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

     ssh-keygen -- OpenSSH authentication key utility

     ssh-keygen [-q] [-b bits] [-C comment] [-f output_keyfile] [-m format]
                [-t dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa]
                [-N new_passphrase] [-O option] [-w provider]
     ssh-keygen -p [-f keyfile] [-m format] [-N new_passphrase]
                [-P old_passphrase]
     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 [-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 [-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 [-f prefix_path]
     ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]
                file ...
     ssh-keygen -Q -f krl_file file ...
     ssh-keygen -Y find-principals -s signature_file -f allowed_signers_file
     ssh-keygen -Y check-novalidate -n namespace -s signature_file
     ssh-keygen -Y sign -f key_file -n namespace file ...
     ssh-keygen -Y verify -f allowed_signers_file -I signer_identity -n
                namespace -s signature_file [-r revocation_file]

     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 in-
     voked 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     Lists, and to test whether given keys have been revoked by one.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.

     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 se-
     ries of words, punctuation, numbers, whitespace, or any string of charac-
     ters you want.  Good passphrases are 10-30 characters long, are not sim-
     ple 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-alphanu-
     meric characters.  The passphrase can be changed later by using the -p

     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, instructions below detail 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 system administration
             scripts to generate new host keys.

     -a rounds
             When saving a private key, this option specifies the number of
             KDF (key derivation function) rounds used.  Higher numbers result
             in slower passphrase verification and increased resistance to
             brute-force password cracking (should the keys be stolen).

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

     -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 ex-
             actly 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

     -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

     -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 op-
             tion allows exporting OpenSSH keys for use by other programs, in-
             cluding 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 ad-
             dresses 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.

     -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

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

     -m key_format
             Specify a key format for key generation, the -i (import), -e (ex-
             port) 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 for-
             mat, but when converting public keys for export the default for-
             mat 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 in-
             cluded in a certificate when signing a key.  Multiple principals
             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 op-
             tions 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:

                     Override the default FIDO application/origin string of
                     "ssh:".  This may be useful when generating host or do-
                     main-specific resident keys.  The specified application
                     string must begin with "ssh:".

                     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.

                     Indicate that the generated private key should not re-
                     quire 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.

                     Indicate that the key should be stored on the FIDO au-
                     thenticator 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.

                     May be used at key generation time to record the attesta-
                     tion certificate returned from FIDO tokens during key
                     generation.  By default this information is discarded.

             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.

     -q      Silence ssh-keygen.

     -R hostname | [hostname]:port
             Removes all keys belonging to the specified hostname (with op-
             tional port number) from a known_hosts file.  This option is use-
             ful 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 be-
             ing 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 ex-
             plicit 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 ex-
             pirty 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 us-
             ing -n.  A file containing the corresponding signature must also
             be supplied using the -s flag.  Successful testing of the signa-
             ture is signalled by ssh-keygen returning a zero exit status.

     -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 name-
             space, used to prevent signature confusion across different do-
             mains 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 NAME-
             SPACE@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 ac-
             cepts 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 for-
             mat 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 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 se-
             rial 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.

     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 de-
     sired length of the primes may be specified by the -O bits option.  For

           # 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 /etc/ssh/moduli.  It is important
     that this file contains moduli of a range of bit lengths and that both
     ends of a connection share common moduli.

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

             Exit after screening the specified number of lines while perform-
             ing DH candidate screening.

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

             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

             Specify the amount of memory to use (in megabytes) when generat-
             ing candidate moduli for DH-GEX.

             Specify start point (in hex) when generating candidate moduli for

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

     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 au-
     thenticate server hosts to users.  To generate a user certificate:

           $ ssh-keygen -s /path/to/ca_key -I key_id /path/to/

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

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

     The host certificate will be output to /path/to/

     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 -D -I key_id

     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 -I key_id

     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-

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

     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-

             Includes an arbitrary certificate critical option or extension.
             The specified name should include a domain suffix, e.g.
             "".  If contents is specified then it is included
             as the contents of the extension/option encoded as a string, oth-
             erwise 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 op-
             tions will cause the certificate to be refused.

             Forces the execution of command instead of any shell or command
             specified by the user when the certificate is used for authenti-

             Disable ssh-agent(1) forwarding (permitted by default).

             Disable port forwarding (permitted by default).

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

             Disable execution of ~/.ssh/rc by sshd(8) (permitted by default).

             Disable X11 forwarding (permitted by default).

             Allows ssh-agent(1) forwarding.

             Allows port forwarding.

             Allows PTY allocation.

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

             Allows X11 forwarding.

             Do not require signatures made using this key require 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.

             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.

     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 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.

     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 ei-
     ther 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 ex-
             pressed 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 op-
     tion 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.

     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

             Indicates that this key is accepted as a certificate authority
             (CA) and that certificates signed by this CA may be accepted for

             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.

     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, ssh-rsa AAAAX1...
        # A certificate authority, trusted for all principals in a domain.
        * cert-authority ssh-ed25519 AAAB4...
        # A key that is accepted only for file signing. namespaces="file" ssh-ed25519 AAA41...

             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.

             Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, au-
             thenticator-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 pri-
             vate key.  ssh(1) will read this file when a login attempt is

             Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, au-
             thenticator-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.

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

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

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

     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                            February 7, 2020                            BSD
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