IABSD.fr/src/usr.bin/ssh/ssh-keygen.1

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• Author : djm
  Date : 2025-12-22 01:49:03
  Hash : f0c5ff0c
  Message : When certificate support was added to OpenSSH, certificates were originally specified to represent any principal if the principals list was empty. This was, in retrospect, a mistake as it created a fail-open situation if a CA could be convinced to accidentally sign a certificate with no principals. This actually happened in a 3rd- party CA product (CVE-2024-7594). Somewhat fortunately, the main pathway for using certificates in sshd (TrustedUserCAKeys) never supported empty-principals certificates, so the blast radius of such mistakes was substantially reduced. This change removes this footcannon and requires all certificates include principals sections. It also fixes interpretation of wildcard principals, and properly enables them for host certificates only. This is a behaviour change that will permanently break uses of certificates with empty principals sections. ok markus@

• usr.bin/ssh/ssh-keygen.1

• .\"	$OpenBSD: ssh-keygen.1,v 1.237 2025/12/22 01:49:03 djm Exp $
  .\"
  .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
  .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  .\"                    All rights reserved
  .\"
  .\" As far as I am concerned, the code I have written for this software
  .\" can be used freely for any purpose.  Any derived versions of this
  .\" software must be clearly marked as such, and if the derived work is
  .\" incompatible with the protocol description in the RFC file, it must be
  .\" called by a name other than "ssh" or "Secure Shell".
  .\"
  .\"
  .\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
  .\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
  .\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
  .\"
  .\" Redistribution and use in source and binary forms, with or without
  .\" modification, are permitted provided that the following conditions
  .\" are met:
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  .Dd $Mdocdate: December 22 2025 $
  .Dt SSH-KEYGEN 1
  .Os
  .Sh NAME
  .Nm ssh-keygen
  .Nd OpenSSH authentication key utility
  .Sh SYNOPSIS
  .Nm ssh-keygen
  .Op Fl q
  .Op Fl a Ar rounds
  .Op Fl b Ar bits
  .Op Fl C Ar comment
  .Op Fl f Ar output_keyfile
  .Op Fl m Ar format
  .Op Fl N Ar new_passphrase
  .Op Fl O Ar option
  .Op Fl t Cm ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
  .Op Fl w Ar provider
  .Op Fl Z Ar cipher
  .Nm ssh-keygen
  .Fl p
  .Op Fl a Ar rounds
  .Op Fl f Ar keyfile
  .Op Fl m Ar format
  .Op Fl N Ar new_passphrase
  .Op Fl P Ar old_passphrase
  .Op Fl Z Ar cipher
  .Nm ssh-keygen
  .Fl i
  .Op Fl f Ar input_keyfile
  .Op Fl m Ar key_format
  .Nm ssh-keygen
  .Fl e
  .Op Fl f Ar input_keyfile
  .Op Fl m Ar key_format
  .Nm ssh-keygen
  .Fl y
  .Op Fl f Ar input_keyfile
  .Nm ssh-keygen
  .Fl c
  .Op Fl a Ar rounds
  .Op Fl C Ar comment
  .Op Fl f Ar keyfile
  .Op Fl P Ar passphrase
  .Nm ssh-keygen
  .Fl l
  .Op Fl v
  .Op Fl E Ar fingerprint_hash
  .Op Fl f Ar input_keyfile
  .Nm ssh-keygen
  .Fl B
  .Op Fl f Ar input_keyfile
  .Nm ssh-keygen
  .Fl D Ar pkcs11
  .Nm ssh-keygen
  .Fl F Ar hostname
  .Op Fl lv
  .Op Fl f Ar known_hosts_file
  .Nm ssh-keygen
  .Fl H
  .Op Fl f Ar known_hosts_file
  .Nm ssh-keygen
  .Fl K
  .Op Fl a Ar rounds
  .Op Fl w Ar provider
  .Nm ssh-keygen
  .Fl R Ar hostname
  .Op Fl f Ar known_hosts_file
  .Nm ssh-keygen
  .Fl r Ar hostname
  .Op Fl g
  .Op Fl f Ar input_keyfile
  .Nm ssh-keygen
  .Fl M Cm generate
  .Op Fl O Ar option
  .Ar output_file
  .Nm ssh-keygen
  .Fl M Cm screen
  .Op Fl f Ar input_file
  .Op Fl O Ar option
  .Ar output_file
  .Nm ssh-keygen
  .Fl I Ar certificate_identity
  .Fl s Ar ca_key
  .Op Fl hU
  .Op Fl D Ar pkcs11_provider
  .Op Fl n Ar principals
  .Op Fl O Ar option
  .Op Fl V Ar validity_interval
  .Op Fl z Ar serial_number
  .Ar
  .Nm ssh-keygen
  .Fl L
  .Op Fl f Ar input_keyfile
  .Nm ssh-keygen
  .Fl A
  .Op Fl a Ar rounds
  .Op Fl f Ar prefix_path
  .Nm ssh-keygen
  .Fl k
  .Fl f Ar krl_file
  .Op Fl u
  .Op Fl s Ar ca_public
  .Op Fl z Ar version_number
  .Ar
  .Nm ssh-keygen
  .Fl Q
  .Op Fl l
  .Fl f Ar krl_file
  .Ar
  .Nm ssh-keygen
  .Fl Y Cm find-principals
  .Op Fl O Ar option
  .Fl s Ar signature_file
  .Fl f Ar allowed_signers_file
  .Nm ssh-keygen
  .Fl Y Cm match-principals
  .Fl I Ar signer_identity
  .Fl f Ar allowed_signers_file
  .Nm ssh-keygen
  .Fl Y Cm check-novalidate
  .Op Fl O Ar option
  .Fl n Ar namespace
  .Fl s Ar signature_file
  .Nm ssh-keygen
  .Fl Y Cm sign
  .Op Fl O Ar option
  .Fl f Ar key_file
  .Fl n Ar namespace
  .Ar
  .Nm ssh-keygen
  .Fl Y Cm verify
  .Op Fl O Ar option
  .Fl f Ar allowed_signers_file
  .Fl I Ar signer_identity
  .Fl n Ar namespace
  .Fl s Ar signature_file
  .Op Fl r Ar revocation_file
  .Sh DESCRIPTION
  .Nm
  generates, manages and converts authentication keys for
  .Xr ssh 1 .
  .Nm
  can create keys for use by SSH protocol version 2.
  .Pp
  The type of key to be generated is specified with the
  .Fl t
  option.
  If invoked without any arguments,
  .Nm
  will generate an Ed25519 key.
  .Pp
  .Nm
  is also used to generate groups for use in Diffie-Hellman group
  exchange (DH-GEX).
  See the
  .Sx MODULI GENERATION
  section for details.
  .Pp
  Finally,
  .Nm
  can be used to generate and update Key Revocation Lists, and to test whether
  given keys have been revoked by one.
  See the
  .Sx KEY REVOCATION LISTS
  section for details.
  .Pp
  Normally each user wishing to use SSH
  with public key authentication runs this once to create the authentication
  key in
  .Pa ~/.ssh/id_ecdsa ,
  .Pa ~/.ssh/id_ecdsa_sk ,
  .Pa ~/.ssh/id_ed25519 ,
  .Pa ~/.ssh/id_ed25519_sk
  or
  .Pa ~/.ssh/id_rsa .
  Additionally, the system administrator may use this to generate host keys,
  as seen in
  .Pa /etc/rc .
  .Pp
  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
  .Dq .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
  .Fl p
  option.
  .Pp
  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.
  .Pp
  .Nm
  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
  .Dq user@host
  when the key is created, but can be changed using the
  .Fl c
  option.
  .Pp
  It is still possible for
  .Nm
  to write the previously-used PEM format private keys using the
  .Fl 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
  .Fl p
  (change passphrase) flag.
  .Pp
  After a key is generated,
  .Nm
  will ask where the keys
  should be placed to be activated.
  .Pp
  The options are as follows:
  .Bl -tag -width Ds
  .It Fl A
  Generate host keys of all default key types (rsa, ecdsa, and
  ed25519) if they do not already exist.
  The host keys are generated with the default key file path,
  an empty passphrase, default bits for the key type, and default comment.
  If
  .Fl 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
  .Pa /etc/rc
  to generate new host keys.
  .It Fl a Ar rounds
  When saving a private key, this option specifies the number of KDF
  (key derivation function, currently
  .Xr 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.
  .It Fl B
  Show the bubblebabble digest of specified private or public key file.
  .It Fl b Ar 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.
  For ECDSA keys, the
  .Fl 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
  .Fl b
  flag will be ignored.
  .It Fl C Ar comment
  Provides a new comment.
  .It Fl 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.
  .It Fl D Ar pkcs11
  Download the public keys provided by the PKCS#11 shared library
  .Ar pkcs11 .
  When used in combination with
  .Fl s ,
  this option indicates that a CA key resides in a PKCS#11 token (see the
  .Sx CERTIFICATES
  section for details).
  .It Fl E Ar fingerprint_hash
  Specifies the hash algorithm used when displaying key fingerprints.
  Valid options are:
  .Dq md5
  and
  .Dq sha256 .
  The default is
  .Dq sha256 .
  .It Fl 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
  .Fl m
  option.
  The default export format is
  .Dq RFC4716 .
  This option allows exporting OpenSSH keys for use by other programs, including
  several commercial SSH implementations.
  .It Fl F Ar hostname | [hostname]:port
  Search for the specified
  .Ar hostname
  (with optional port number)
  in a
  .Pa 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
  .Fl H
  option to print found keys in a hashed format.
  .It Fl f Ar filename
  Specifies the filename of the key file.
  .It Fl g
  Use generic DNS format when printing fingerprint resource records using the
  .Fl r
  command.
  .It Fl H
  Hash a
  .Pa 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
  .Nm ssh
  and
  .Nm 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.
  .It Fl h
  When signing a key, create a host certificate instead of a user
  certificate.
  See the
  .Sx CERTIFICATES
  section for details.
  .It Fl I Ar certificate_identity
  Specify the key identity when signing a public key.
  See the
  .Sx CERTIFICATES
  section for details.
  .It Fl i
  This option will read an unencrypted private (or public) key file
  in the format specified by the
  .Fl 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
  .Dq RFC4716 .
  .It Fl 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.
  See the
  .Sx FIDO AUTHENTICATOR
  section for more information.
  .It Fl k
  Generate a KRL file.
  In this mode,
  .Nm
  will generate a KRL file at the location specified via the
  .Fl 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
  .Sx KEY REVOCATION LISTS
  section.
  .It Fl L
  Prints the contents of one or more certificates.
  .It Fl l
  Show fingerprint of specified public key file.
  .Nm
  will try to find the matching public key file and prints its fingerprint.
  If combined with
  .Fl v ,
  a visual ASCII art representation of the key is supplied with the
  fingerprint.
  .It Fl M Cm generate
  Generate candidate Diffie-Hellman Group Exchange (DH-GEX) parameters for
  eventual use by the
  .Sq diffie-hellman-group-exchange-*
  key exchange methods.
  The numbers generated by this operation must be further screened before
  use.
  See the
  .Sx MODULI GENERATION
  section for more information.
  .It Fl M Cm 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
  .Pa /etc/moduli
  file.
  See the
  .Sx MODULI GENERATION
  section for more information.
  .It Fl m Ar key_format
  Specify a key format for key generation, the
  .Fl i
  (import),
  .Fl e
  (export) conversion options, and the
  .Fl 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:
  .Dq RFC4716
  (RFC 4716/SSH2 public or private key),
  .Dq PKCS8
  (PKCS8 public or private key)
  or
  .Dq 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
  .Dq RFC4716 .
  Setting a format of
  .Dq PEM
  when generating or updating a supported private key type will cause the
  key to be stored in the legacy PEM private key format.
  .It Fl N Ar new_passphrase
  Provides the new passphrase.
  .It Fl n Ar 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.
  See the
  .Sx CERTIFICATES
  section for details.
  .It Fl O Ar option
  Specify a key/value option.
  These are specific to the operation that
  .Nm
  has been requested to perform.
  .Pp
  When signing certificates, one of the options listed in the
  .Sx CERTIFICATES
  section may be specified here.
  .Pp
  When performing moduli generation or screening, one of the options
  listed in the
  .Sx MODULI GENERATION
  section may be specified.
  .Pp
  When generating FIDO authenticator-backed keys, the options listed in the
  .Sx FIDO AUTHENTICATOR
  section may be specified.
  .Pp
  When performing signature-related options using the
  .Fl Y
  flag, the following options are accepted:
  .Bl -tag -width Ds
  .It Cm hashalg Ns = Ns Ar algorithm
  Selects the hash algorithm to use for hashing the message to be signed.
  Valid algorithms are
  .Dq sha256
  and
  .Dq sha512.
  The default is
  .Dq sha512.
  .It Cm print-pubkey
  Print the full public key to standard output after signature verification.
  .It Cm verify-time Ns = Ns Ar timestamp
  Specifies a time to use when validating signatures instead of the current
  time.
  The time may be specified as a date or time in the YYYYMMDD[Z] or
  in YYYYMMDDHHMM[SS][Z] formats.
  Dates and times will be interpreted in the current system time zone unless
  suffixed with a Z character, which causes them to be interpreted in the
  UTC time zone.
  .El
  .Pp
  When generating SSHFP DNS records from public keys using the
  .Fl r
  flag, the following options are accepted:
  .Bl -tag -width Ds
  .It Cm hashalg Ns = Ns Ar algorithm
  Selects a hash algorithm to use when printing SSHFP records using the
  .Fl D
  flag.
  Valid algorithms are
  .Dq sha1
  and
  .Dq sha256 .
  The default is to print both.
  .El
  .Pp
  The
  .Fl O
  option may be specified multiple times.
  .It Fl P Ar passphrase
  Provides the (old) passphrase.
  .It Fl 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.
  .It Fl Q
  Test whether keys have been revoked in a KRL.
  If the
  .Fl l
  option is also specified then the contents of the KRL will be printed.
  .It Fl q
  Silence
  .Nm ssh-keygen .
  .It Fl R Ar hostname | [hostname]:port
  Removes all keys belonging to the specified
  .Ar hostname
  (with optional port number)
  from a
  .Pa known_hosts
  file.
  This option is useful to delete hashed hosts (see the
  .Fl H
  option above).
  .It Fl r Ar hostname
  Print the SSHFP fingerprint resource record named
  .Ar hostname
  for the specified public key file.
  .It Fl s Ar ca_key
  Certify (sign) a public key using the specified CA key.
  See the
  .Sx CERTIFICATES
  section for details.
  .Pp
  When generating a KRL,
  .Fl s
  specifies a path to a CA public key file used to revoke certificates directly
  by key ID or serial number.
  See the
  .Sx KEY REVOCATION LISTS
  section for details.
  .It Fl t Cm ecdsa | ecdsa-sk | ed25519 | ed25519-sk | rsa
  Specifies the type of key to create.
  The possible values are
  .Dq ecdsa ,
  .Dq ecdsa-sk ,
  .Dq ed25519 (the default),
  .Dq ed25519-sk ,
  or
  .Dq rsa .
  .Pp
  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
  .Dq ssh-rsa
  (SHA1 signatures, not recommended),
  .Dq rsa-sha2-256 ,
  and
  .Dq rsa-sha2-512
  (the default for RSA keys).
  .It Fl U
  When used in combination with
  .Fl s
  or
  .Fl Y Cm sign ,
  this option indicates that a CA key resides in an
  .Xr ssh-agent 1 .
  See the
  .Sx CERTIFICATES
  section for more information.
  .It Fl u
  Update a KRL.
  When specified with
  .Fl k ,
  keys listed via the command line are added to the existing KRL rather than
  a new KRL being created.
  .It Fl V Ar 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.
  .Pp
  The start time may be specified as:
  .Bl -bullet -compact
  .It
  The string
  .Dq always
  to indicate the certificate has no specified start time.
  .It
  A date or time in the system time zone formatted as YYYYMMDD or
  YYYYMMDDHHMM[SS].
  .It
  A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z.
  .It
  A relative time before the current system time consisting of a minus sign
  followed by an interval in the format described in the
  TIME FORMATS section of
  .Xr sshd_config 5 .
  .It
  A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a hexadecimal
  number beginning with
  .Dq 0x .
  .El
  .Pp
  The end time may be specified similarly to the start time:
  .Bl -bullet -compact
  .It
  The string
  .Dq forever
  to indicate the certificate has no specified end time.
  .It
  A date or time in the system time zone formatted as YYYYMMDD or
  YYYYMMDDHHMM[SS].
  .It
  A date or time in the UTC time zone as YYYYMMDDZ or YYYYMMDDHHMM[SS]Z.
  .It
  A relative time after the current system time consisting of a plus sign
  followed by an interval in the format described in the
  TIME FORMATS section of
  .Xr sshd_config 5 .
  .It
  A raw seconds since epoch (Jan 1 1970 00:00:00 UTC) as a hexadecimal
  number beginning with
  .Dq 0x .
  .El
  .Pp
  For example:
  .Bl -tag -width Ds
  .It +52w1d
  Valid from now to 52 weeks and one day from now.
  .It -4w:+4w
  Valid from four weeks ago to four weeks from now.
  .It 20100101123000:20110101123000
  Valid from 12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011.
  .It 20100101123000Z:20110101123000Z
  Similar, but interpreted in the UTC time zone rather than the system time zone.
  .It -1d:20110101
  Valid from yesterday to midnight, January 1st, 2011.
  .It 0x1:0x2000000000
  Valid from roughly early 1970 to May 2033.
  .It -1m:forever
  Valid from one minute ago and never expiring.
  .El
  .It Fl v
  Verbose mode.
  Causes
  .Nm
  to print debugging messages about its progress.
  This is helpful for debugging moduli generation.
  Multiple
  .Fl v
  options increase the verbosity.
  The maximum is 3.
  .It Fl w Ar 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.
  .It Fl Y Cm find-principals
  Find the principal(s) associated with the public key of a signature,
  provided using the
  .Fl s
  flag in an authorized signers file provided using the
  .Fl f
  flag.
  The format of the allowed signers file is documented in the
  .Sx ALLOWED SIGNERS
  section below.
  If one or more matching principals are found, they are returned on
  standard output.
  .It Fl Y Cm match-principals
  Find principal matching the principal name provided using the
  .Fl I
  flag in the authorized signers file specified using the
  .Fl f
  flag.
  If one or more matching principals are found, they are returned on
  standard output.
  .It Fl Y Cm check-novalidate
  Checks that a signature generated using
  .Nm
  .Fl Y Cm sign
  has a valid structure.
  This does not validate if a signature comes from an authorized signer.
  When testing a signature,
  .Nm
  accepts a message on standard input and a signature namespace using
  .Fl n .
  A file containing the corresponding signature must also be supplied using the
  .Fl s
  flag.
  Successful testing of the signature is signalled by
  .Nm
  returning a zero exit status.
  .It Fl Y Cm sign
  Cryptographically sign a file or some data using an SSH key.
  When signing,
  .Nm
  accepts zero or more files to sign on the command-line - if no files
  are specified then
  .Nm
  will sign data presented on standard input.
  Signatures are written to the path of the input file with
  .Dq .sig
  appended, or to standard output if the message to be signed was read from
  standard input.
  .Pp
  The key used for signing is specified using the
  .Fl f
  option and may refer to either a private key, or a public key with the private
  half available via
  .Xr 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
  .Fl n
  flag.
  Namespaces are arbitrary strings, and may include:
  .Dq file
  for file signing,
  .Dq email
  for email signing.
  For custom uses, it is recommended to use names following a
  NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces.
  .It Fl Y Cm verify
  Request to verify a signature generated using
  .Nm
  .Fl Y Cm sign
  as described above.
  When verifying a signature,
  .Nm
  accepts a message on standard input and a signature namespace using
  .Fl n .
  A file containing the corresponding signature must also be supplied using the
  .Fl s
  flag, along with the identity of the signer using
  .Fl I
  and a list of allowed signers via the
  .Fl f
  flag.
  The format of the allowed signers file is documented in the
  .Sx ALLOWED SIGNERS
  section below.
  A file containing revoked keys can be passed using the
  .Fl 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
  .Nm
  returning a zero exit status.
  .It Fl y
  This option will read a private
  OpenSSH format file and print an OpenSSH public key to stdout.
  .It Fl Z Ar 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
  .Qq ssh -Q cipher .
  The default is
  .Dq aes256-ctr .
  .It Fl z Ar serial_number
  Specifies a serial number to be embedded in the certificate to distinguish
  this certificate from others from the same CA.
  If the
  .Ar serial_number
  is prefixed with a
  .Sq +
  character, then the serial number will be incremented for each certificate
  signed on a single command-line.
  The default serial number is zero.
  .Pp
  When generating a KRL, the
  .Fl z
  flag is used to specify a KRL version number.
  .El
  .Sh MODULI GENERATION
  .Nm
  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).
  .Pp
  Generation of primes is performed using the
  .Fl M Cm generate
  option.
  The desired length of the primes may be specified by the
  .Fl O Cm bits
  option.
  For example:
  .Pp
  .Dl # ssh-keygen -M generate -O bits=2048 moduli-2048.candidates
  .Pp
  By default, the search for primes begins at a random point in the
  desired length range.
  This may be overridden using the
  .Fl O Cm start
  option, which specifies a different start point (in hex).
  .Pp
  Once a set of candidates have been generated, they must be screened for
  suitability.
  This may be performed using the
  .Fl M Cm screen
  option.
  In this mode
  .Nm
  will read candidates from standard input (or a file specified using the
  .Fl f
  option).
  For example:
  .Pp
  .Dl # ssh-keygen -M screen -f moduli-2048.candidates moduli-2048
  .Pp
  By default, each candidate will be subjected to 100 primality tests.
  This may be overridden using the
  .Fl O Cm 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
  .Fl O Cm generator
  option.
  Valid generator values are 2, 3, and 5.
  .Pp
  Screened DH groups may be installed in
  .Pa /etc/moduli .
  It is important that this file contains moduli of a range of bit lengths.
  .Pp
  A number of options are available for moduli generation and screening via the
  .Fl O
  flag:
  .Bl -tag -width Ds
  .It Ic lines Ns = Ns Ar number
  Exit after screening the specified number of lines while performing DH
  candidate screening.
  .It Ic start-line Ns = Ns Ar line-number
  Start screening at the specified line number while performing DH candidate
  screening.
  .It Ic checkpoint Ns = Ns Ar 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.
  .It Ic start Ns = Ns Ar hex-value
  Specify start point (in hex) when generating candidate moduli for DH-GEX.
  .It Ic generator Ns = Ns Ar value
  Specify desired generator (in decimal) when testing candidate moduli for DH-GEX.
  .El
  .Sh CERTIFICATES
  .Nm
  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
  .Xr ssl 8 .
  .Pp
  .Nm
  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:
  .Pp
  .Dl $ ssh-keygen -s /path/to/ca_key -I id -n user \e
  .Dl \ \ \ \ \ \ /path/to/user_key.pub
  .Pp
  The resultant certificate will be placed in
  .Pa /path/to/user_key-cert.pub .
  The argument to
  .Fl I
  is a key identifier that will be used in logs and may be used to revoke
  keys.
  The argument to
  .Fl n
  is one or more (comma-separated) principals, typically usernames, that
  the certificate represents.
  A host certificate requires the
  .Fl h
  option:
  .Pp
  .Dl $ ssh-keygen -s /path/to/ca_key -I id -h -n foo.example.org \e
  .Dl \ \ \ \ \ \ /path/to/host_key.pub
  .Pp
  For host certificates, the principals specified using the
  .Fl n
  argument are hostnames and may contain wildcard characters.
  .Pp
  The host certificate will be output to
  .Pa /path/to/host_key-cert.pub .
  .Pp
  It is possible to sign using a CA key stored in a PKCS#11 token by
  providing the token library using
  .Fl D
  and identifying the CA key by providing its public half as an argument
  to
  .Fl s :
  .Pp
  .Dl $ ssh-keygen -s ca_key.pub -D libpkcs11.so -I id -n user \e
  .Dl \ \ \ \ \ \ user_key.pub
  .Pp
  Similarly, it is possible for the CA key to be hosted in an
  .Xr ssh-agent 1 .
  This is indicated by the
  .Fl U
  flag and, again, the CA key must be identified by its public half.
  .Pp
  .Dl $ ssh-keygen -Us ca_key.pub -I id -n user user_key.pub
  .Pp
  In all cases,
  .Ar key_id
  is a "key identifier" that is logged by the server when the certificate
  is used for authentication.
  .Pp
  Certificates are limited to be valid for a set of principal (user/host)
  names.
  To generate a certificate for a specified set of principals:
  .Pp
  .Dl $ ssh-keygen -s ca_key -I id -n user1,user2 user_key.pub
  .Dl $ ssh-keygen -s ca_key -I id -h -n host.domain host_key.pub
  .Pp
  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.
  .Pp
  The options that are valid for user certificates are:
  .Pp
  .Bl -tag -width Ds -compact
  .It Ic clear
  Clear all enabled permissions.
  This is useful for clearing the default set of permissions so permissions may
  be added individually.
  .Pp
  .It Ic critical : Ns Ar name Ns Op Ns = Ns Ar contents
  .It Ic extension : Ns Ar name Ns Op Ns = Ns Ar contents
  Includes an arbitrary certificate critical option or extension.
  The specified
  .Ar name
  should include a domain suffix, e.g.\&
  .Dq name@example.com .
  If
  .Ar 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.
  .Pp
  .It Ic force-command Ns = Ns Ar command
  Forces the execution of
  .Ar command
  instead of any shell or command specified by the user when
  the certificate is used for authentication.
  .Pp
  .It Ic no-agent-forwarding
  Disable
  .Xr ssh-agent 1
  forwarding (permitted by default).
  .Pp
  .It Ic no-port-forwarding
  Disable port forwarding (permitted by default).
  .Pp
  .It Ic no-pty
  Disable PTY allocation (permitted by default).
  .Pp
  .It Ic no-user-rc
  Disable execution of
  .Pa ~/.ssh/rc
  by
  .Xr sshd 8
  (permitted by default).
  .Pp
  .It Ic no-x11-forwarding
  Disable X11 forwarding (permitted by default).
  .Pp
  .It Ic permit-agent-forwarding
  Allows
  .Xr ssh-agent 1
  forwarding.
  .Pp
  .It Ic permit-port-forwarding
  Allows port forwarding.
  .Pp
  .It Ic permit-pty
  Allows PTY allocation.
  .Pp
  .It Ic permit-user-rc
  Allows execution of
  .Pa ~/.ssh/rc
  by
  .Xr sshd 8 .
  .Pp
  .It Ic permit-X11-forwarding
  Allows X11 forwarding.
  .Pp
  .It Ic 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
  .Cm ecdsa-sk
  and
  .Cm ed25519-sk .
  .Pp
  .It Ic source-address Ns = Ns Ar address_list
  Restrict the source addresses from which the certificate is considered valid.
  The
  .Ar address_list
  is a comma-separated list of one or more address/netmask pairs in CIDR
  format.
  .Pp
  .It Ic verify-required
  Require signatures made using this key indicate that the user was first
  verified, e.g. by PIN or on-token biometrics.
  This option only makes sense for the FIDO authenticator algorithms
  .Cm ecdsa-sk
  and
  .Cm ed25519-sk .
  .El
  .Pp
  At present, no standard options are valid for host keys.
  .Pp
  Finally, certificates may be defined with a validity lifetime.
  The
  .Fl 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
  .Ux
  Epoch to the distant future.
  .Pp
  For certificates to be used for user or host authentication, the CA
  public key must be trusted by
  .Xr sshd 8
  or
  .Xr ssh 1 .
  Refer to those manual pages for details.
  .Sh FIDO AUTHENTICATOR
  .Nm
  is able to generate FIDO authenticator-backed keys, after which
  they may be used much like any other key type supported by OpenSSH, so
  long as the hardware authenticator is attached when the keys are used.
  FIDO authenticators generally require the user to explicitly authorise
  operations by touching or tapping them.
  FIDO keys consist of two parts: a key handle part stored in the
  private key file on disk, and a per-device private key that is unique
  to each FIDO authenticator and that cannot be exported from the
  authenticator hardware.
  These are combined by the hardware at authentication time to derive
  the real key that is used to sign authentication challenges.
  Supported key types are
  .Cm ecdsa-sk
  and
  .Cm ed25519-sk .
  .Pp
  The options that are valid for FIDO keys are:
  .Bl -tag -width Ds
  .It Cm application
  Override the default FIDO application/origin string of
  .Dq ssh: .
  This may be useful when generating host or domain-specific resident keys.
  The specified application string must begin with
  .Dq ssh: .
  .It Cm challenge Ns = Ns Ar path
  Specifies a path to a challenge string that will be passed to the
  FIDO authenticator 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).
  .It Cm device
  Explicitly specify a
  .Xr fido 4
  device to use, rather than letting the authenticator middleware select one.
  .It Cm no-touch-required
  Indicate that the generated private key should not require touch
  events (user presence) when making signatures.
  Note that
  .Xr sshd 8
  will refuse such signatures by default, unless overridden via
  an authorized_keys option.
  .It Cm resident
  Indicate that the key handle should be stored on the FIDO
  authenticator itself.
  This makes it easier to use the authenticator on multiple computers.
  Resident keys may be supported on FIDO2 authenticators and typically
  require that a PIN be set on the authenticator prior to generation.
  Resident keys may be loaded off the authenticator using
  .Xr ssh-add 1 .
  Storing both parts of a key on a FIDO authenticator increases the likelihood
  of an attacker being able to use a stolen authenticator device.
  .It Cm 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.
  .It Cm verify-required
  Indicate that this private key should require user verification for
  each signature.
  Not all FIDO authenticators support this option.
  Currently PIN authentication is the only supported verification method,
  but other methods may be supported in the future.
  .It Cm write-attestation Ns = Ns Ar path
  May be used at key generation time to record the attestation data
  returned from FIDO authenticators during key generation.
  This information is potentially sensitive.
  By default, this information is discarded.
  .El
  .Sh KEY REVOCATION LISTS
  .Nm
  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.
  .Pp
  KRLs may be generated using the
  .Fl 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).
  .Pp
  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.
  .Bl -tag -width Ds
  .It Cm serial : Ar serial_number Ns Op - Ns Ar 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
  .Nm
  command line using the
  .Fl s
  option.
  .It Cm id : Ar key_id
  Revokes a certificate with the specified key ID string.
  The CA key must have been specified on the
  .Nm
  command line using the
  .Fl s
  option.
  .It Cm key : Ar public_key
  Revokes the specified key.
  If a certificate is listed, then it is revoked as a plain public key.
  .It Cm sha1 : Ar public_key
  Revokes the specified key by including its SHA1 hash in the KRL.
  .It Cm sha256 : Ar 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.
  .It Cm hash : Ar fingerprint
  Revokes a key using a fingerprint hash, as obtained from an
  .Xr sshd 8
  authentication log message or the
  .Nm
  .Fl l
  flag.
  Only SHA256 fingerprints are supported here and resultant KRLs are
  not supported by OpenSSH versions prior to 7.9.
  .El
  .Pp
  KRLs may be updated using the
  .Fl u
  flag in addition to
  .Fl k .
  When this option is specified, keys listed via the command line are merged into
  the KRL, adding to those already there.
  .Pp
  It is also possible, given a KRL, to test whether it revokes a particular key
  (or keys).
  The
  .Fl 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
  .Nm
  will exit with a non-zero exit status.
  A zero exit status will only be returned if no key was revoked.
  .Sh ALLOWED SIGNERS
  When verifying signatures,
  .Nm
  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
  .Xr 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
  .Ql #
  are ignored as comments.
  .Pp
  The principals field is a pattern-list (see PATTERNS in
  .Xr 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
  .Fl I
  option must match a principals pattern in order for the corresponding key to be
  considered acceptable for verification.
  .Pp
  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):
  .Bl -tag -width Ds
  .It Cm 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.
  .It Cm namespaces Ns = Ns "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.
  .It Cm valid-after Ns = Ns "timestamp"
  Indicates that the key is valid for use at or after the specified timestamp,
  which may be a date or time in the YYYYMMDD[Z] or YYYYMMDDHHMM[SS][Z] formats.
  Dates and times will be interpreted in the current system time zone unless
  suffixed with a Z character, which causes them to be interpreted in the UTC
  time zone.
  .It Cm valid-before Ns = Ns "timestamp"
  Indicates that the key is valid for use at or before the specified timestamp.
  .El
  .Pp
  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.
  .Pp
  An example allowed signers file:
  .Bd -literal -offset 3n
  # 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...
  .Ed
  .Sh ENVIRONMENT
  .Bl -tag -width Ds
  .It Ev 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.
  .El
  .Sh FILES
  .Bl -tag -width Ds -compact
  .It Pa ~/.ssh/id_ecdsa
  .It Pa ~/.ssh/id_ecdsa_sk
  .It Pa ~/.ssh/id_ed25519
  .It Pa ~/.ssh/id_ed25519_sk
  .It Pa ~/.ssh/id_rsa
  Contains the 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
  .Nm
  but it is offered as the default file for the private key.
  .Xr ssh 1
  will read this file when a login attempt is made.
  .Pp
  .It Pa ~/.ssh/id_ecdsa.pub
  .It Pa ~/.ssh/id_ecdsa_sk.pub
  .It Pa ~/.ssh/id_ed25519.pub
  .It Pa ~/.ssh/id_ed25519_sk.pub
  .It Pa ~/.ssh/id_rsa.pub
  Contains the ECDSA, authenticator-hosted ECDSA, Ed25519,
  authenticator-hosted Ed25519 or RSA public key for authentication.
  The contents of this file should be added to
  .Pa ~/.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.
  .Pp
  .It Pa /etc/moduli
  Contains Diffie-Hellman groups used for DH-GEX.
  The file format is described in
  .Xr moduli 5 .
  .El
  .Sh SEE ALSO
  .Xr ssh 1 ,
  .Xr ssh-add 1 ,
  .Xr ssh-agent 1 ,
  .Xr moduli 5 ,
  .Xr sshd 8
  .Rs
  .%R RFC 4716
  .%T "The Secure Shell (SSH) Public Key File Format"
  .%D 2006
  .Re
  .Sh 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.