Has keygen

Use the ssh-keygen command to generate a public/private authentication key pair. Authentication keys allow a user to connect to a remote system without supplying a password. Keys must be generated for each user separately. If you generate key pairs as the root user, only the root can use the keys.

The following example creates the public and private parts of an RSA key:

Use the –t option to specify the type of key to create. Possible values are “rsa1” for protocol version 1, and “dsa“, “ecdsa“, or “rsa” for protocol version 2.

You have the option of specifying a passphrase to encrypt the private part of the key. If you encrypt your personal key, you must supply the passphrase each time you use the key. This prevents an attacker, who has access to your private key and can impersonate you and access all the computers you have access to, from being able to do so. The attacker still needs to supply the passphrase.

The ssh-key command in the example generated two keys in the ~/.ssh directory:

To log on to, or copy files to, a remote system without supplying a password, copy the public key (~/.ssh/id_rsa.pub in this example) to ~/.ssh/authorized_keys on the remote system. Set the remote ~/.ssh directory permissions to 700. You can then use the ssh or scp tools to access the remote system without supplying a password.

To allow multiple connections, append the public key to the authorized_keys file on the remote system instead of copying it. The following example appends the public key:

You can improve system security even further by disabling the standard password authentication, and enforcing the key-based authentication. To do so, set the PasswordAuthentication option to no in the /etc/ssh/sshd_config configuration file as follows:

This disallows users whose keys are not in the authorized_keys file of the specific user on the server to connect via ssh. The connection is denied and the following message appears:

Setting the PasswordAuthentication option to yes, which is the default, permits a user to use a password for authentication.

Name

ssh-keygen - authentication key generation, management and conversion

Synopsis

ssh-keygen [-q] [-bbits] -ttype [-Nnew_passphrase] [-Ccomment] [-foutput_keyfile]

ssh-keygen -p [-Pold_passphrase] [-Nnew_passphrase] [-fkeyfile]

ssh-keygen -i [-finput_keyfile]

ssh-keygen -e [-finput_keyfile]

ssh-keygen -y [-finput_keyfile]

ssh-keygen -c [-Ppassphrase] [-Ccomment] [-fkeyfile]

ssh-keygen -l [-finput_keyfile]

ssh-keygen -B [-finput_keyfile]

ssh-keygen -Dreader

ssh-keygen -Fhostname [-fknown_hosts_file] [-l]

ssh-keygen -H [-fknown_hosts_file]

ssh-keygen -Rhostname [-fknown_hosts_file]

ssh-keygen -Ureader [-finput_keyfile]

ssh-keygen -rhostname [-finput_keyfile] [-g]

ssh-keygen -Goutput_file [-v] [-bbits] [-Mmemory] [-Sstart_point]

ssh-keygen -Toutput_file-finput_file [-v] [-anum_trials] [-Wgenerator]

ssh-keygen [-n] [-Dsmartcard]

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 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 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 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:

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.

-bbits
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.

-Ccomment
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.

-Dreader
Download the RSA public key stored in the smartcard in reader.

-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 exporting keys for use by several commercial SSH implementations.

-Fhostname
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.

-ffilename
Specifies the filename of the key file.

-Goutput_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.

-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 Format. This option allows importing keys from several commercial SSH implementations.

-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.

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

-n' Extract the public key from smartcard.

-Nnew_passphrase
Provides the new passphrase.

-Ppassphrase
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.

-Rhostname
Removes all keys belonging to hostname from a known_hosts file. This option is useful to delete hashed hosts (see the -H option above).

-rhostname
Print the SSHFP fingerprint resource record named hostname for the specified public key file.

-Sstart
Specify start point (in hex) when generating candidate moduli for DH-GEX.

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

-ttype
Specifies the type of key to create. The possible values are ''rsa1'' for protocol version 1 and ''rsa'' or ''dsa'' for protocol version 2.

-Ureader
Upload an existing RSA private key into the smartcard in reader.

-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.

-Wgenerator
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.

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 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 moduli.

Files

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/ssh/moduli
Contains Diffie-Hellman groups used for DH-GEX. The file format is described in moduli(5).

Environment

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 6 bytes. This setting is not recommended on the computers without the hardware 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)

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.

BSD April 14, 2013 BSD

Referenced By

In their most basic functions, key generators try to emulate the key sequence required by a program for installation. This allows bootlegged copies of software to be distributed, often for free, and then installed by anyone without the need to pay for an actual license for a product which should include the product key. The creation of key generators and cracking software is rampant because of the retail cost of legitimate software. For example, productivity tools like Microsoft Office or Adobe Suite cost hundreds to thousands of dollars for a single copy, which is too expensive for many people around the world. This results in the massive distribution of key generators and cracked copies of software.

Most modern software has other means of validation than a product key to ensure that the software is legally licensed and not pirated. A key generator may allow the user to install the software but validation over the Internet would then stop the software from working. However, hackers and crackers use more than the key generator in order to illegally use software. Some keygens are equipped with spoofing servers that intercept the communication between the software and the actual servers, providing it with the validation reply it is expecting from the real servers, thereby tricking the software into thinking that it has been validated.