# ssh-keygen

## NAME



           **ssh-keygen** — 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 “.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 **-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
           “user@host” 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: “md5” and “sha256”.  The
                   default is “sha256”.
    
           **-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
                   “RFC4716”.  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 “RFC4716”.
    
           **-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
                   ‘diffie-hellman-group-exchange-*’ 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: “RFC4716” (RFC 4716/SSH2
                   public or private key), “PKCS8” (PKCS8 public or private
                   key) or “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 “RFC4716”.  Setting a format of “PEM” 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
                           “sha256” and “sha512.” The default is “sha512.”
    
                   **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 “sha1” and “sha256”.  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 “ecdsa”, “ecdsa-sk”, “ed25519 (the default),”
                   “ed25519-sk”, or “rsa”.
    
                   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 “ssh-rsa”
                   (SHA1 signatures, not recommended), “rsa-sha2-256”, and
                   “rsa-sha2-512” (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:
                   **•** The string “always” to indicate the certificate has no
                       specified start time.
                   **•** A date or time in the system time zone formatted as
                       YYYYMMDD or YYYYMMDDHHMM[SS].
                   **•** A date or time in the UTC time zone as YYYYMMDDZ or
                       YYYYMMDDHHMM[SS]Z.
                   **•** 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).
                   **•** A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as
                       a hexadecimal number beginning with “0x”.
    
                   The end time may be specified similarly to the start time:
                   **•** The string “forever” to indicate the certificate has
                       no specified end time.
                   **•** A date or time in the system time zone formatted as
                       YYYYMMDD or YYYYMMDDHHMM[SS].
                   **•** A date or time in the UTC time zone as YYYYMMDDZ or
                       YYYYMMDDHHMM[SS]Z.
                   **•** 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).
                   **•** A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as
                       a hexadecimal number beginning with “0x”.
    
                   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 “.sig” 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:
                   “file” for file signing, “email” 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 “aes256-ctr”.
    
           **-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
                   ‘+’ 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. “name@example.com”.  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
                   “ssh:”.  This may be useful when generating host or
                   domain-specific resident keys.  The specified application
                   string must begin with “ssh:”.
    
           **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 ‘#’ 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).
    


* * *