Generate Authentication Keys in Linux with 'ssh-keygen'

Use 'ssh-keygen' to issue public/private key pairs for secure communication

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Ssh-keygen generates, manages and converts authentication keys for ssh. 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.

How the Process Works

sample key

Normally each user using SSH with RSA or DSA authentication runs this command 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 command 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. The passphrase can be changed later by using the -p option.

A passphrase is similar to a password, except it can be a phrase with a series of words, punctuation, numbers, white space, or any string of characters you want. Good passphrases are 10 to 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.

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.

Synopsis

The command takes any of the following forms:

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

The options used as flags for the command include:

  • -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.