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     ssh-keygen — authentication key generation, management and conversion


     ssh-keygen [-q] [-b bits] -t type [-N new_passphrase] [-C comment] [-f output_keyfile]
     ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
     ssh-keygen -i [-m key_format] [-f input_keyfile]
     ssh-keygen -e [-m key_format] [-f input_keyfile]
     ssh-keygen -y [-f input_keyfile]
     ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile]
     ssh-keygen -l [-f input_keyfile]
     ssh-keygen -B [-f input_keyfile]
     ssh-keygen -D pkcs11
     ssh-keygen -F hostname [-f known_hosts_file] [-l]
     ssh-keygen -H [-f known_hosts_file]
     ssh-keygen -R hostname [-f known_hosts_file]
     ssh-keygen -r hostname [-f input_keyfile] [-g]
     ssh-keygen -G output_file [-v] [-b bits] [-M memory] [-S start_point]
     ssh-keygen -T output_file -f input_file [-v] [-a num_trials] [-W generator]
     ssh-keygen -s ca_key -I certificate_identity [-h] [-n principals] [-O option]
                [-V validity_interval] [-z serial_number] file ...
     ssh-keygen -L [-f input_keyfile]
     ssh-keygen -A


     ssh-keygen generates, manages and converts authentication keys for ssh(1).  ssh-keygen can
     create RSA keys for use by SSH protocol version 1 and DSA, ECDSA or RSA 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 for use in SSH protocol 2

     ssh-keygen is also used to generate groups for use in Diffie-Hellman group exchange (DH-
     GEX).  See the MODULI GENERATION section for details.

     Normally each user wishing to use SSH with public key authentication runs this once to
     create the authentication key in ~/.ssh/identity, ~/.ssh/id_ecdsa, ~/.ssh/id_dsa or
     ~/.ssh/id_rsa.  Additionally, the system administrator may use this to generate host keys.

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

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

     For RSA1 keys, there is also a comment field in the key file that is only for convenience to
     the user to help identify the key.  The comment can tell what the key is for, or whatever is
     useful.  The comment is initialized to “user@host” when the key is created, but can be
     changed using the -c option.

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

     The options are as follows:

     -A      For each of the key types (rsa1, rsa, dsa and ecdsa) 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.  This is used by system
             administration scripts to generate new host keys.

     -a trials
             Specifies the number of primality tests to perform when screening DH-GEX candidates
             using the -T command.

     -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 768 bits and the default is 2048 bits.  Generally, 2048 bits is considered
             sufficient.  DSA keys must be exactly 1024 bits as specified by FIPS 186-2.  For
             ECDSA keys, the -b flag determines they 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.

     -C comment
             Provides a new comment.

     -c      Requests changing the comment in the private and public key files.  This operation
             is only supported for RSA1 keys.  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 RSA 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      This option will read a private or public OpenSSH key file and print to stdout the
             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
             Search for the specified hostname 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 output_file
             Generate candidate primes for DH-GEX.  These primes must be screened for safety
             (using the -T option) before use.

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

     -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”.

     -L      Prints the contents of a certificate.

     -l      Show fingerprint of specified public key file.  Private RSA1 keys are also
             supported.  For RSA and DSA keys ssh-keygen tries to find the matching public key
             file and prints its fingerprint.  If combined with -v, an ASCII art representation
             of the key is supplied with the fingerprint.

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

     -m key_format
             Specify a key format for the -i (import) or -e (export) conversion options.  The
             supported key formats are: “RFC4716” (RFC 4716/SSH2 public or private key), “PKCS8”
             (PEM PKCS8 public key) or “PEM” (PEM public key).  The default conversion format is

     -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 certificate option when signing a key.  This option may be specified
             multiple times.  Please see the CERTIFICATES section for details.  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.

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

                     Disable ssh-agent(1) forwarding (permitted by default).

                     Disable port forwarding (permitted by default).

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

                     Disable execution of ~/.ssh/rc by sshd(8) (permitted by default).

                     Disable X11 forwarding (permitted by default).

                     Allows ssh-agent(1) forwarding.

                     Allows port forwarding.

                     Allows PTY allocation.

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

                     Allows X11 forwarding.

                     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 options are valid for host keys.

     -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      Silence ssh-keygen.

     -R hostname
             Removes all keys belonging to hostname 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 start
             Specify start point (in hex) when generating candidate moduli for DH-GEX.

     -s ca_key
             Certify (sign) a public key using the specified CA key.  Please see the CERTIFICATES
             section for details.

     -T output_file
             Test DH group exchange candidate primes (generated using the -G option) for safety.

     -t type
             Specifies the type of key to create.  The possible values are “rsa1” for protocol
             version 1 and “dsa”, “ecdsa” or “rsa” for protocol version 2.

     -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 a date in YYYYMMDD
             format, a time in YYYYMMDDHHMMSS format or a relative time (to the current time)
             consisting of a minus sign followed by a relative time in the format described in
             the TIME FORMATS section of sshd_config(5).  The end time may be specified as a
             YYYYMMDD date, a YYYYMMDDHHMMSS time or a relative time starting with a plus

             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).

     -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 generator
             Specify desired generator when testing candidate moduli for DH-GEX.

     -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.  The default serial number is zero.


     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 -G option.  The desired length of the primes may
     be specified by the -b option.  For example:

           # ssh-keygen -G moduli-2048.candidates -b 2048

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

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

           # ssh-keygen -T moduli-2048 -f moduli-2048.candidates

     By default, each candidate will be subjected to 100 primality tests.  This may be overridden
     using the -a 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 -W
     option.  Valid generator values are 2, 3, and 5.

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


     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/

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

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

     The host certificate will be output to /path/to/

     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

           $ ssh-keygen -s -D -I key_id

     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
           $ ssh-keygen -s ca_key -I key_id -h -n host.domain

     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.  For a list of valid certificate options, see the documentation for the -O
     option above.

     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 have a maximum
     validity interval.

     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.


             Contains the protocol version 1 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 3DES.  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.

             Contains the protocol version 1 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 RSA authentication.  There is no need to keep the contents of
             this file secret.

             Contains the protocol version 2 DSA, ECDSA 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.

             Contains the protocol version 2 DSA, ECDSA 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.

             Contains Diffie-Hellman groups used for DH-GEX.  The file format is described in


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

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


     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.