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 |
-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 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.
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).
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.