Public And Private Keys Rsa Key Generation

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Contents

  1. Generating RSA Keys
  • Jun 22, 2012 SSH keys provide a more secure way of logging into a virtual private server with SSH than using a password alone. With SSH keys, users can log into a server without a password. This tutorial explains how to generate, use, and upload an SSH Key Pair.
  • We will see two aspects of the RSA cryptosystem, firstly generation of key pair and secondly encryption-decryption algorithms. Generation of RSA Key Pair. Each person or a party who desires to participate in communication using encryption needs to generate a pair of keys, namely public key and private key.
  • The RSA private key is used to generate digital signatures, and the RSA public key is used to verify digital signatures. The RSA public key is also used for key encryption of DES or AES DATA keys and the RSA private key for key recovery.
  • An Online RSA Public and Private Key Generator. Sep 6 th, 2013. I was recently in a meeting where a person needed to generate a private and public key for RSA encryption, but they were using a PC (Windows). This is something that is easily done via a terminal using ssh-keygen on Mac and Linux.
  • Generate RSA keys with SSH by using PuTTYgen. Last updated on: 2016-06-23; Authored by: Rackspace Support; One effective way of securing SSH access to your cloud server is to use a public-private key pair. This means that a public key is placed on the server and a private key is placed on your local workstation. Using a key pair makes it.

Parent page: Internet and Networking >> SSH

Public key authentication is more secure than password authentication. This is particularly important if the computer is visible on the internet. If you don't think it's important, try logging the login attempts you get for the next week. My computer - a perfectly ordinary desktop PC - had over 4,000 attempts to guess my password and almost 2,500 break-in attempts in the last week alone.

With public key authentication, the authenticating entity has a public key and a private key. Each key is a large number with special mathematical properties. The private key is kept on the computer you log in from, while the public key is stored on the .ssh/authorized_keys file on all the computers you want to log in to. When you log in to a computer, the SSH server uses the public key to 'lock' messages in a way that can only be 'unlocked' by your private key - this means that even the most resourceful attacker can't snoop on, or interfere with, your session. As an extra security measure, most SSH programs store the private key in a passphrase-protected format, so that if your computer is stolen or broken in to, you should have enough time to disable your old public key before they break the passphrase and start using your key. Wikipedia has a more detailed explanation of how keys work.

Public key authentication is a much better solution than passwords for most people. In fact, if you don't mind leaving a private key unprotected on your hard disk, you can even use keys to do secure automatic log-ins - as part of a network backup, for example. Different SSH programs generate public keys in different ways, but they all generate public keys in a similar format:

Key-based authentication is the most secure of several modes of authentication usable with OpenSSH, such as plain password and Kerberos tickets. Key-based authentication has several advantages over password authentication, for example the key values are significantly more difficult to brute-force, or guess than plain passwords, provided an ample key length. Other authentication methods are only used in very specific situations.

SSH can use either 'RSA' (Rivest-Shamir-Adleman) or 'DSA' ('Digital Signature Algorithm') keys. Both of these were considered state-of-the-art algorithms when SSH was invented, but DSA has come to be seen as less secure in recent years. RSA is the only recommended choice for new keys, so this guide uses 'RSA key' and 'SSH key' interchangeably.

Key-based authentication uses two keys, one 'public' key that anyone is allowed to see, and another 'private' key that only the owner is allowed to see. To securely communicate using key-based authentication, one needs to create a key pair, securely store the private key on the computer one wants to log in from, and store the public key on the computer one wants to log in to.

Using key based logins with ssh is generally considered more secure than using plain password logins. This section of the guide will explain the process of generating a set of public/private RSA keys, and using them for logging into your Ubuntu computer(s) via OpenSSH.

The first step involves creating a set of RSA keys for use in authentication.

This should be done on the client.

To create your public and private SSH keys on the command-line:

You will be prompted for a location to save the keys, and a passphrase for the keys. This passphrase will protect your private key while it's stored on the hard drive:

Your public key is now available as .ssh/id_rsa.pub in your home folder.

Congratulations! You now have a set of keys. Now it's time to make your systems allow you to login with them

Choosing a good passphrase

You need to change all your locks if your RSA key is stolen. Otherwise the thief could impersonate you wherever you authenticate with that key.

An SSH key passphrase is a secondary form of security that gives you a little time when your keys are stolen. If your RSA key has a strong passphrase, it might take your attacker a few hours to guess by brute force. That extra time should be enough to log in to any computers you have an account on, delete your old key from the .ssh/authorized_keys file, and add a new key.

Your SSH key passphrase is only used to protect your private key from thieves. It's never transmitted over the Internet, and the strength of your key has nothing to do with the strength of your passphrase.

The decision to protect your key with a passphrase involves convenience x security. Note that if you protect your key with a passphrase, then when you type the passphrase to unlock it, your local computer will generally leave the key unlocked for a time. So if you use the key multiple times without logging out of your local account in the meantime, you will probably only have to type the passphrase once.

If you do adopt a passphrase, pick a strong one and store it securely in a password manager. You may also write it down on a piece of paper and keep it in a secure place. If you choose not to protect the key with a passphrase, then just press the return when ssh-keygen asks.

Key Encryption Level

Note: The default is a 2048 bit key. You can increase this to 4096 bits with the -b flag (Increasing the bits makes it harder to crack the key by brute force methods).

Password Authentication

The main problem with public key authentication is that you need a secure way of getting the public key onto a computer before you can log in with it. If you will only ever use an SSH key to log in to your own computer from a few other computers (such as logging in to your PC from your laptop), you should copy your SSH keys over on a memory stick, and disable password authentication altogether. If you would like to log in from other computers from time to time (such as a friend's PC), make sure you have a strong password.

The key you need to transfer to the host is the public one. If you can log in to a computer over SSH using a password, you can transfer your RSA key by doing the following from your own computer:

Where <username> and <host> should be replaced by your username and the name of the computer you're transferring your key to.

Due to this bug, you cannot specify a port other than the standard port 22. You can work around this by issuing the command like this: ssh-copy-id '<username>@<host> -p <port_nr>'. If you are using the standard port 22, you can ignore this tip.

Another alternative is to copy the public key file to the server and concatenate it onto the authorized_keys file manually. It is wise to back that up first:

You can make sure this worked by doing:

You should be prompted for the passphrase for your key:

Enter passphrase for key '/home/<user>/.ssh/id_rsa':

Enter your passphrase, and provided host is configured to allow key-based logins, you should then be logged in as usual.

Encrypted Home Directory

If you have an encrypted home directory, SSH cannot access your authorized_keys file because it is inside your encrypted home directory and won't be available until after you are authenticated. Therefore, SSH will default to password authentication.

To solve this, create a folder outside your home named /etc/ssh/<username> (replace '<username>' with your actual username). This directory should have 755 permissions and be owned by the user. Move the authorized_keys file into it. The authorized_keys file should have 644 permissions and be owned by the user.

Public And Private Keys Rsa Key Generation

Then edit your /etc/ssh/sshd_config and add:

Finally, restart ssh with:

The next time you connect with SSH you should not have to enter your password.

username@host's password:

If you are not prompted for the passphrase, and instead get just the

prompt as usual with password logins, then read on. There are a few things which could prevent this from working as easily as demonstrated above. On default Ubuntu installs however, the above examples should work. If not, then check the following condition, as it is the most frequent cause:

On the host computer, ensure that the /etc/ssh/sshd_config contains the following lines, and that they are uncommented;

If not, add them, or uncomment them, restart OpenSSH, and try logging in again. If you get the passphrase prompt now, then congratulations, you're logging in with a key!

Permission denied (publickey)

If you're sure you've correctly configured sshd_config, copied your ID, and have your private key in the .ssh directory, and still getting this error:

Permission denied (publickey).

Chances are, your /home/<user> or ~/.ssh/authorized_keys permissions are too open by OpenSSH standards. You can get rid of this problem by issuing the following commands:

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Error: Agent admitted failure to sign using the key.

Public

This error occurs when the ssh-agent on the client is not yet managing the key. Issue the following commands to fix:

This command should be entered after you have copied your public key to the host computer.

Debugging and sorting out further problems

The permissions of files and folders is crucial to this working. You can get debugging information from both the client and server.

if you think you have set it up correctly , yet still get asked for the password, try starting the server with debugging output to the terminal.

To connect and send information to the client terminal

No matter how your public key was generated, you can add it to your Ubuntu system by opening the file .ssh/authorized_keys in your favourite text editor and adding the key to the bottom of the file. You can also limit the SSH features that the key can use, such as disallowing port-forwarding or only allowing a specific command to be run. This is done by adding 'options' before the SSH key, on the same line in the authorized_keys file. For example, if you maintain a CVS repository, you could add a line like this:

When the user with the specified key logged in, the server would automatically run /usr/bin/cvs server, ignoring any requests from the client to run another command such as a shell. For more information, see the sshd man page. /755

Several tools exist to generate SSH public/private key pairs. The following sections show how to generate an SSH key pair on UNIX, UNIX-like and Windows platforms.

Generating an SSH Key Pair on UNIX and UNIX-Like Platforms Using the ssh-keygen Utility

UNIX and UNIX-like platforms (including Solaris and Linux) include the ssh-keygen utility to generate SSH key pairs.

Public And Private Keys Rsa Key Generation 10

To generate an SSH key pair on UNIX and UNIX-like platforms using the ssh-keygen utility:

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  1. Navigate to your home directory:
  2. Run the ssh-keygen utility, providing as filename your choice of file name for the private key:

    The ssh-keygen utility prompts you for a passphrase for the private key.

  3. Enter a passphrase for the private key, or press Enter to create a private key without a passphrase:

    Note:

    While a passphrase is not required, you should specify one as a security measure to protect the private key from unauthorized use. When you specify a passphrase, a user must enter the passphrase every time the private key is used.

    The ssh-keygen utility prompts you to enter the passphrase again.

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  4. Enter the passphrase again, or press Enter again to continue creating a private key without a passphrase:
  5. The ssh-keygen utility displays a message indicating that the private key has been saved as filename and the public key has been saved as filename.pub. It also displays information about the key fingerprint and randomart image.

Generating an SSH Key Pair on Windows Using the PuTTYgen Program

Public And Private Keys Rsa Key Generation Windows

The PuTTYgen program is part of PuTTY, an open source networking client for the Windows platform.

To generate an SSH key pair on Windows using the PuTTYgen program:

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  1. Download and install PuTTY or PuTTYgen.

    To download PuTTY or PuTTYgen, go to http://www.putty.org/ and click the You can download PuTTY here link.

  2. Run the PuTTYgen program.
  3. Set the Type of key to generate option to SSH-2 RSA.
  4. In the Number of bits in a generated key box, enter 2048.
  5. Click Generate to generate a public/private key pair.

    As the key is being generated, move the mouse around the blank area as directed.

  6. (Optional) Enter a passphrase for the private key in the Key passphrase box and reenter it in the Confirm passphrase box.

    Note:

    While a passphrase is not required, you should specify one as a security measure to protect the private key from unauthorized use. When you specify a passphrase, a user must enter the passphrase every time the private key is used.

  7. Click Save private key to save the private key to a file. To adhere to file-naming conventions, you should give the private key file an extension of .ppk (PuTTY private key).

    Note:

    The .ppk file extension indicates that the private key is in PuTTY's proprietary format. You must use a key of this format when using PuTTY as your SSH client. It cannot be used with other SSH client tools. Refer to the PuTTY documentation to convert a private key in this format to a different format.
  8. Select all of the characters in the Public key for pasting into OpenSSH authorized_keys file box.

    Make sure you select all the characters, not just the ones you can see in the narrow window. If a scroll bar is next to the characters, you aren't seeing all the characters.

  9. Right-click somewhere in the selected text and select Copy from the menu.
  10. Open a text editor and paste the characters, just as you copied them. Start at the first character in the text editor, and do not insert any line breaks.
  11. Save the text file in the same folder where you saved the private key, using the .pub extension to indicate that the file contains a public key.
  12. If you or others are going to use an SSH client that requires the OpenSSH format for private keys (such as the ssh utility on Linux), export the private key:
    1. On the Conversions menu, choose Export OpenSSH key.
    2. Save the private key in OpenSSH format in the same folder where you saved the private key in .ppk format, using an extension such as .openssh to indicate the file's content.