How to regenerate new ssh server keys This is an unusual topic since most distribution create these keys for you during the installation of the OpenSSH server package. But it may be useful to be able generate new server keys from time to time, this happen to me when I duplicate Virtual Private Server which contains an installed ssh package. May 27, 2010  H ow do I generate ssh RSA keys under Linux operating systems? You need to use the ssh-keygen command as follows to generate RSA keys (open terminal and type the following command): ssh-keygen -t rsa OR ssh-keygen. I'm working on a (Debian) Dreamhost VPS, and it seems to want password-based authentication only: adding my RSA and DSA public keys to /.ssh/authorizedkeys did not change the behavior of requirin. Using SSH keys for authentication when connecting to your cloud server greatly simplifies the login process and increase the server security. Here's how to set up your first SSH keys using OpenSSH or PuTTYTray. Mar 29, 2020  This article describes how to generate SSH keys on Debian 10 systems. We will also show you how to set up an SSH key-based authentication and connect to remote Linux servers without entering a password. Creating SSH keys on Debian # The chances are that you already have an SSH key pair on your Debian client machine. Jun 09, 2018 Why regenerate new ssh server keys? Most Linux and Unix distribution create ssh keys for you during the installation of the OpenSSH server package. But it may be useful to be able re-generate new server keys from time to time.

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2. Debian Generate Ssh Server Keys Download
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4. Generate Ssh Public Key
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Links

Original URL: http://metasploit.com/users/hdm/tools/debian-openssl/ (Mirror)

Exploit:

Recommend Tool: Crowbar (able to brute force SSH keys)

Testing Method: ssh-vulnkey & dowkd.pl

CVE (CVE-2008-0166):

Information

The Bug

On May 13th, 2008 the Debian project announced that Luciano Bello found an interesting vulnerability in the OpenSSL package they were distributing. The bug in question was caused by the removal of the following line of code from md_rand.c

These lines were removed because they caused the Valgrind and Purify tools to produce warnings about the use of uninitialized data in any code that was linked to OpenSSL. You can see one such report to the OpenSSL team here. Removing this code has the side effect of crippling the seeding process for the OpenSSL PRNG. Instead of mixing in random data for the initial seed, the only 'random' value that was used was the current process ID. On the Linux platform, the default maximum process ID is 32,768, resulting in a very small number of seed values being used for all PRNG operations.

The Impact

All SSL and SSH keys generated on Debian-based systems (Ubuntu, Kubuntu, etc) between September 2006 and May 13th, 2008 may be affected. In the case of SSL keys, all generated certificates will be need to recreated and sent off to the Certificate Authority to sign. Any Certificate Authority keys generated on a Debian-based system will need be regenerated and revoked. All system administrators that allow users to access their servers with SSH and public key authentication need to audit those keys to see if any of them were created on a vulnerable system. Any tools that relied on OpenSSL's PRNG to secure the data they transferred may be vulnerable to an offline attack. Any SSH server that uses a host key generated by a flawed system is subject to traffic decryption and a man-in-the-middle attack would be invisible to the users. This flaw is ugly because even systems that do not use the Debian software need to be audited in case any key is being used that was created on a Debian system. The Debian and Ubuntu projects have released a set of tools for identifying vulnerable keys. You can find these listed in the references section below.

The Toys

The blacklists published by Debian and Ubuntu demonstrate just how small the key space is. When creating a new OpenSSH key, there are only 32,767 possible outcomes for a given architecture, key size, and key type. The reason is that the only 'random' data being used by the PRNG is the ID of the process. In order to generate the actual keys that match these blacklists, we need a system containing the correct binaries for the target platform and a way to generate keys with a specific process ID. To solve the process ID issue, I wrote a shared library that could be preloaded and that returns a user-specified value for the getpid() libc call.

The next step was to build a chroot environment that contained the actual binaries and libraries from a vulnerable system. I took a snapshot from a Ubuntu system on the local network. You can find the entire chroot environment here In order to generate an OpenSSH key with a specific type, bit count, and process ID, I wrote a shell script that could be executed from within the chroot environment. You can find this shell script here. This script is placed into the root directory of the extracted Ubuntu file system. In order to generate a key, this script is called with the following command line:

This will generate a new OpenSSH 1024-bit DSA key with the value of getpid() always returning the number '1'. We now have our first pre-generated SSH key. If we continue this process for all PIDs up to 32,767 and then repeat it for 2048-bit RSA keys, we have covered the valid key ranges for x86 systems running the buggy version of the OpenSSL library. With this key set, we can compromise any user account that has a vulnerable key listed in the authorized_keys file. This key set is also useful for decrypting a previously-captured SSH session, if the SSH server was using a vulnerable host key. Links to the pre-generated key sets for 1024-bit DSA and 2048-bit RSA keys (x86) are provided in the downloads section below.

The interesting thing about these keys is how they are tied to the process ID. Since most Debian-based systems use sequential process ID values (incrementing from system boot and wrapping back around as needed), the process ID of a given key can also indicate how soon from the system boot that key was generated. If we look at the inverse of that, we can determine which keys to use during a brute force based on the target we are attacking. When attempting to guess a key generated at boot time (like a SSH host key), those keys with PID values less than 200 would be the best choices for a brute force. When attacking a user-generated key, we can assume that most of the valid user keys were created with a process ID greater than 500 and less than 10,000. This optimization can significantly speed up a brute force attack on a remote user account over the SSH protocol.

In the near future, this site will be updated to include a brute force tool that can be used quickly gain access to any SSH account that allows public key authentication using a vulnerable key. The keys in the data files below use the following naming convention:

To obtain the private key file for any given public key, you need to know the key fingerprint. The easiest way to obtain this fingerprint is through the following command:

If we look at the public key in an editor, we can also infer that the key type is RSA. In order to locate the private key for this public key, we need to extract the data files, and look for a file named:

In the example above, the fingerprint is represented in hexadecimal with the colons removed, and the process ID is indicated as '26670'. If we want to authenticate to a vulnerable system that uses this public key for authentication, we would run the following command:

Frequently Asked Questions

Q: How long did it take to generate these keys?

A: I used 31 Xeon cores clocked at 2.33Ghz. It took two hours to generate the 1024-bit DSA and 2048-bit RSA keys for x86. The 4096-bit RSA keys took about 6 hours to generate. The 8192-bit RSA key generation would take about 100 hours at its current rate and will likely be stopped before completion.

Q: Will you share your code for distributing the key-generation across multiple processors?

A: Nope. The code is hardcoded for this specific cluster and is too poorly-written to be worth cleaning up.

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Q: How long does it take a crack a SSH user account using these keys?

A: This depends on the speed of the network and the configuration of the SSH server. It should be possible to try all 32,767 keys of both DSA-1024 and RSA-2048 within a couple hours, but be careful of anti-brute-force scripts on the target server.

Q: I use 16384-bit RSA keys, can these be broken?

A: Yes, its just a matter of time and processing power. The 8192-bit RSA keyset would take about 3100 hours of CPU time to generate all 32,767 keys (100 hours on the 31 cores I'm using now). I imagine the 16384-bit RSA keyset would take closer to 100,000 hours of CPU time. One thing to keep in mind is that most keys are within a much smaller range, based on the process ID seed, and the entire set would not need to be generated to cover the majority of user keys (most keys are within the first 3,000 process IDs).

Copyright © 2008 H D Moore

Related

Introduction

SSH, or secure shell, is an encrypted protocol used to administer and communicate with servers. When working with a Debian server, chances are you will spend most of your time in a terminal session connected to your server through SSH.

In this guide, we’ll focus on setting up SSH keys for a vanilla Debian 10 installation. SSH keys provide an easy, secure way of logging into your server and are recommended for all users.

Step 1 — Create the RSA Key Pair

The first step is to create a key pair on the client machine (usually your computer):

By default ssh-keygen will create a 2048-bit RSA key pair, which is secure enough for most use cases (you may optionally pass in the -b 4096 flag to create a larger 4096-bit key).

After entering the command, you should see the following output:

Press enter to save the key pair into the .ssh/ subdirectory in your home directory, or specify an alternate path.

If you had previously generated an SSH key pair, you may see the following prompt:

Warning: If you choose to overwrite the key on disk, you will not be able to authenticate using the previous key anymore. Be very careful when selecting yes, as this is a destructive process that cannot be reversed.

You should then see the following prompt:

Here you optionally may enter a secure passphrase, which is highly recommended. A passphrase adds an additional layer of security to prevent unauthorized users from logging in. To learn more about security, consult our tutorial on How To Configure SSH Key-Based Authentication on a Linux Server.

You should then see the following output:

You now have a public and private key that you can use to authenticate. The next step is to place the public key on your server so that you can use SSH-key-based authentication to log in.

Step 2 — Copy the Public Key to Debian Server

The quickest way to copy your public key to the Debian host is to use a utility called ssh-copy-id. Due to its simplicity, this method is highly recommended if available. If you do not have ssh-copy-id available to you on your client machine, you may use one of the two alternate methods provided in this section (copying via password-based SSH, or manually copying the key).

Copying Public Key Using ssh-copy-id

The ssh-copy-id tool is included by default in many operating systems, so you may have it available on your local system. For this method to work, you must already have password-based SSH access to your server.

To use the utility, you simply need to specify the remote host that you would like to connect to and the user account that you have password SSH access to. This is the account to which your public SSH key will be copied.

The syntax is:

You may see the following message:

This means that your local computer does not recognize the remote host. This will happen the first time you connect to a new host. Type “yes” and press ENTER to continue.

Next, the utility will scan your local account for the id_rsa.pub key that we created earlier. When it finds the key, it will prompt you for the password of the remote user’s account:

Type in the password (your typing will not be displayed for security purposes) and press ENTER. The utility will connect to the account on the remote host using the password you provided. It will then copy the contents of your ~/.ssh/id_rsa.pub key into a file in the remote account’s home ~/.ssh directory called authorized_keys.

Key generator download. You should see the following output:

At this point, your id_rsa.pub key has been uploaded to the remote account. You can continue on to Step 3.

Copying Public Key Using SSH

If you do not have ssh-copy-id available, but you have password-based SSH access to an account on your server, you can upload your keys using a conventional SSH method.

We can do this by using the cat command to read the contents of the public SSH key on our local computer and piping that through an SSH connection to the remote server.

On the other side, we can make sure that the ~/.ssh directory exists and has the correct permissions under the account we’re using.

We can then output the content we piped over into a file called authorized_keys within this directory. We’ll use the >> redirect symbol to append the content instead of overwriting it. This will let us add keys without destroying previously added keys.

The full command looks like this:

You may see the following message:

This means that your local computer does not recognize the remote host. This will happen the first time you connect to a new host. Type “yes” and press ENTER to continue.

Afterwards, you should be prompted to enter the remote user account password:

After entering your password, the content of your id_rsa.pub key will be copied to the end of the authorized_keys file of the remote user’s account. Continue on to Step 3 if this was successful.

Copying Public Key Manually

If you do not have password-based SSH access to your server available, you will have to complete the above process manually.

We will manually append the content of your id_rsa.pub file to the ~/.ssh/authorized_keys file on your remote machine.

To display the content of your id_rsa.pub key, type this into your local computer:

You will see the key’s content, which should look something like this:

Access your remote host using whichever method you have available.

Once you have access to your account on the remote server, you should make sure the ~/.ssh directory exists. This command will create the directory if necessary, or do nothing if it already exists:

Now, you can create or modify the authorized_keys file within this directory. You can add the contents of your id_rsa.pub file to the end of the authorized_keys file, creating it if necessary, using this command:

In the above command, substitute the public_key_string with the output from the cat ~/.ssh/id_rsa.pub command that you executed on your local system. It should start with ssh-rsa AAAA...

Finally, we’ll ensure that the ~/.ssh directory and authorized_keys file have the appropriate permissions set:

This recursively removes all “group” and “other” permissions for the ~/.ssh/ directory.

If you’re using the root account to set up keys for a user account, it’s also important that the ~/.ssh directory belongs to the user and not to root:

In this tutorial our user is named sammy but you should substitute the appropriate username into the above command.

We can now attempt passwordless authentication with our Debian server.

Step 3 — Authenticate to Debian Server Using SSH Keys

If you have successfully completed one of the procedures above, you should be able to log into the remote host without the remote account’s password.

The basic process is the same:

If this is your first time connecting to this host (if you used the last method above), you may see something like this:

This means that your local computer does not recognize the remote host. Type “yes” and then press ENTER to continue.

If you did not supply a passphrase for your private key, you will be logged in immediately. If you supplied a passphrase for the private key when you created the key, you will be prompted to enter it now (note that your keystrokes will not display in the terminal session for security). After authenticating, a new shell session should open for you with the configured account on the Debian server.

If key-based authentication was successful, continue on to learn how to further secure your system by disabling password authentication.

Step 4 — Disable Password Authentication on your Server

If you were able to log into your account using SSH without a password, you have successfully configured SSH-key-based authentication to your account. However, your password-based authentication mechanism is still active, meaning that your server is still exposed to brute-force attacks.

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Before completing the steps in this section, make sure that you either have SSH-key-based authentication configured for the root account on this server, or preferably, that you have SSH-key-based authentication configured for a non-root account on this server with sudo privileges. This step will lock down password-based logins, so ensuring that you will still be able to get administrative access is crucial.

Once you’ve confirmed that your remote account has administrative privileges, log into your remote server with SSH keys, either as root or with an account with sudo privileges. Then, open up the SSH daemon’s configuration file:

Inside the file, search for a directive called PasswordAuthentication. This may be commented out. Uncomment the line and set the value to “no”. This will disable your ability to log in via SSH using account passwords:

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Save and close the file when you are finished by pressing CTRL + X, then Y to confirm saving the file, and finally ENTER to exit nano. To actually implement these changes, we need to restart the sshd service:

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As a precaution, open up a new terminal window and test that the SSH service is functioning correctly before closing this session:

Once you have verified your SSH service, you can safely close all current server sessions.

The SSH daemon on your Debian server now only responds to SSH keys. Password-based authentication has successfully been disabled.

Conclusion

You should now have SSH-key-based authentication configured on your server, allowing you to sign in without providing an account password.

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If you’d like to learn more about working with SSH, take a look at our SSH Essentials Guide.