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 [-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 reader
ssh-keygen -F hostname [-f known_hosts_file]
ssh-keygen -H [-f known_hosts_file]
ssh-keygen -R hostname [-f known_hosts_file]
ssh-keygen -U reader [-f input_keyfile]
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 generates, manages and converts authentication keys for ssh(1) . ssh-keygen can create RSA keys for use by SSH protocol version 1 and RSA or DSA 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 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 RSA or DSA authentication runs this once to create the authentication key in ~/.ssh/identity, ~/.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 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 nonalphanumeric 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 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 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 activated.
The options are as follows:
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 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/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.
~/.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 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/identity.pub
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.
~/.ssh/id_dsa
Contains the protocol version 2 DSA 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
Contains the protocol version 2 DSA 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.
~/.ssh/id_rsa
Contains the protocol version 2 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_rsa.pub
Contains the protocol version 2 RSA public key for authentication.
The contents of this file should be added to
~/.ssh/authorized_keys on all machines where the user wishes to
log in using public key authentication. There is no need to keep
the contents of this file secret.
/etc/moduli
Contains Diffie-Hellman groups used for DH-GEX. The file format
is described in moduli(5)
.
ssh(1) , ssh-add(1) , ssh-agent(1) , moduli(5) , sshd(8)
J. Galbraith and R. Thayer, SECSH Public Key File Format, draft-ietfsecsh-publickeyfile-01.txt, March 2001, work in progress material.
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.