ssh-keygen is also used to generate groups for use in Diffie-Hellman group exchange (DH-GEX). See the Sx 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:

-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.

-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 reader

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 a `SECSH Public Key File Format' to stdout. This option allows exporting keys for use by 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 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 `SECSH 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.

-M memory

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

-N new_passphrase

Provides the new passphrase.

-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.

-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 ``rsa'' or ``dsa'' for protocol version 2.

-U reader

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.

-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.

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).

J. Galbraith R. Thayer "SECSH Public Key File Format" draft-ietf-secsh-publickeyfile-01.txt March 2001 work in progress material