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ecdsa_sk.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_ed25519_sk.pub
     ~/.ssh/id_rsa.pub
             Contains the ECDSA, authenticator-hosted ECDSA, Ed25519,
             authenticator-hosted Ed25519 or RSA public key for
             authentication.  The contents of this file should be added to
             ~/.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.

     /etc/moduli
             Contains Diffie-Hellman groups used for DH-GEX.  The file format
             is described in moduli(5).

SEE ALSO
     ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)

     The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.

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.

OpenBSD 7.5                     August 17, 2024                    OpenBSD 7.5