SSH keys serve as a means of identifying yourself to an SSH server using. One immediate advantage this method has over traditional password authentication is that you can be authenticated by the server without ever having to send your password over the network. Anyone eavesdropping on your connection will not be able to intercept and crack your password because it is never actually transmitted. Additionally, using SSH keys for authentication virtually eliminates the risk posed by brute-force password attacks by drastically reducing the chances of the attacker correctly guessing the proper credentials. As well as offering additional security, SSH key authentication can be more convenient than the more traditional password authentication. When used with a program known as an SSH agent, SSH keys can allow you to connect to a server, or multiple servers, without having to remember or enter your password for each system. SSH keys are not without their drawbacks and may not be appropriate for all environments, but in many circumstances they can offer some strong advantages. A general understanding of how SSH keys work will help you decide how and when to use them to meet your needs. This article assumes you already have a basic understanding of the protocol and have the package. Contents • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Background SSH keys are always generated in pairs with one known as the private key and the other as the public key. The private key is known only to you and it should be safely guarded. By contrast, the public key can be shared freely with any SSH server to which you wish to connect. If an SSH server has your public key on file and sees you requesting a connection, it uses your public key to construct and send you a challenge. This challenge is an encrypted message and it must be met with the appropriate response before the server will grant you access. What makes this coded message particularly secure is that it can only be understood by the private key holder. While the public key can be used to encrypt the message, it cannot be used to decrypt that very same message. Only you, the holder of the private key, will be able to correctly understand the challenge and produce the proper response. This challenge-response phase happens behind the scenes and is invisible to the user. As long as you hold the private key, which is typically stored in the ~/.ssh/ directory, your SSH client should be able to reply with the appropriate response to the server. A private key is a guarded secret and as such it is advisable to store it on disk in an encrypted form. When the encrypted private key is required, a passphrase must first be entered in order to decrypt it. While this might superficially appear as though you are providing a login password to the SSH server, the passphrase is only used to decrypt the private key on the local system. The passphrase is not transmitted over the network. Generating an SSH key pair An SSH key pair can be generated by running the ssh-keygen command, defaulting to 2048-bit RSA (and SHA256) which the man page says is ' generally considered sufficient' and should be compatible with virtually all clients and servers: $ ssh-keygen Generating public/private rsa key pair. Enter file in which to save the key (/home//.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home//.ssh/id_rsa. Your public key has been saved in /home//.ssh/id_rsa.pub. Ssh Keygen No Passphrase Promptly DefinitionSsh-keygen -b 2048 -t rsa -f /tmp/sshkey -q.except that it asks me for the passphrase that would encrypt the keys. This make -at present- the automatisation difficult. I could provide a passphrase via the command line argument -N thepassphrase, so to keep the prompt from appearing. 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. Note: An alternate way of naming key files is to specify one or more key filenames at the end of the ssh-keygen command. -O key Uses the specified OpenSSH public or private key to create a public or private key in Reflection format. A passphrase is similar to a password. However, a password generally refers to something used to authenticate or log into a system. However, a password generally refers to something used to authenticate or log into a system. Cd ~/.ssh ssh-keygen -t rsa Just press the 'Enter' key in response to the prompts. Your interaction should look like this (where your username replaces 'xxx99'): [[email protected]]$ ssh-keygen -t rsa Generating public/private rsa key pair. The key fingerprint is: SHA256:gGJtSsV8BM+7w018d39Ji57F8iO6c0N2GZq3/RY2NhI username@hostname The key's randomart image is: +---[RSA 2048]----+| ooo.|| oo+.|| + +.+|| o + + E.||.. Oo*=O||..+=o+|| o=ooo+| +----[SHA256]-----+ The was as an easier means of visually identifying the key fingerprint. Note: If you set a passphrase for your key, it is strongly encouraged to use the -o option to ssh-keygen. Ssh Enter Passphrase For KeyThis will save your private key in the new OpenSSH format, which has greatly increased resistance to brute-force password cracking, but is not supported by versions of OpenSSH prior to 6.5. According to, Ed25519 keys always use the new private key format. Also use the -a switch to specify the number of KDF rounds on the password encryption. You can also add an optional comment field to the public key with the -C switch, to more easily identify it in places such as ~/.ssh/known_hosts, ~/.ssh/authorized_keys and ssh-add -L output. For example: $ ssh-keygen -C '$(whoami)@$(hostname)-$(date -I)' will add a comment saying which user created the key on which machine and when. Choosing the authentication key type OpenSSH supports several signing algorithms (for authentication keys) which can be divided in two groups depending on the mathematical properties they exploit: • and, which rely on the of factoring the product of two large prime numbers, • and, which rely on the elliptic curve problem. () (ECC) algorithms are a to public key cryptosystems.
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