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ssh-keygen(1)             BSD General Commands Manual            ssh-keygen(1)


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 dsa | 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 check-novalidate [-O option] -n namespace -s signature_file
     ssh-keygen -Y sign -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 RSA 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_dsa,
     ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519,
     ~/.ssh/id_ed25519_sk or ~/.ssh/id_rsa.  Additionally, the system adminis-
     trator 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 char-
     acters 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 for-
     mat 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      For each of the key types (rsa, dsa, ecdsa and ed25519) for which
             host keys do not exist, generate the host keys 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 result-
             ing 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.  Gen-
             erally, 3072 bits is considered sufficient.  DSA keys must be
             exactly 1024 bits as specified by FIPS 186-2.  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 pri-
             vate 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 finger-
             prints.  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.  Please see the CERTIFICATES section for details.

     -I certificate_identity
             Specify the key identity when signing a public key.  Please 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 authen-
             ticator.

     -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.  For RSA and DSA
             keys ssh-keygen tries 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) parame-
             ters 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 genera-
             tors.  The results of this operation may be added to the
             /opt/local/opt/local/etc/ssh/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 opera-
             tion.  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 princi-
             pals may be specified, separated by commas.  Please 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 a key that will be hosted on a FIDO authentica-
             tor, this flag may be used to specify key-specific options.
             Those supported at present 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 token 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 token middleware select one.

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

             resident
                     Indicate that the key should be stored on the FIDO
                     authenticator itself.  Resident keys may be supported on
                     FIDO2 tokens and typically require that a PIN be set on
                     the token prior to generation.  Resident keys may be
                     loaded off the token using ssh-add(1).

             user    A username to be associated with a resident key, overrid-
                     ing 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 veri-
                     fication for each signature.  Not all FIDO tokens 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 attesta-
                     tion data returned from FIDO tokens during key genera-
                     tion.  Please note that this information is potentially
                     sensitive.  By default, this information is discarded.

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

             print-pubkey
                     Print the full public key to standard output after signa-
                     ture 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 in YYYYMMDD format or a time in YYYYMMD-
                     DHHMM[SS] format.

             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.  Please
             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 dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
             Specifies the type of key to create.  The possible values are
             ``dsa'', ``ecdsa'', ``ecdsa-sk'', ``ed25519'', ``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 recom-
             mended), ``rsa-sha2-256'', and ``rsa-sha2-512'' (the default).

     -U      When used in combination with -s, 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 com-
             mand 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 valid-
             ity 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 indi-
             cate the certificate has no specified start time, a date in
             YYYYMMDD format, a time in YYYYMMDDHHMM[SS] format, a relative
             time (to the current time) consisting of a minus sign followed by
             an interval in the format described in the TIME FORMATS section
             of sshd_config(5).

             The end time may be specified as a YYYYMMDD date, a YYYYMMD-
             DHHMM[SS] time, a relative time starting with a plus character or
             the string ``forever'' to indicate that the certificate has no
             expiry date.

             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),
             ``-1d:20110101'' (valid from yesterday to midnight, January 1st,
             2011), ``-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 genera-
             tion.  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 signa-
             ture, 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 stan-
             dard 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 sta-
             tus.

     -Y sign
             Cryptographically sign a file or some data using a 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 names-
             pace, used to prevent signature confusion across different
             domains of use (e.g. file signing vs email signing) must be pro-
             vided 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 verifi-
             cation by an authorized signer is signalled by ssh-keygen return-
             ing 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 ver-
             sion 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 suitabil-
     ity (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 spec-
     ified 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
     /opt/local/opt/local/etc/ssh/moduli.  It is important that this file con-
     tains 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 perform-
             ing 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 per-
             forming 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 generat-
             ing 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 cer-
     tificates 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 pro-
     viding 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 identi-
     fied 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 princi-
     pals:

           $ 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 dis-
     able 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 individu-
             ally.

     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 con-
             tents (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 authenti-
             cation.

     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 demonstra-
             tion of user presence (e.g. by having the user touch the authen-
             ticator).  This option only makes sense for the FIDO authentica-
             tor algorithms ecdsa-sk and ed25519-sk.

     source-address=address_list
             Restrict the source addresses from which the certificate is con-
             sidered 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 cer-
     tificate that is presented at a time outside this range will not be con-
     sidered 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 pub-
     lic key must be trusted by sshd(8) or ssh(1).  Please refer to those man-
     ual pages for details.


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 con-
     tents 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 orig-
     inal certificate on hand.  A KRL specification consists of lines contain-
     ing one of the following directives followed by a colon and some direc-
     tive-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, key-
     type, 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 corre-
     sponding key to be considered acceptable for verification.

     The options (if present) consist of comma-separated option specifica-
     tions.  No spaces are permitted, except within double quotes.  The fol-
     lowing 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 com-
             mand-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 in YYYYMMDD format or a time in
             YYYYMMDDHHMM[SS] format.

     valid-before=timestamp
             Indicates that the key is valid for use at or before the speci-
             fied 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_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_ecdsa_sk
     ~/.ssh/id_ed25519
     ~/.ssh/id_ed25519_sk
     ~/.ssh/id_rsa
             Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519,
             authenticator-hosted Ed25519 or RSA authentication identity of
             the user.  This file should not be readable by anyone but the
             user.  It is possible to specify a passphrase when generating the
             key; that passphrase will be used to encrypt the private part of
             this file using 128-bit AES.  This file is not automatically
             accessed by 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_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ecdsa_sk.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_ed25519_sk.pub
     ~/.ssh/id_rsa.pub
             Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519,
             authenticator-hosted Ed25519 or RSA public key for authentica-
             tion.  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.

     /opt/local/opt/local/etc/ssh/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 cre-
     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
     versions 1.5 and 2.0.

BSD                             August 21, 2021                            BSD

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