Ubuntu Manpage: intro - Introduction to the Mandos system

NAME

       intro - Introduction to the Mandos system

DESCRIPTION

       This is the the Mandos system, which allows computers to have encrypted root file systems
       and at the same time be capable of remote and/or unattended reboots.

       The computers run a small client program in the initial RAM disk environment which will
       communicate with a server over a network. All network communication is encrypted using
       TLS. The clients are identified by the server using a TLS public key; each client has one
       unique to it. The server sends the clients an encrypted password. The encrypted password
       is decrypted by the clients using a separate OpenPGP key, and the password is then used to
       unlock the root file system, whereupon the computers can continue booting normally.

INTRODUCTION

You know how it is. You’ve heard of it happening. The Man comes and takes away your
servers, your friends’ servers, the servers of everybody in the same hosting facility. The
servers of their neighbors, and their neighbors’ friends. The servers of people who owe
them money. And like that, they’re gone. And you doubt you’ll ever see them again.

That is why your servers have encrypted root file systems. However, there’s a downside.
There’s no going around it: rebooting is a pain. Dragging out that rarely-used keyboard
and screen and unraveling cables behind your servers to plug them in to type in that
password is messy, especially if you have many servers. There are some people who do
clever things like using serial line consoles and daisy-chain it to the next server, and
keep all the servers connected in a ring with serial cables, which will work, if your
servers are physically close enough. There are also other out-of-band management
solutions, but with all these, you still have to be on hand and manually type in the
password at boot time. Otherwise the server just sits there, waiting for a password.

Wouldn’t it be great if you could have the security of encrypted root file systems and
still have servers that could boot up automatically if there was a short power outage
while you were asleep? That you could reboot at will, without having someone run over to
the server to type in the password?

Well, with Mandos, you (almost) can! The gain in convenience will only be offset by a
small loss in security. The setup is as follows:

The server will still have its encrypted root file system. The password to this file
system will be stored on another computer (henceforth known as the Mandos server) on the
same local network. The password will not be stored in plaintext, but encrypted with
OpenPGP. To decrypt this password, a key is needed. This key (the Mandos client key) will
not be stored there, but back on the original server (henceforth known as the Mandos
client) in the initial RAM disk image. Oh, and all network Mandos client/server
communications will be encrypted, using TLS (SSL).

So, at boot time, the Mandos client will ask for its encrypted data over the network,
decrypt the data to get the password, use the password to decrypt the root file system,
and the client can then continue booting.

Now, of course the initial RAM disk image is not on the encrypted root file system, so
anyone who had physical access could take the Mandos client computer offline and read the
disk with their own tools to get the authentication keys used by a client. But, by then
the Mandos server should notice that the original server has been offline for too long,
and will no longer give out the encrypted key. The timing here is the only real weak
point, and the method, frequency and timeout of the server’s checking can be adjusted to
any desired level of paranoia.

(The encrypted keys on the Mandos server is on its normal file system, so those are safe,
provided the root file system of that server is encrypted.)

FREQUENTLY ASKED QUESTIONS

Couldn’t the security be defeated by...

Grabbing the Mandos client key from the initrd really quickly?
This, as mentioned above, is the only real weak point. But if you set the timing values
tight enough, this will be really difficult to do. An attacker would have to physically
disassemble the client computer, extract the key from the initial RAM disk image, and then
connect to a still online Mandos server to get the encrypted key, and do all this before
the Mandos server timeout kicks in and the Mandos server refuses to give out the key to
anyone.

Now, as the typical procedure seems to be to barge in and turn off and grab all computers,
to maybe look at them months later, this is not likely. If someone does that, the whole
system will lock itself up completely, since Mandos servers are no longer running.

For sophisticated attackers who could do the clever thing, and had physical access to the
server for enough time, it would be simpler to get a key for an encrypted file system by
using hardware memory scanners and reading it right off the memory bus.

Replay attacks?
Nope, the network stuff is all done over TLS, which provides protection against that.

Man-in-the-middle?
No. The server only gives out the passwords to clients which have in the TLS handshake
proven that they do indeed hold the private key corresponding to that client.

How about sniffing the network traffic and decrypting it later by physically grabbing the
Mandos client and using its key?
We only use PFS (Perfect Forward Security) key exchange algorithms in TLS, which protects
against this.

Physically grabbing the Mandos server computer?
You could protect that computer the old-fashioned way, with a
must-type-in-the-password-at-boot method. Or you could have two computers be the Mandos
server for each other.

Multiple Mandos servers can coexist on a network without any trouble. They do not clash,
and clients will try all available servers. This means that if just one reboots then the
other can bring it back up, but if both reboot at the same time they will stay down until
someone types in the password on one of them.

Faking checker results?
If the Mandos client does not have an SSH server, the default is for the Mandos server to
use “fping”, the replies to which could be faked to eliminate the timeout. But this could
easily be changed to any shell command, with any security measures you like. If the Mandos
client has an SSH server, the default configuration (as generated by mandos-keygen with
the --password option) is for the Mandos server to use an ssh-keyscan command with strict
keychecking, which can not be faked. Alternatively, IPsec could be used for the ping
packets, making them secure.

SECURITY

       So, in summary: The only weakness in the Mandos system is from people who have:

        1. The power to come in and physically take your servers, and

        2. The cunning and patience to do it carefully, one at a time, and quickly, faking Mandos
           client/server responses for each one before the timeout.

       While there are some who may be threatened by people who have both these attributes, they
       do not, probably, constitute the majority.

       If you do face such opponents, you must figure that they could just as well open your
       servers and read the file system keys right off the memory by running wires to the memory
       bus.

       What Mandos is designed to protect against is not such determined, focused, and competent
       attacks, but against the early morning knock on your door and the sudden absence of all
       the servers in your server room. Which it does nicely.

PLUGINS

       In the early designs, the mandos-client(8mandos) program (which retrieves a password from
       the Mandos server) also prompted for a password on the terminal, in case a Mandos server
       could not be found. Other ways of retrieving a password could easily be envisoned, but
       this multiplicity of purpose was seen to be too complex to be a viable way to continue.
       Instead, the original program was separated into mandos-client(8mandos) and password-
       prompt(8mandos), and a plugin-runner(8mandos) exist to run them both in parallel, allowing
       the first successful plugin to provide the password. This opened up for any number of
       additional plugins to run, all competing to be the first to find a password and provide it
       to the plugin runner.

       Four additional plugins are provided:

       plymouth(8mandos)
           This prompts for a password when using plymouth(8).

       usplash(8mandos)
           This prompts for a password when using usplash(8).

       splashy(8mandos)
           This prompts for a password when using splashy(8).

       askpass-fifo(8mandos)
           To provide compatibility with the "askpass" program from cryptsetup, this plugin
           listens to the same FIFO as askpass would do.

       More plugins can easily be written and added by the system administrator; see the section
       called "WRITING PLUGINS" in plugin-runner(8mandos) to learn the plugin requirements.

SYSTEMD

       More advanced startup systems like systemd(1), already have their own plugin-like
       mechanisms for allowing multiple agents to independently retrieve a password and deliver
       it to the subsystem requesting a password to unlock the root file system. On these
       systems, it would make no sense to run plugin-runner(8mandos), the plugins of which would
       largely duplicate the work of (and conflict with) the existing systems prompting for
       passwords.

       As for systemd(1) in particular, it has its own Password Agents[1] system. Mandos uses
       this via its password-agent(8mandos) program, which is run instead of plugin-
       runner(8mandos) when systemd(1) is used during system startup.

BUGS

       Please report bugs to the Mandos development mailing list: <mandos-dev@recompile.se>
       (subscription required). Note that this list is public. The developers can be reached
       privately at <mandos@recompile.se> (OpenPGP key fingerprint 153A 37F1 0BBA 0435 987F 2C4A
       7223 2973 CA34 C2C4 for encrypted mail).

COPYRIGHT

       Copyright © 2011-2020 Teddy Hogeborn, Björn Påhlsson

       This manual page is part of Mandos.

       Mandos is free software: you can redistribute it and/or modify it under the terms of the
       GNU General Public License as published by the Free Software Foundation, either version 3
       of the License, or (at your option) any later version.

       Mandos is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
       without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU General Public License for more details.

       You should have received a copy of the GNU General Public License along with Mandos. If
       not, see http://www.gnu.org/licenses/.

NOTES

        1. Password Agents
           https://systemd.io/PASSWORD_AGENTS/

        2. Mandos
           https://www.recompile.se/mandos