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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]
     ssh-keygen -p [-a rounds] [-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 [-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 -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]

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     whether given keys have been revoked by one.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
     administrator 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 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      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 resulting 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).
             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.  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 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.  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
             authenticator.

     -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) 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/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 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.
             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 authenticator, 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 signatures.  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, 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 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 attestation data returned
                     from FIDO tokens during key generation.  Please note that this information
                     is potentially sensitive.  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.  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 recommended), “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 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 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 YYYYMMDDHHMM[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 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 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 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 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 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/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:

     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
     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).  Please refer to those manual 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 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.

     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 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/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 created OpenSSH.  Markus Friedl contributed the support
     for SSH protocol versions 1.5 and 2.0.