ssh-keygen

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
     administrator may use this to generate host keys, as seen in /etc/rc.

     Normally this program generates the key and asks for a file in which to
     store the private key.  The public key is stored in a file with the same
     name but “.pub” appended.  The program also asks for a passphrase.  The
     passphrase may be empty to indicate no passphrase (host keys must have an
     empty passphrase), or it may be a string of arbitrary length.  A passphrase
     is similar to a password, except it can be a phrase with a series of words,
     punctuation, numbers, whitespace, or any string of characters you want.
     Good passphrases are 10-30 characters long, are not simple sentences or
     otherwise easily guessable (English prose has only 1-2 bits of entropy per
     character, and provides very bad passphrases), and contain a mix of upper
     and lowercase letters, numbers, and non-alphanumeric characters.  The
     passphrase can be changed later by using the -p option.

     There is no way to recover a lost passphrase.  If the passphrase is lost or
     forgotten, a new key must be generated and the corresponding public key
     copied to other machines.

     ssh-keygen will by default write keys in an OpenSSH-specific format.  This
     format is preferred as it offers better protection for keys at rest as well
     as allowing storage of key comments within the private key file itself.
     The key comment may be useful to help identify the key.  The comment is
     initialized to “user@host” when the key is created, but can be changed
     using the -c option.

     It is still possible for ssh-keygen to write the previously-used PEM format
     private keys using the -m flag.  This may be used when generating new keys,
     and existing new-format keys may be converted using this option in
     conjunction with the -p (change passphrase) flag.

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

     The options are as follows:

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

     -a rounds
             When saving a private key, this option specifies the number of KDF
             (key derivation function, currently bcrypt_pbkdf(3)) rounds used.
             Higher numbers result in slower passphrase verification and
             increased resistance to brute-force password cracking (should the
             keys be stolen).  The default is 16 rounds.

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

     -b bits
             Specifies the number of bits in the key to create.  For RSA keys,
             the minimum size is 1024 bits and the default is 3072 bits.
             Generally, 3072 bits is considered sufficient.  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.

             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
                     signature verification.

             verify-time=timestamp
                     Specifies a time to use when validating signatures instead
                     of the current time.  The time may be specified as a date
                     in YYYYMMDD format or a time in YYYYMMDDHHMM[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
             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 cipher
             Specifies the cipher to use for encryption when writing an OpenSSH-
             format private key file.  The list of available ciphers may be
             obtained using "ssh -Q cipher".  The default is “aes256-ctr”.

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

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

MODULI GENERATION
     ssh-keygen may be used to generate groups for the Diffie-Hellman Group
     Exchange (DH-GEX) protocol.  Generating these groups is a two-step process:
     first, candidate primes are generated using a fast, but memory intensive
     process.  These candidate primes are then tested for suitability (a CPU-
     intensive process).

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

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

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

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

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

     By default, each candidate will be subjected to 100 primality tests.  This
     may be overridden using the -O prime-tests option.  The DH generator value
     will be chosen automatically for the prime under consideration.  If a
     specific generator is desired, it may be requested using the -O generator
     option.  Valid generator values are 2, 3, and 5.

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

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

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

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

     checkpoint=filename
             Write the last line processed to the specified file while
             performing DH candidate screening.  This will be used to skip lines
             in the input file that have already been processed if the job is
             restarted.

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

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

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

CERTIFICATES
     ssh-keygen supports signing of keys to produce certificates that may be
     used for user or host authentication.  Certificates consist of a public
     key, some identity information, zero or more principal (user or host) names
     and a set of options that are signed by a Certification Authority (CA) key.
     Clients or servers may then trust only the CA key and verify its signature
     on a certificate rather than trusting many user/host keys.  Note that
     OpenSSH certificates are a different, and much simpler, format to the X.509
     certificates used in ssl(8).

     ssh-keygen supports two types of certificates: user and host.  User
     certificates authenticate users to servers, whereas host certificates
     authenticate server hosts to users.  To generate a user certificate:

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

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

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

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

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

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

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

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

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

     Certificates may be limited to be valid for a set of principal (user/host)
     names.  By default, generated certificates are valid for all users or
     hosts.  To generate a certificate for a specified set of principals:

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

     Additional limitations on the validity and use of user certificates may be
     specified through certificate options.  A certificate option may disable
     features of the SSH session, may be valid only when presented from
     particular source addresses or may force the use of a specific command.

     The options that are valid for user certificates are:

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

     critical:name[=contents]
     extension:name[=contents]
             Includes an arbitrary certificate critical option or extension.
             The specified name should include a domain suffix, e.g.
             “name@example.com”.  If contents is specified then it is included
             as the contents of the extension/option encoded as a string,
             otherwise the extension/option is created with no contents (usually
             indicating a flag).  Extensions may be ignored by a client or
             server that does not recognise them, whereas unknown critical
             options will cause the certificate to be refused.

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

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

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

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

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

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

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

     permit-port-forwarding
             Allows port forwarding.

     permit-pty
             Allows PTY allocation.

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

     permit-X11-forwarding
             Allows X11 forwarding.

     no-touch-required
             Do not require signatures made using this key include demonstration
             of user presence (e.g. by having the user touch the authenticator).
             This option only makes sense for the FIDO authenticator algorithms
             ecdsa-sk and ed25519-sk.

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

     verify-required
             Require signatures made using this key indicate that the user was
             first verified.  This option only makes sense for the FIDO
             authenticator algorithms ecdsa-sk and ed25519-sk.  Currently PIN
             authentication is the only supported verification method, but other
             methods may be supported in the future.

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

     Finally, certificates may be defined with a validity lifetime.  The -V
     option allows specification of certificate start and end times.  A
     certificate that is presented at a time outside this range will not be
     considered valid.  By default, certificates are valid from the UNIX Epoch
     to the distant future.

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

     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 specified
             timestamp.

     When verifying signatures made by certificates, the expected principal name
     must match both the principals pattern in the allowed signers file and the
     principals embedded in the certificate itself.

     An example allowed signers file:

        # Comments allowed at start of line
        user1@example.com,user2@example.com ssh-rsa AAAAX1...
        # A certificate authority, trusted for all principals in a domain.
        *@example.com cert-authority ssh-ed25519 AAAB4...
        # A key that is accepted only for file signing.
        user2@example.com namespaces="file" ssh-ed25519 AAA41...

ENVIRONMENT
     SSH_SK_PROVIDER
             Specifies a path to a library that will be used when loading any
             FIDO authenticator-hosted keys, overriding the default of using the
             built-in USB HID support.

FILES
     ~/.ssh/id_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.

BSD                              August 11, 2021                             BSD