ssh-keygen(1) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | MODULI GENERATION | CERTIFICATES | KEY REVOCATION LISTS | ALLOWED SIGNERS | ENVIRONMENT | FILES | SEE ALSO | AUTHORS | COLOPHON

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

NAME         top

     ssh-keygen — OpenSSH authentication key utility

SYNOPSIS         top

     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         top

     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/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         top

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

CERTIFICATES         top

     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         top

     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         top

     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         top

     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         top

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

SEE ALSO         top

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

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

AUTHORS         top

     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.

COLOPHON         top

     This page is part of the openssh (Portable OpenSSH) project.
     Information about the project can be found at
     http://www.openssh.com/portable.html.  If you have a bug report for
     this manual page, see ⟨http://www.openssh.com/report.html⟩.  This
     page was obtained from the tarball openssh-8.7p1.tar.gz fetched
     from ⟨http://ftp.eu.openbsd.org/pub/OpenBSD/OpenSSH/portable/⟩ on
     2021-08-27.  If you discover any rendering problems in this HTML
     version of the page, or you believe there is a better or more up-
     to-date source for the page, or you have corrections or
     improvements to the information in this COLOPHON (which is not part
     of the original manual page), send a mail to man-pages@man7.org

BSD                          August 11, 2021                         BSD