ssh-keygen

- authentication key generation

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 | PKCS#11-URI]

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 | PKCS#11-URI]

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]

Description

The ssh-keygen utility 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 can also generate fingerprints or convert the public keys from the X.509v3 certificates specified as PKCS#11 URIs.

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. The system administrator can also use this to generate host keys..

Ordinarily, 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 with the ``.pub'' extension appended. The program also asks for a passphrase. The passphrase can be empty to indicate no passphrase (host keys must have empty passphrases), or it can be a string of arbitrary length. Good passphrases are 10-30 characters long, are not simple sentences or otherwise easy to guess, and contain a mix of uppercase and lowercase letters, numbers, and non-alphanumeric characters. (English prose has only 1-2 bits of entropy per word and provides very poor passphrases.) If a passphrase is set, it must be at least 4 characters long.

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, you have to generate a new key and copy the corresponding public key to other machines.

For RSA, 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 to place the keys to activate them.

Options

The following options are supported:

-b bits

Specifies the number of bits in the key to create. The minimum number is 512 bits. Generally, 2048 bits is considered sufficient. Key sizes above that no longer improve security but make things slower. The default is 2048 bits.

-B

Shows the bubblebabble digest of the specified private or public key file.

-c

Requests changing the comment in the private and public key files. The program prompts for the file containing the private keys, for the passphrase if the key has one, and for the new comment.

This option only applies to rsa1 (SSHv1) keys.

-C comment

Provides the new comment.

-e

This option reads a private or public OpenSSH key file and prints the key in a “SECSH” Public Key File Format to stdout. This option allows exporting keys for use by several other SSH implementations.

-f

Specifies the filename of the key file.

-F

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 can also be used in conjunction with the -H option to print found keys in a hashed format.

-H

Hash a known_hosts file. This replaces all host names 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 does not modify existing hashed host names and is therefore safe to use on files that mix hashed and non-hashed names.

-i

This option reads an unencrypted private (or public) key file in SSH2-compatible format and prints 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 other SSH implementations.

-l

Shows the fingerprint of the specified private or public key file.

-N new_passphrase

Provides the new passphrase.

-p

Requests changing the passphrase of a private key file instead of creating a new private key. The program prompts for the file containing the private key, for the old passphrase, and prompts twice for the new passphrase.

-P passphrase

Provides the (old) passphrase.

-q

Silences ssh-keygen.

-t type

Specifies the algorithm used for the key, where type is one of rsa, dsa, and rsa1. Type rsa1 is used only for the SSHv1 protocol.

-R hostname

Removes all keys belonging to hostname from a known_hosts file. This option is useful to delete hashed hosts. See -H.

-x

Obsolete. Replaced by the -e option.

-X

Obsolete. Replaced by the -i option.

-y

This option reads a private OpenSSH format file and prints an OpenSSH public key to stdout.

-8

Specifies that ssh-keygen will generate the keys in PKCS#8 format. The supported type of key to be generated is rsa or dsa.

Exit Status

The following exit values are returned:

0

Successful completion.

1

An error occurred.

Files

$HOME/.ssh/identity

This file contains the RSA private key for the SSHv1 protocol. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase is used to encrypt the private part of this file using 128–bit AES. This file is not automatically accessed by ssh-keygen, but it is offered as the default file for the private key. sshd(1M) reads this file when a login attempt is made.

$HOME/.ssh/identity.pub

This file contains the RSA public key for the SSHv1 protocol. The contents of this file should be added to $HOME/.ssh/authorized_keys on all machines where you wish to log in using RSA authentication. There is no need to keep the contents of this file secret.

$HOME/.ssh/id_dsa

$HOME/.ssh/id_rsa

These files contain, respectively, the DSA or RSA private key for the SSHv2 protocol. These files should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase is used to encrypt the private part of the file using 3DES. Neither of these files is automatically accessed by ssh-keygen but is offered as the default file for the private key. sshd(1M) reads this file when a login attempt is made.

$HOME/.ssh/id_dsa.pub

$HOME/.ssh/id_rsa.pub

These files contain, respectively, the DSA or RSA public key for the SSHv2 protocol. The contents of these files should be added, respectively, to $HOME/.ssh/authorized_keys on all machines where you wish to log in using DSA or RSA authentication. There is no need to keep the contents of these files secret.

Attributes

See attributes(5) for descriptions of the following attributes:

See Also

ssh(1), ssh-add(1), ssh-agent(1), sshd(1M), attributes(5)