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

NAME
     ssh-keygen - authentication key generation, management and conversion

SYNOPSIS
     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 [-f input_keyfile]
     ssh-keygen -e [-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 [-n] [-D smartcard]
     ssh-keygen -s ca_key -I certificate_identity [-h] [-Z principals] [-O option]
                [-V validity_interval] [-z serial_number] file ...
     ssh-keygen -L [-f input_keyfile]

DESCRIPTION
     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 spec-
     ified with the -t option.  If invoked without any arguments, ssh-keygen will generate
     an RSA key for use in SSH protocol 2 connections.

     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, 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 sen-
     tences 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 lower-
     case 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 forgot-
     ten, a new key must be generated and copied to the corresponding public key to other
     machines.

     For RSA1 keys, there is also a comment field in the key file that is only for conve-
     nience 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 activated.

     The options are as follows:

     -a trials
             Specifies the number of primality tests to perform when screening DH-GEX can-
             didates 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 con-
             sidered sufficient.  DSA keys must be exactly 1024 bits as specified by FIPS
             186-2.

     -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 stored in the pkcs11 provider.

     -e      This option will read a private or public OpenSSH key file and print the key
             in RFC 4716 SSH Public Key File Format to stdout.  This option allows export-
             ing keys for use by several commercial SSH implementations.

     -F hostname
             Search for the specified hostname in a known_hosts file, listing any occur-
             rences 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 command.

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

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

     -I      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
             SSH2-compatible format and print an OpenSSH compatible private (or public)
             key to stdout.  ssh-keygen also reads the RFC 4716 SSH Public Key File For-
             mat.  This option allows importing keys from several commercial SSH implemen-
             tations.

     -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 rep-
             resentation 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.

     -n      Extract the public key from smartcard.

     -N new_passphrase
             Provides the new passphrase.

     -Z principals
             Specify one or more principals (user or host names) to be included in a cer-
             tificate 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 speci-
             fied multiple times.  Please see the CERTIFICATES section for details.  The
             options that are valid for user certificates are:

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

             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)

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

             permit-x11-forwarding
                     Allows X11 forwarding.

             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).

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

             source-address=address_list
                     Restrict the source addresses from which the certificate is consid-
                     ered valid from.  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.  Used by /etc/rc when creating a new key.

     -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 pro-
             tocol 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 begin-
             ning 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 speci-
             fied as a date in YYYYMMDD format, a time in YYYYMMDDHHMMSS format or a rela-
             tive time (to the current time) consisting of a minus sign followed by a rel-
             ative time in the format described in the TIME FORMATS section of
             ssh_config(5).  The end time may be specified as a YYYYMMDD date, a YYYYMMD-
             DHHMMSS time or a relative time starting with a plus character.

             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 mid-
             night, 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 pub-
             lic 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.

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

     Generation of primes is performed using the -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 hex).

     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 candi-
     dates 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 moduli.

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

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

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

     The resultant certificate will be placed in /path/to/user_key-cert.pub.  A host cer-
     tificate 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.  In both 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 cer-
     tificate for a specified set of principals:

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

FILES
     ~/.ssh/identity
             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 automati-
             cally 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/identity.pub
             Contains the protocol version 1 RSA public key for authentication.  The con-
             tents 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.

     ~/.ssh/id_dsa
     ~/.ssh/id_ecdsa
     ~/.ssh/id_rsa
             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 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.

     ~/.ssh/id_dsa.pub
     ~/.ssh/id_ecdsa.pub
     ~/.ssh/id_rsa.pub
             Contains the protocol version 2 DSA, ECDSA 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/ssh/moduli
             Contains Diffie-Hellman groups used for DH-GEX.  The file format is described
             in moduli(5).

ENVIRONMENT
     SSH_USE_STRONG_RNG
             The reseeding of the OpenSSL random generator is usually done from
             /dev/urandom.  If the SSH_USE_STRONG_RNG environment variable is set to value
             other than 0 the OpenSSL random generator is reseeded from /dev/random.  The
             number of bytes read is defined by the SSH_USE_STRONG_RNG value.  Minimum is
             14 bytes.  This setting is not recommended on the computers without the hard-
             ware random generator because insufficient entropy causes the connection to
             be blocked until enough entropy is available.

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

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

AUTHORS
     OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen.
     Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song
     removed many bugs, re-added newer features and created OpenSSH.  Markus Friedl con-
     tributed the support for SSH protocol versions 1.5 and 2.0.

BSD                            December 14, 2017                           BSD

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