Ssh key authentication idc online com

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SSH KEY AUTHENTICATION

The SSH protocol has support for multiple different types of authentication. By default, one of the authentication mechanisms used by SSH is password authentication. SSH will allow you to connect to a given account on a computer if you know the password for that user on that computer. By default SSH will also try to use an SSH key-pair for authentication instead of a password, and in fact, it will try to use a key-pair before it tries to use a password. To use SSH key authentication you need to do the following:

1. Generate an SSH key-pair on the computer you will be SSHing from (you only have to do this once, you can use the same key-pair to authenticate to multiple computers safely).

2. Give the public key from that key-pair to the person managing the computer you want to SSH to (never share your private key with anyone!).

3. Wait for the administrator of the remote computer to add your public key to the list of allowed keys within the account you will be SSHing to.

Once those steps have been completed you will be able to log in to the remote computer without having to know the password of the user you will be connecting as. Lets look at these steps in detail now. To play along you'll need two computers, one to SSH from, and one to SSH to.

Generating an SSH Key-Pair

The process stars on the computer you will be SSHing from. You need to open a terminal as the user who will be SSHing to the remote computer, and in that terminal type the command:

ssh-keygen -t rsa

This will create an SSH key-pair and offer to store the two halves in the default locations (press enter to accept the defaults):

The private key: ~/.ssh/id_rsa The public key: ~/.ssh/id_rsa.pub If you already have a set of keys and don't want to replace them, you can use thef flag to specify a different location to save the private key (the public key will get stored in the same folder and with the same name, but with .pub appended to it). When you run the ssh-keygen command you will be asked to enter a password.

This is the password that will secure the private key. This is a very important safety measure, because it means that if your private key is lost or stolen, it cannot

be used unless the attacker also knows the matching password. The ssh-keygen

command will accept a blank password, but this is to be strongly discouraged because it leaves your private key unprotected.

It should also be noted that if you forget the password protecting your private key, you won't be able to use that key-pair any more, and you'll need to generate a fresh key-pair!

Once you enter a password ssh-keygen will generate a public and private key,

tell you where it has saved them, tell you the key's fingerprint (a big long

hexadecimal string separated with :s), and it will show you the key's random art

image. This is representation of the key as a little ASCII art graphic. This is much more memorable to humans than the fingerprint, show us two different pictures and we'll spot the difference in seconds, show us two different strings of hex and we'll find it very hard to spot subtle differences! To get a sense of how difficult an SSH key is to brute-force attack, you can have a look at the private key you just generated with the command:

cat ~/.ssh/id_rsa

And the public key with the command:

cat ~/.ssh/id_rsa.pub

Asside ? Base64 Encoding

If you are wondering what format the keys are stored in, it's the very commonly used base64 encoding. This is a very robust format that ignores characters like line breaks and spaces which could get introduced if a key were to be copied and

pasted into an email or something like that.

Granting Access With an SSH Public Key

The next step in the process is to share your public key with the person administering the computer you will be SSHing to. You can do this by attaching the public key to an email, or simply copying and pasting it's content into an email. If we know the password of the remote account we will be connecting to, we can also copy the key over ourselves, but more on that later. To grant a remote user access to a given account a computer administrator needs to add the remote user's public key to a special file in the home directory of the local user the remote user will be connecting as. That special file

is~/.ssh/authorised_keys (or, if the key is only to be used over the SSH2 protocol,~/.ssh/authorized_keys2). The ~/.ssh/authorized_keys file should contain one public key per line.

You can grant access to as many users as you like by adding as many public keys are you like. SSH is an absolute stickler about the permissions on

the authorized_keys file, including the permissions on the folder that contains it, i.e. ~/.ssh/. No one other than the owner of the account (and root)

should have write permissions to either the containing folder, or the file itself.

Because public keys are not sensitive information, SSH does not care of other users can read what is effectively public information, but the ability to write to that file would allow any other user on the system to grant themselves access to that account by adding their own public key to the list. To prevent this from happening, SSH will not accept a key if it's contained in a file that is writeable by anyone but the owner of the account. An example of working permissions on an account with

the username bart is show below:

[bart@www ~]$ ls -al ~/.ssh total 20 drwx------ 2 bart bart 4096 May 5 2014 . drwxr-xr-x 16 bart bart 4096 Mar 15 14:32 .. -rw-r--r-- 1 bart bart 670 Feb 14 2013 authorized_keys -rw-r--r-- 1 bart bart 660 May 5 2014 known_hosts [bart@www ~]$

Remember that in a list of the contents of the folder ~/.ssh, the permissions on that folder itself are the permissions on the special file .. I have highlighted the

command, and the two important sets of permissions in bold.

Simplifying the Process with ssh-copy-id

It takes time and effort to manually copy across your public key, and to make sure all the file permissions are correct.

Assuming you know the password to log in to the remote computer, you can

automate the process with the ssh-copy-id utility.

This utility comes as standard on all the Linux distributions I have used, but

annoyingly, OS X's version of SSH does not come with ssh-copy-id. All is not

lost though, because the open source community are here to help!

OS X users can install ssh-copy-id onto their Mac using the free and open

source project ssh-copy-id-for-OSX.

If you follow the link above you'll see that installing ssh-copy-id onto your

mac is as simple as running the command:

curl -L | sh

The above command has to be run from an admin account, and it uses sudo for

the install, so you will be prompted for your password.

What ever OS you are on, once you have ssh-copy-id installed, copying over

your public key becomes as easy as running the command below

(replacing user and computer as appropriate):

ssh-copy-id user@computer

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