- IT, Information Security and Software Dev
- пятница, 23 августа 2013 г.
- What Is ssh-keygen?
- SSH Keys and Public Key Authentication
- Creating an SSH Key Pair for User Authentication
- Copying the Public Key to the Server
- Adding the Key to SSH Agent
- Creating Host Keys
- Using X.509 Certificates for Host Authentication
- Using OpenSSH’s Proprietary Certificates
- Key Management Requires Attention
- Make Sure There Is Enough Randomness
- Command and Option Summary
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IT, Information Security and Software Dev
Using SSH is a great way to remotely manage a server and to securely transfer data to and from it. You basically connect using SSH with your username and password. In that case you authenticate with something you know, which in this case is the password.
But you can also authenticate with something you have, like an SSH key. You can use this to authenticate without a password, and even configure the server to only allow SSH keys to prevent password attacks.
This tutorial shows how to configure the SSH server, generate SSH keys for each user and optionally disable password logins.
Verify that the server has a host key
On Linux, check if either the DSA host key file /etc/ssh/ssh_host_dsa_key or the RSA host key file /etc/ssh/ssh_host_rsa_key exists. If one or the other exists, you can only use that kind of key to connect to SSH. If you really want both, you can generate each like this (as root):
Of course you can use different file names if you like.
After settings these up, edit the SSHD config file (sshd_config) and make sure that there’s a HostKey item for each of these host keys and that these items are uncommented (don’t have a #-character in front of the line).
Also make sure that RSAAuthentication and PubKeyAuthentication are set to yes and are uncommented:
For security reasons you must make sure that the host key files (NOT the public key files with .pub in their names) are only readable and writable by root. These PRIVATE keys should be kept secret from anyone else. You can fix that with CHMOD:
Save the SSHD configuration file and restart the SSH Server. Consult your OS to do this. On Linux this is usually done with either:
Now make sure that the home folder of the user account you wish to login has a .ssh folder. Log on as the specific user, and then run:
If it gives an error about directory already exists, then you can just ignore that message. it just means that the folder is already there.
Generating an SSH keypair
Now you should generate an SSH keypair for each computer that should connect to the server. If that computer runs on Linux, BSD or Mac OS X you can do that directly from that computer. Otherwise you need to generate it using 3rd party tools or on a different Linux box and then move the keys to the specific computer afterwards.
First of all, make sure that you are logged in with the user account you wish to use when connecting to the server. Again, verify that the .ssh folder exists in your home folder.
Now run the following command in a terminal for an RSA keypair (replace RSA with DSA for a DSA keypair):
This will create two files in your .ssh folder: id_rsa (your private key) and id_rsa.pub (your public key). You should carefully guard your private key (make it read/write for your user account only (chmod 600)). Your public key should be stored on the server. We get to that in a minute.
If you are using Microsoft Windows, you should use a 3rd party utility or install the Cygwin UNIX layer. In case you would decide to generate the keypair on a different system, make sure that you import the PRIVATE key in your SSH application and REMOVE it from the original system where you’ve generated it.
Importing your public key
Your goal here is to save the contents of your public key into the file .ssh/authorized_keys in the home folder of the user account you wish to logon on the server. If the file doesn’t exist, you can create it. If you have an SFTP client with a file manager, this should not be a problem.
As an alternative, you can use SCP:
This copies your public key to the home folder of the server user account using the file name public_key. Now log on to the server using SSH. Enter the following command to add the contents of your public key to the authorized_hosts file:
You can now remove the public key you’ve uploaded to the server, as it’s no longer neccesary:
Tip: If you have to manage many public keys for one user, you can add information to each line of authorized_keys like the name of the computer using comments, like this:
Disable password logins
At this point you can login to SSH using either your password or your private key. You can still login using a password if you don’t have your private key at hand. If you want to disable password logons, follow these steps.
Before you continue, make sure that you can log on to SSH without the need of a password. If you disable password authentication and your keys aren’t working, this will render SSH useless and require you to access the server in a different way. That said, make sure to secure your private key. If all private keys are lost, you’ll end up in the same scenario.
Edit your SSHD config file as root (sshd_config) and set the line PasswordAuthentication to no, and make sure it’s uncommented:
Restart your SSH Server to save your settings.
If you are having issues, make sure to note any error messages you may receive when asking for help. Some tips:
– All files in your home folder should be owned by you (chown -R username
/), and only be accessible to you (chmod 700
/.ssh). All other files should be owned by root and root only.
– The server should have the SSH Server AND Client installed, while the clients only need to have the SSH Client installed.
– This guide is targeted for users of Linux, BSD and Mac OS X. Other platforms like Microsoft Windows, iOS, Android etc. need to use 3rd party tools for an SSH Client and/or SSH Server, but the basics apply here as well.
– Key authentication is a great solution when using SSH connections in automated scripts. We don’t want to store a password in plain text on a system, now do we? 😉
Дядя, дядя, наши сети, притащили.
пятница, 23 августа 2013 г.
Удалить устаревшие ключи SSH, управление ключами
/.ssh/known_hosts . Узнать, где какой ключ нельзя (ибо несекьюрно).
Ключ сервера хранится
в /etc/ssh/ssh_host_rsa_key и /etc/ssh/ssh_host_rsa_key.pub . Их можно:
а) скопировать со старого сервера на новый.
б) сгенерировать с помощью ssh-keygen. Пароля при этом задавать не надо (т.е. пустой). Ключ с паролем ssh-сервер использовать не сможет.
This page is about the OpenSSH version of `ssh-keygen`. For Tectia SSH, see here. If you wish to generate keys for PuTTY, see PuTTYgen on Windows or PuTTYgen on Linux.
What Is ssh-keygen?
Ssh-keygen is a tool for creating new authentication key pairs for SSH. Such key pairs are used for automating logins, single sign-on, and for authenticating hosts.
SSH Keys and Public Key Authentication
The SSH protocol uses public key cryptography for authenticating hosts and users. The authentication keys, called SSH keys, are created using the keygen program.
SSH introduced public key authentication as a more secure alternative to the older .rhosts authentication. It improved security by avoiding the need to have password stored in files, and eliminated the possibility of a compromised server stealing the user’s password.
However, SSH keys are authentication credentials just like passwords. Thus, they must be managed somewhat analogously to user names and passwords. They should have a proper termination process so that keys are removed when no longer needed.
Creating an SSH Key Pair for User Authentication
The simplest way to generate a key pair is to run ssh-keygen without arguments. In this case, it will prompt for the file in which to store keys. Here’s an example:
First, the tool asked where to save the file. SSH keys for user authentication are usually stored in the user’s .ssh directory under the home directory. However, in enterprise environments, the location is often different. The default key file name depends on the algorithm, in this case id_rsa when using the default RSA algorithm. It could also be, for example, id_dsa or id_ecdsa .
Then it asks to enter a passphrase. The passphrase is used for encrypting the key, so that it cannot be used even if someone obtains the private key file. The passphrase should be cryptographically strong. Our online random password generator is one possible tool for generating strong passphrases.
Choosing an Algorithm and Key Size
SSH supports several public key algorithms for authentication keys. These include:
- rsa — an old algorithm based on the difficulty of factoring large numbers. A key size of at least 2048 bits is recommended for RSA; 4096 bits is better. RSA is getting old and significant advances are being made in factoring. Choosing a different algorithm may be advisable. It is quite possible the RSA algorithm will become practically breakable in the foreseeable future. All SSH clients support this algorithm.
- dsa — an old US government Digital Signature Algorithm. It is based on the difficulty of computing discrete logarithms. A key size of 1024 would normally be used with it. DSA in its original form is no longer recommended.
- ecdsa — a new Digital Signature Algorithm standarized by the US government, using elliptic curves. This is probably a good algorithm for current applications. Only three key sizes are supported: 256, 384, and 521 (sic!) bits. We would recommend always using it with 521 bits, since the keys are still small and probably more secure than the smaller keys (even though they should be safe as well). Most SSH clients now support this algorithm.
- ed25519 — this is a new algorithm added in OpenSSH. Support for it in clients is not yet universal. Thus its use in general purpose applications may not yet be advisable.
The algorithm is selected using the -t option and key size using the -b option. The following commands illustrate:
Specifying the File Name
Normally, the tool prompts for the file in which to store the key. However, it can also be specified on the command line using the -f option.
Copying the Public Key to the Server
To use public key authentication, the public key must be copied to a server and installed in an authorized_keys file. This can be conveniently done using the ssh-copy-id tool. Like this:
Once the public key has been configured on the server, the server will allow any connecting user that has the private key to log in. During the login process, the client proves possession of the private key by digitally signing the key exchange.
Adding the Key to SSH Agent
ssh-agent is a program that can hold a user’s private key, so that the private key passphrase only needs to be supplied once. A connection to the agent can also be forwarded when logging into a server, allowing SSH commands on the server to use the agent running on the user’s desktop.
For more information on using and configuring the SSH agent, see the ssh-agent page.
Creating Host Keys
The tool is also used for creating host authentication keys. Host keys are stored in the /etc/ssh/ directory.
Host keys are just ordinary SSH key pairs. Each host can have one host key for each algorithm. The host keys are almost always stored in the following files:
The host keys are usually automatically generated when an SSH server is installed. They can be regenerated at any time. However, if host keys are changed, clients may warn about changed keys. Changed keys are also reported when someone tries to perform a man-in-the-middle attack. Thus it is not advisable to train your users to blindly accept them. Changing the keys is thus either best done using an SSH key management tool that also changes them on clients, or using certificates.
Using X.509 Certificates for Host Authentication
OpenSSH does not support X.509 certificates. Tectia SSH does support them. X.509 certificates are widely used in larger organizations for making it easy to change host keys on a period basis while avoiding unnecessary warnings from clients. They also allow using strict host key checking, which means that the clients will outright refuse a connection if the host key has changed.
Using OpenSSH’s Proprietary Certificates
OpenSSH has its own proprietary certificate format, which can be used for signing host certificates or user certificates. For user authentication, the lack of highly secure certificate authorities combined with the inability to audit who can access a server by inspecting the server makes us recommend against using OpenSSH certificates for user authentication.
However, OpenSSH certificates can be very useful for server authentication and can achieve similar benefits as the standard X.509 certificates. However, they need their own infrastructure for certificate issuance. See more information on certificate authentication.
Key Management Requires Attention
It is easy to create and configure new SSH keys. In the default configuration, OpenSSH allows any user to configure new keys. The keys are permanent access credentials that remain valid even after the user’s account has been deleted.
In organizations with more than a few dozen users, SSH keys easily accumulate on servers and service accounts over the years. We have seen enterprises with several million keys granting access to their production servers. It only takes one leaked, stolen, or misconfigured key to gain access.
In any larger organization, use of SSH key management solutions is almost necessary. SSH keys should also be moved to root-owned locations with proper provisioning and termination processes. For more information, see how to manage SSH keys. A widely used SSH key management tool for OpenSSH is Universal SSH Key Manager.
Practically all cybersecurity regulatory frameworks require managing who can access what. SSH keys grant access, and fall under this requirement. This, organizations under compliance mandates are required to implement proper management processes for the keys. NIST IR 7966 is a good starting point.
Make Sure There Is Enough Randomness
It is important to ensure there is enough unpredictable entropy in the system when SSH keys are generated. There have been incidents when thousands of devices on the Internet have shared the same host key when they were improperly configured to generate the key without proper randomness.
General Purpose Systems
On general purpose computers, randomness for SSH key generation is usually not a problem. It may be something of an issue when initially installing the SSH server and generating host keys, and only people building new Linux distributions or SSH installation packages generally need to worry about it.
Our recommendation is to collect randomness during the whole installation of the operating system, save that randomness in a random seed file. Then boot the system, collect some more randomness during the boot, mix in the saved randomness from the seed file, and only then generate the host keys. This maximizes the use of the available randomness. And make sure the random seed file is periodically updated, in particular make sure that it is updated after generating the SSH host keys.
Many modern general-purpose CPUs also have hardware random number generators. This helps a lot with this problem. The best practice is to collect some entropy in other ways, still keep it in a random seed file, and mix in some entropy from the hardware random number generator. This way, even if one of them is compromised somehow, the other source of randomness should keep the keys secure.
Embedded Devices and Internet of Things
Available entropy can be a real problem on small IoT devices that don’t have much other activity on the system. They may just not have the mechanical randomness from disk drive mechanical movement timings, user-caused interrupts, or network traffic. Furthermore, embedded devices often run on low-end processors that may not have a hardware random number generator.
The availability of entropy is also critically important when such devices generate keys for HTTPS.
Our recommendation is that such devices should have a hardware random number generator. If the CPU does not have one, it should be built onto the motherboard. The cost is rather small.
Command and Option Summary
Here’s a summary of commonly used options to the keygen tool:
This option specifies the number of bits in the key. The regulations that govern the use case for SSH may require a specific key length to be used. In general, 2048 bits is considered to be sufficient for RSA keys.
This option allows reformatting of existing keys between the OpenSSH key file format and the format documented in RFC 4716, “SSH Public Key File Format”.
-p “Change the passphrase” This option allows changing the passphrase of a private key file with [-P old_passphrase] and [-N new_passphrase], [-f keyfile].
-t “Type” This option specifies the type of key to be created. Commonly used values are: — rsa for RSA keys — dsa for DSA keys — ecdsa for elliptic curve DSA keys
-i Input When ssh-keygen is required to access an existing key, this option designates the file.
-f File Specifies name of the file in which to store the created key.
-N New Provides a new passphrase for the key.
-P Passphrase Provides the (old) passphrase when reading a key.
-c Comment Changes the comment for a keyfile.
-p Change the passphrase of a private key file.
-q Silence ssh-keygen.
-v Verbose mode.
-l Fingerprint Print the fingerprint of the specified public key.
-B Bubble babble Shows a bubble babble (Tectia format) fingerprint of a keyfile.
-F Search for a specified hostname in a known_hosts file.
-R Remove all keys belonging to a hostname from a known_hosts file.
-y Read a private OpenSSH format file and print an OpenSSH public key to stdout.
This only listed the most commonly used options. For full usage, including the more exotic and special-purpose options, use the man ssh-keygen command.
Alert on SSH Keys
By Tatu Ylonen, Inventor of SSH