c# create barcode from string Using Public/Private Key Based Authentication in an SSH Environment in Font

Drawing QR Code in Font Using Public/Private Key Based Authentication in an SSH Environment

Using Public/Private Key Based Authentication in an SSH Environment
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When SSH key-based authentication is used, you must make sure that, for all users who need to use this technology, the public key is available on the servers they want to log in to. When logging in, the user creates an authentication request that s signed with the user s private key. This authentication request is matched to the public key of the same user on the server where that user wants to be authenticated. If it matches, the user is allowed access; if it doesn t, user access is denied. Public/private key based authentication is enabled by default on Ubuntu Server, so it s only when no keys are present that the server prompts users for a password. The following steps provide a summary of what happens when a user tries to establish an SSH session with a server: 1. If public key authentication is enabled (the default), SSH checks the .ssh directory in the user s home directory to see if a private key is present. 2. If a private key is found, SSH creates a packet with some data in it (the salt), encrypts that packet with the private key, and sends it to the server. The public key is also sent with this packet.
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CHAPTER 8 MAKING CONNECTION
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3. The server now checks if a file with the name authorized_keys exists in the home directory of the user. If it doesn t, the user can t be authenticated with his keys. If the file does exist and the public key is an allowed key (and also is identical to the key that was previously stored on the server), the server uses this key to check the signature. 4. If the signature is verified, the user is granted access. If the signature can t be verified, the server prompts the user for a password instead. All this sounds pretty complicated, but it really isn t. Everything happens transparently, if it has been set up right. Also, there s hardly any noticeable delay when establishing a connection. It normally takes no more than a second.
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Setting Up SSH for Key-Based Authentication
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The best way to explain how to set up SSH for key-based authentication is by working through an example. In the following procedure, key-based authentication is enabled for the user root. 1. On the desktop where root is working, use the command ssh-keygen -t dsa -b 1024. This generates a public/private key pair of 1,024 bits. Listing 8-19 shows what happens. Listing 8-19. Generating a Public/Private Key Pair with ssh-keygen workstation # ssh-keygen -t dsa -b 1024 Generating public/private dsa key pair. Enter file in which to save the key (/root/.ssh/id_dsa) : Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /root/.ssh/id_dsa. Your public key has been saved in /root/.ssh/id_dsa.pub. The key fingerprint is: 59:63:b5:a0:c5:2c:b5:b8:2f:99:80:5b:43:77:3c:dd root@workstation I ll explain what happens. The user in this example uses the ssh-keygen command to generate a public and a private key. The encryption algorithm used to generate this key is DSA, which is considered more secure than its alternative, RSA. The option -b 1024 specifies that 1024-bit encryption should be used for the key. You re possibly aware that, the longer this number, the more secure it is. Notice, however, that a many-bits encryption algorithm also requires more system resources to use it. After generating the keys, the command prompts you to save it somewhere. By default, a directory with the name .ssh is created in your home directory, and, within this directory, a file with the name id_dsa. This file contains the private key. Next, you re prompted to enter a passphrase, which is an important extra layer of protection that can be added to the key. Because anyone who has access to your private key (which isn t that easy) can forge your identity, your private key should always be protected with a passphrase. After entering the same passphrase twice, the private key is saved and the related public key is generated and saved in the file /root/.ssh/id_dsa.pub. Also, a key fingerprint is generated. This fingerprint is a summary of your key, a checksum that s calculated on the key
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