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] [-b bits] [-C comment] [-f output_keyfile] [-m format]
                [-t dsa | ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa]
                [-N new_passphrase] [-O option] [-w provider]
     ssh-keygen -p [-f keyfile] [-m format] [-N new_passphrase]
                [-P old_passphrase]
     ssh-keygen -i [-f input_keyfile] [-m key_format]
     ssh-keygen -e [-f input_keyfile] [-m key_format]
     ssh-keygen -y [-f input_keyfile]
     ssh-keygen -c [-C comment] [-f keyfile] [-P passphrase]
     ssh-keygen -l [-v] [-E fingerprint_hash] [-f input_keyfile]
     ssh-keygen -B [-f input_keyfile]
     ssh-keygen -D pkcs11
     ssh-keygen -F hostname [-lv] [-f known_hosts_file]
     ssh-keygen -H [-f known_hosts_file]
     ssh-keygen -K [-w provider]
     ssh-keygen -R hostname [-f known_hosts_file]
     ssh-keygen -r hostname [-g] [-f input_keyfile]
     ssh-keygen -M generate [-O option] output_file
     ssh-keygen -M screen [-f input_file] [-O option] output_file
     ssh-keygen -I certificate_identity -s ca_key [-hU] [-D pkcs11_provider]
                [-n principals] [-O option] [-V validity_interval]
                [-z serial_number] file ...
     ssh-keygen -L [-f input_keyfile]
     ssh-keygen -A [-f prefix_path]
     ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]
                file ...
     ssh-keygen -Q [-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         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 admin‐
     istrator 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, num‐
     bers, 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 generat‐
     ing new keys, and existing new-format keys may be converted using this
     option in conjunction with the -p (change passphrase) flag.

     After a key is generated, instructions below detail where the keys
     should be placed to be activated.

     The options are as follows:

     -A      For each of the key types (rsa, dsa, ecdsa and ed25519) for
             which host keys do not exist, generate the host keys with the
             default key file path, an empty passphrase, default bits for
             the key type, and default comment.  If -f has also been speci‐
             fied, its argument is used as a prefix to the default path for
             the resulting host key files.  This is used by /etc/rc to gen‐
             erate 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 resis‐
             tance to brute-force password cracking (should the keys be
             stolen).

     -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 indi‐
             cates that a CA key resides in a PKCS#11 token (see the
             CERTIFICATES section for details).

     -E fingerprint_hash
             Specifies the hash algorithm used when displaying key finger‐
             prints.  Valid options are: “md5” and “sha256”.  The default is
             “sha256”.

     -e      This option will read a private or public OpenSSH key file and
             print to stdout a public key in one of the formats specified by
             the -m option.  The default export format is “RFC4716”.  This
             option allows exporting OpenSSH keys for use by other programs,
             including several commercial SSH implementations.

     -F hostname | [hostname]:port
             Search for the specified hostname (with optional port number)
             in a known_hosts file, listing any occurrences found.  This
             option is useful to find hashed host names or addresses and may
             also be used in conjunction with the -H option to print found
             keys in a hashed format.

     -f filename
             Specifies the filename of the key file.

     -g      Use generic DNS format when printing fingerprint resource
             records using the -r command.

     -H      Hash a known_hosts file.  This replaces all hostnames and
             addresses with hashed representations within the specified
             file; the original content is moved to a file with a .old suf‐
             fix.  These hashes may be used normally by ssh and sshd, but
             they do not reveal identifying information should the file's
             contents be disclosed.  This option will not modify existing
             hashed hostnames and is therefore safe to use on files that mix
             hashed and non-hashed names.

     -h      When signing a key, create a host certificate instead of a user
             certificate.  Please see the CERTIFICATES section for details.

     -I certificate_identity
             Specify the key identity when signing a public key.  Please see
             the CERTIFICATES section for details.

     -i      This option will read an unencrypted private (or public) key
             file in the format specified by the -m option and print an
             OpenSSH compatible private (or public) key to stdout.  This
             option allows importing keys from other software, including
             several commercial SSH implementations.  The default import
             format is “RFC4716”.

     -K      Download resident keys from a FIDO authenticator.  Public and
             private key files will be written to the current directory for
             each downloaded key.

     -k      Generate a KRL file.  In this mode, ssh-keygen will generate a
             KRL file at the location specified via the -f flag that revokes
             every key or certificate presented on the command line.
             Keys/certificates to be revoked may be specified by public key
             file or using the format described in the KEY REVOCATION LISTS
             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 infor‐
             mation.

     -M screen
             Screen candidate parameters for Diffie-Hellman Group Exchange.
             This will accept a list of candidate numbers and test that they
             are safe (Sophie Germain) primes with acceptable group genera‐
             tors.  The results of this operation may be added to the
             /etc/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 oper‐
             ation.  The latter may be used to convert between OpenSSH pri‐
             vate key and PEM private key formats.  The supported key for‐
             mats are: “RFC4716” (RFC 4716/SSH2 public or private key),
             “PKCS8” (PKCS8 public or private key) or “PEM” (PEM public
             key).  By default OpenSSH will write newly-generated private
             keys in its own format, but when converting public keys for
             export the default format is “RFC4716”.  Setting a format of
             “PEM” when generating or updating a supported private key type
             will cause the key to be stored in the legacy PEM private key
             format.

     -N new_passphrase
             Provides the new passphrase.

     -n principals
             Specify one or more principals (user or host names) to be
             included in a certificate when signing a key.  Multiple princi‐
             pals may be specified, separated by commas.  Please see the
             CERTIFICATES section for details.

     -O option
             Specify a key/value option.  These are specific to the opera‐
             tion 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 speci‐
             fied.

             When generating a key that will be hosted on a FIDO authentica‐
             tor, this flag may be used to specify key-specific options.
             Those supported at present are:

             application
                     Override the default FIDO application/origin string of
                     “ssh:”.  This may be useful when generating host or
                     domain-specific resident keys.  The specified applica‐
                     tion string must begin with “ssh:”.

             challenge=path
                     Specifies a path to a challenge string that will be
                     passed to the FIDO token during key generation.  The
                     challenge string may be used as part of an out-of-band
                     protocol for key enrollment (a random challenge is used
                     by default).

             device  Explicitly specify a fido(4) device to use, rather than
                     letting the token middleware select one.

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

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

             user    A username to be associated with a resident key, over‐
                     riding the empty default username.  Specifying a user‐
                     name may be useful when generating multiple resident
                     keys for the same application name.

             write-attestation=path
                     May be used at key generation time to record the attes‐
                     tation certificate returned from FIDO tokens during key
                     generation.  By default this information is discarded.

             The -O option may be specified multiple times.

     -P passphrase
             Provides the (old) passphrase.

     -p      Requests changing the passphrase of a private key file instead
             of creating a new private key.  The program will prompt for the
             file containing the private key, for the old passphrase, and
             twice for the new passphrase.

     -Q      Test whether keys have been revoked in a KRL.  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 avail‐
             able 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 com‐
             mand line are added to the existing KRL rather than a new KRL
             being created.

     -V validity_interval
             Specify a validity interval when signing a certificate.  A
             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 indi‐
             cate an explicit time interval.

             The start time may be specified as the string “always” to indi‐
             cate the certificate has no specified start time, a date in
             YYYYMMDD format, a time in YYYYMMDDHHMM[SS] format, a relative
             time (to the current time) consisting of a minus sign followed
             by an interval in the format described in the TIME FORMATS sec‐
             tion of sshd_config(5).

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

             For example: “+52w1d” (valid from now to 52 weeks and one day
             from now), “-4w:+4w” (valid from four weeks ago to four weeks
             from now), “20100101123000:20110101123000” (valid from 12:30
             PM, January 1st, 2010 to 12:30 PM, January 1st, 2011),
             “-1d:20110101” (valid from yesterday to midnight, January 1st,
             2011).  “-1m:forever” (valid from one minute ago and never
             expiring).

     -v      Verbose mode.  Causes ssh-keygen to print debugging messages
             about its progress.  This is helpful for debugging moduli gen‐
             eration.  Multiple -v options increase the verbosity.  The max‐
             imum 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 sig‐
             nature, 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 signa‐
             ture must also be supplied using the -s flag.  Successful test‐
             ing 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 signa‐
             ture namespace, used to prevent signature confusion across dif‐
             ferent 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 fol‐
             lowing 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 signa‐
             ture 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 con‐
             taining 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 sig‐
             nalled 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         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 suit‐
     ability (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 genera‐
     tor value will be chosen automatically for the prime under considera‐
     tion.  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 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 per‐
             forming DH candidate screening.

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

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

     memory=mbytes
             Specify the amount of memory to use (in megabytes) when gener‐
             ating 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 sim‐
     pler, 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 pro‐
     viding the token library using -D and identifying the CA key by provid‐
     ing 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 individ‐
             ually.

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

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

     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 require demon‐
             stration 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.

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

     Finally, certificates may be defined with a validity lifetime.  The -V
     option allows specification of certificate start and end times.  A cer‐
     tificate that is presented at a time outside this range will not be
     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         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 com‐
     plete 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 num‐
             bers 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 particu‐
     lar 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 com‐
     mand 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 cor‐
     responding key to be considered acceptable for verification.

     The options (if present) consist of comma-separated option specifica‐
     tions.  No spaces are permitted, except within double quotes.  The fol‐
     lowing option specifications are supported (note that option keywords
     are case-insensitive):

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

     namespaces="namespace-list"
             Specifies a pattern-list of namespaces that are accepted for
             this key.  If this option is present, the signature namespace
             embedded in the signature object and presented on the verifica‐
             tion 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         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 automat‐
             ically accessed by ssh-keygen but it is offered as the default
             file for the private key.  ssh(1) will read this file when a
             login attempt is made.

     ~/.ssh/id_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_ecdsa_sk.pub
     ~/.ssh/id_ed25519.pub
     ~/.ssh/id_ed25519_sk.pub
     ~/.ssh/id_rsa.pub
             Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519,
             authenticator-hosted Ed25519 or RSA public key for authentica‐
             tion.  The contents of this file should be added to
             ~/.ssh/authorized_keys on all machines where the user wishes to
             log in using public key authentication.  There is no need to
             keep the contents of this file secret.

     /etc/moduli
             Contains Diffie-Hellman groups used for DH-GEX.  The file for‐
             mat 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.  Informa‐
     tion 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-8.3p1.tar.gz fetched from
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     sion 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                             April 3, 2020                            BSD