Authentication by Cryptographic Key (SSH, The Secure Shell: The Definitive Guide)

2.4. Authentication by Cryptographic Key

In our running example, the user pat is authenticated by the SSH server via login password. Passwords, however, have serious drawbacks:

In order for a password to be secure, it should be long and random, but such passwords are hard to memorize.
A password sent across the network, even protected by an SSH secure channel, can be captured when it arrives on the remote host if that host has been compromised.
Most operating systems support only a single password per account. For shared accounts (e.g., a superuser account), this presents difficulties:
- Password changes are inconvenient because the new password must be communicated to all people with access to the account.
- Tracking usage of the account becomes difficult because the operating system doesn't distinguish between the different users of the account.

To address these problems, SSH supports public-key authentication: instead of relying on the password scheme of the host operating system, SSH may use cryptographic keys. [Section 3.2.2, "Public- and Secret-Key Cryptography"] Keys are more secure than passwords in general and address all the weaknesses mentioned earlier.

2.4.1. A Brief Introduction to Keys

A key is a digital identity. It's a unique string of binary data that means, "This is me, honestly, I swear." And with a little cryptographic magic, your SSH client can prove to a server that its key is genuine, and you are really you.

An SSH identity uses a pair of keys, one private and one public. The private key is a closely guarded secret only you have. Your SSH clients use it to prove your identity to servers. The public key is, like the name says, public. You place it freely into your accounts on SSH server machines. During authentication, the SSH client and server have a little conversation about your private and public key. If they match (according to a cryptographic test), your identity is proven, and authentication succeeds.

The following sequence demonstrates the conversation between client and server. [Section 3.4.1, "Establishing the Secure Connection"] (It occurs behind the scenes, so you don't need to memorize it or anything; we just thought you might be interested.)

Your client says, "Hey server, I'd like to connect by SSH to an account on your system, specifically, the account owned by user smith."
The server says, "Well, maybe. First, I challenge you to prove your identity!" And the server sends some data, known as a challenge, to the client.
Your client says, "I accept your challenge. Here is proof of my identity. I made it myself by mathematically using your challenge and my private key." This response to the server is called an authenticator.
The server says, "Thanks for the authenticator. I will now examine the smith account to see if you may enter." Specifically, the server checks smith's public keys to see if the authenticator "matches" any of them. (The "match" is another cryptographic operation.) If so, the server says, "OK, come on in!" Otherwise, the authentication fails.

Before you can use public-key authentication, some setup is required:

You need a private key and a public key, known collectively as a key pair. You also need a secret passphrase to protect your private key. [Section 2.4.2, "Generating Key Pairs with ssh-keygen"]
You need to install your public key on an SSH server machine. [Section 2.4.3, "Installing a Public Key on an SSH ServerMachine"]

2.4.2. Generating Key Pairs with ssh-keygen

To use cryptographic authentication, you must first generate a key pair for yourself, consisting of a private key (your digital identity that sits on the client machine) and a public key (that sits on the server machine). To do this, use the ssh-keygen program. Its behavior differs for SSH1, SSH2, and OpenSSH. On an SSH1 system, the program is called ssh-keygen or ssh-keygen1. When you invoke it, ssh-keygen creates an RSA key pair and asks you for a secret passphrase to protect the private key.[8]

[8]RSA is an encryption algorithm for SSH keys, among other things. [Section 3.9.1, "Public-Key Algorithms"] DSA is another, as you'll see later.

$ ssh-keygen1
Initializing random number generator...
Generating p:  ..................................++ (distance 1368)
Generating q:  ....++ (distance 58)
Computing the keys...
Testing the keys...
Key generation complete.
Enter file in which to save the key (/home/pat/.ssh/identity): 
Enter passphrase: **************
Enter the same passphrase again: **************
Your identification has been saved in identity.
Your public key is:
1024 35 11272721957877936880509167858732970485872567486703821636830\
1950099934876023218886571857276011133767701853088352661186539160906\
9214986989240214507621864063548908730298546478215446737245984456708\
9631066077107611074114663544313782992987840457273825436579285836220\
2493395730648451296601594344979290457421809236729 path@shell.isp.com
Your public key has been saved in identity.pub.

On SSH2 systems, the command is either ssh-keygen or ssh-keygen2, and its behavior is a bit different and produces either a DSA key (the default) or an RSA key:

$ ssh-keygen2
Generating 1024-bit dsa key pair
   1 ..oOo.oOo.oO
   2 o.oOo.oOo.oO
   3 o.oOo.oOo.oO
   4 o.oOo.oOo.oO
Key generated.
1024-bit dsa, created by pat@shell.isp.com Mon Mar 20 13:01:15 2000
Passphrase : **************
Again      : **************
Private key saved to /home/pat/.ssh2/id_dsa_1024_a
Public key saved to /home/pat/.ssh2/id_dsa_1024_a.pub

The OpenSSH version of ssh-keygen also can produce either RSA or DSA keys, defaulting to RSA. Its operation is similar to that of ssh-keygen1.

Normally, ssh-keygen performs all necessary mathematics to generate a key, but on some operating systems you might be asked to assist it. Key generation requires some random numbers, and if your operating system doesn't supply a random-number generator, you may be asked to type some random text. ssh-keygen uses the timings of your keystrokes to initialize its internal random-number generator. On a 300-MHz Pentium system running Linux, generating a 1024-bit RSA key takes about three seconds; if your hardware is slower than this or heavily loaded, generation may take significantly longer, up to a minute or more. It can also take longer if the process runs out of random bits, and ssh-keygen has to wait to collect more.

ssh-keygen then creates your local SSH directory (~/.ssh for SSH1 and OpenSSH or ~/.ssh2 for SSH2) if it doesn't already exist, and stores the private and public components of the generated key in two files there. By default, their names are identity and identity.pub (SSH1, OpenSSH) or id_dsa_1024_a and id_dsa_1024_a.pub (SSH2). SSH clients consider these to be your default identity for authentication purposes.

WARNING: Never reveal your private key and passphrase to anyone else. They are just as sensitive as your login password. Anyone possessing them can log in as you!

When created, the identity file is readable only by your account, and its contents are further protected by encrypting them with the passphrase you supplied during generation. We say "passphrase" instead of "password" both to differentiate it from a login password, and to stress that spaces and punctuation are allowed and encouraged. We recommend a passphrase at least 10 -15 characters long and not a grammatical sentence.

ssh-keygen has numerous options for managing keys: changing the passphrase, choosing a different name for the key file, and so forth. [Section 6.2, "Creating an Identity"]

2.4.3. Installing a Public Key on an SSH ServerMachine

When passwords are used for authentication, the host operating system maintains the association between the username and the password. For cryptographic keys, you must set up a similar association manually. After creating the key pair on the local host, you must install your public key in your account on the remote host. A remote account may have many public keys installed for accessing it in various ways.

Returning to our running example, you must install a public key into the "pat" account on shell.isp.com. This is done by editing a file in the SSH configuration directory: ~/.ssh/authorized_keys for SSH1 and OpenSSH [9] or ~/.ssh2/authorization for SSH2.

[9]OpenSSH uses authorized_keys2 for SSH-2 connections. For simplicity, we'll discuss OpenSSH later. [Section 8.2.3, "OpenSSH Authorization Files "]

For SSH1 or OpenSSH, create or edit the file ~/.ssh/authorized_keys and append your public key, i.e., the contents of the identity.pub file you generated on the local machine. A typical authorized_keys file contains a list of public key data, one key per line. The example contains only two public keys, each on its own line of the file, but they are too long to fit on this page. The line breaks inside the long numbers are printing artifact; if they were actually in the file, it would be incorrectly formatted and wouldn't work:

1024 35 8697511247987525784866526224505474204292260357215616159982327587956883143
362147028876494426516682677550219425827002174890309672203219700937187777979705864
107549106608811204142046600066790196940691100768682518506600601481676686828742807
11088849408310989234142475694298520575977312478025518391 my personal key
1024 37 1140868200916227508775331982659387253607752793422843620910258618820621996
941824516069319525136671585267698112659690736259150374130846896838697083490981532
877352706061107257845462743793679411866715467672826112629198483320167783914580965
674001731023872042965273839192998250061795483568436433123392629 my work key

These are RSA public keys: the first number in each entry is the number of bits in the key, while the second and third are RSA-specific parameters called the public exponent and modulus. After these comes an arbitrary amount of text treated as a comment. [Section 8.2.1, "SSH1 Authorization Files "]

For SSH2, you need to edit two files, one on the client machine and one on the server machine. On the client machine, create or edit the file ~/.ssh2/identification and insert a line to identify your private key file:

IdKey id_dsa_1024_a

On the server machine, create or edit the file ~/.ssh2/authorization, which contains information about public keys, one per line. But unlike SSH1's authorized_keys file, which contains copies of the public keys, the authorization file lists only the filename of the key:

Key id_dsa_1024_a.pub

Finally, copy id_dsa_1024_a.pub from your local machine to the remote SSH2 server machine, placing it in ~/.ssh2.

Regardless of which SSH implementation you use, make sure your remote SSH directory and associated files are writable only by your account:[10]

[10]We make files world-readable and directories world-searchable, to avoid NFS problems. [Section 10.7.2, "NFS Access Problems"]

# SSH1, OpenSSH
$ chmod 755 ~/.ssh
$ chmod 644 ~/.ssh/authorized_keys

# OpenSSH only
$ chmod 644 ~/.ssh/authorized_keys2

# SSH2 only
$ chmod 755 ~/.ssh2
$ chmod 644 ~/.ssh2/id_dsa_1024_a.pub
$ chmod 644 ~/.ssh2/authorization

The SSH server is picky about file and directory permissions and may refuse authentication if the remote account's SSH configuration files have insecure permissions. [Section 5.4.2.1, "Acceptable permissions for user files"]

You are now ready to use your new key to access the "pat" account:

# SSH1, SSH2, OpenSSH; output shown is for SSH1
$ ssh -l pat shell.isp.com
Enter passphrase for RSA key 'Your Name <you@local.org>': ************
Last login: Mon May 24 19:44:21 1999 from quincunx.nefertiti.org
You have new mail.
shell.isp.com>

If all goes well, you are logged into the remote account. Figure 2-2 shows the entire process.

Figure 2-2. Public-key authentication

Note the similarity to the earlier example with password authentication. [Section 2.2, "Remote Terminal Sessions with ssh"] On the surface, the only difference is that you provide the passphrase to your private key, instead of providing your login password. Underneath, however, something quite different is happening. In password authentication, the password is transmitted to the remote host. With cryptographic authentication, the passphrase serves only to decrypt the private key to create an authenticator. [Section 2.4.1, "A Brief Introduction to Keys"]

Public-key authentication is more secure than password authentication because:

It requires two secret components -- the identity file on disk, and the passphrase in your head -- so both must be captured in order for an adversary to access your account. Password authentication requires only one component, the password, which might be easier to steal.
Neither the passphrase nor the key is sent to the remote host, just the authenticator discussed earlier. Therefore, no secret information is transmitted off the client machine.
Machine-generated cryptographic keys are infeasible to guess. Human-generated passwords are routinely cracked by a password-guessing technique called a dictionary attack. A dictionary attack may be mounted on the passphrase as well, but this requires stealing the private key file first.

A host's security can be greatly increased by disabling password authentication altogether and permitting only SSH connections by key.

2.4.4. If You Change Your Key

Suppose you have generated a key pair, identity and identity.pub, and copied identity.pub to a bunch of SSH server machines. All is well. Then one day, you decide to change your identity, so you run ssh-keygen a second time, overwriting identity and identity.pub. Guess what? Your previous public key file is now invalid, and you must copy the new public key to all those SSH server machines again. This is a maintenance headache, so think carefully before changing (destroying!) a key pair. Some caveats:

You are not limited to one key pair. You can generate as many as you like, stored in different files, and use them for diverse purposes. [Section 6.4, "Multiple Identities"]
If you just want to change your passphrase, you don't have to generate a new key pair. ssh-keygen has command-line options for replacing the passphrase of an existing key: -p for SSH1 and OpenSSH [Section 6.2.1, "Generating RSA Keys for SSH1"] and -e for SSH2 [Section 6.2.2, "Generating RSA/DSA Keys for SSH2"]. In this case your public key remains valid since the private key hasn't changed, just the passphrase for decrypting it.


2.3. Adding Complexity to the Example		2.5. The SSH Agent