Home | About Chris Hope | Contact Chris Hope | Search:

Manpage for ssh-keygen

Posted March 5th, 2004 in Linux/Unix/BSD and Man Pages (Updated May 24th, 2005)

Private and public RSA and DSA keys can be generated on Unix based systems (such as Linux and FreeBSD) for use with SSH. The ssh-keygen command allows you to generate, manage and convert these authentication keys. You can read about generating key files in my Create RSA and DSA Keys for SSH article.

The following is the manpage for ssh-keygen:

SSH-KEYGEN(1) System 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 reader
ssh-keygen -U reader [-f input_keyfile]
ssh-keygen -r hostname [-f input_keyfile] [-g]
ssh-keygen -G output_file [-b bits] [-M memory] [-S start_point]
ssh-keygen -T output_file -f input_file [-a num_trials] [-W generator]

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.

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 $HOME/.ssh/identity,
$HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa. Additionally, the system admin
istrator 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 char
acters 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 initial�­
ized 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 bits
Specifies the number of bits in the key to create. Minimum is
512 bits. Generally, 1024 bits is considered sufficient. The
default is 1024 bits.

-c Requests changing the comment in the private and public key
files. This operation is only supported for RSA1 keys. The pro
gram will prompt for the file containing the private keys, for
the passphrase if the key has one, and for the new comment.

-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.

-g Use generic DNS resource record format.

-f filename
Specifies the filename of the key file.

-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.

-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.

-y This option will read a private OpenSSH format file and print an
OpenSSH public key to stdout.

-t type
Specifies the type of the key to create. The possible values are
``rsa1'' for protocol version 1 and ``rsa'' or ``dsa'' for proto
col version 2.

-B Show the bubblebabble digest of specified private or public key
file.

-C comment
Provides the new comment.

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

-G output_file
Generate candidate primes for DH-GEX. These primes must be
screened for safety (using the -T option) before use.

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

-N new_passphrase
Provides the new passphrase.

-P passphrase
Provides the (old) passphrase.

-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.

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

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

-r hostname
Print DNS resource record with the specified hostname.

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 pro�­
cess: first, candidate primes are generated using a fast, but memory
intensive process. These candidate primes are then tested for suitabil�­
ity (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 speci
fies 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.

FILES
$HOME/.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.

$HOME/.ssh/identity.pub
Contains the protocol version 1 RSA public key for authentica
tion. The contents of this file should be added to
$HOME/.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.

$HOME/.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.

$HOME/.ssh/id_dsa.pub
Contains the protocol version 2 DSA public key for authentica
tion. The contents of this file should be added to
$HOME/.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.

$HOME/.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.

$HOME/.ssh/id_rsa.pub
Contains the protocol version 2 RSA public key for authentica
tion. The contents of this file should be added to
$HOME/.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).

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

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

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 cre
ated OpenSSH. Markus Friedl contributed the support for SSH protocol
versions 1.5 and 2.0.

BSD September 25, 1999 BSD