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ssh-keygen - authentication key generation

Synopsis

ssh-keygen [-b bits] [-f file] [-N new_passphrase] [-C comment]

ssh-keygen -p [-P old_passphrase] [-N new_passphrase]

ssh-keygen -c [-P passphrase] [-C comment]

ssh-keygen -u [-f file] [-P passphrase]

Ssh-keygen generates and manages authentication keys for ssh. Normally each user wishing to use ssh with RSA authentication runs this once to create the authentication key in %HOME%/.ssh/identity. Additionally, the system administrator may use this to generate host keys.

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 empty passphrase), or it may be a string of arbitrary length. Good passphrases are 10-30 characters long and are not simple sentences or otherwise easily guessable (English prose has only 1-2 bits of entropy per word, and provides very bad passphrases). 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 will have to generate a new key and copy the corresponding public key to other machines.

USING GOOD, UNGUESSABLE PASSPHRASES IS STRONGLY RECOMMENDED. EMPTY PASSPHRASES SHOULD NOT BE USED UNLESS YOU UNDERSTAND WHAT YOU ARE DOING.

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.

The cipher to be used when encrypting keys with passphrase is defined in ssh.h. Using the -u option, keys encrypted in any supported cipher can be updated to use this default cipher.

Options

Specifies the number of bits in the key to create. Minimum is 512 bits. Generally 1024 bits is consid ered 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. The program will prompt for the file containing the private keys, for passphrase if the key has one, and for the new comment.
-f: Specifies the file name in which to load/store the key.
-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.
-u: Requests that the key's cipher is changed to the current default cipher (determined at compile-time currently 3DES).
-C: Provides the new comment.
-N: Provides the new passphrase.
-P: Provides the (old) passphrase.

Files

%HOME%/.ssh/random_seed
Used for seeding the random number generator. This file should not be readable by anyone but the user. This file is created the first time the program is run, and is updated every time.

%HOME%/.ssh/identity
Contains the 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.

%HOME%/.ssh/identity.pub
Contains the public key for authentication. 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.

Authors

Tatu Ylönen <ylo@cs.hut.fi>;
OS/2 adaptation by Hans-Michael Stahl <hmstahl@berlin.snafu.de>