1. Technology
Linux / Unix Command: ssh-keygen
Command Library

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 ]  

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.

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 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:

-b bits
Specifies the number of bits in the key to create. Minimum is 512 bits. Generally 1024 bits is considered sufficient, and key sizes above that no longer improve security but make things slower. 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 program 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.
-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 protocol 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
-N new_passphrase
Provides the new passphrase.
-P passphrase
Provides the (old) passphrase.
-U reader
Upload an existing RSA private key into the smartcard in reader

 

SEE ALSO

ssh(1), ssh-add1, ssh-agent1, sshd(8)
J. Galbraith R. Thayer "SECSH Public Key File Format" draft-ietf-secsh-publickeyfile-01.txt March 2001 work in progress material


Important: Use the man command (% man) to see how a command is used on your particular computer.

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