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-*- org -*-

* Mandos
  - Have your cake and eat it too!
  
  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 it to get the password, use it to decrypt
  the root file, and 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.)

* FAQ - 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 OpenPGP
   private key corresponding to that client.

** 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 reboots at the same time they will
   stay down until someone types in the password on one of them.

** Faking ping replies?
   The default for the server is 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.
   It could, for instance, be changed to an SSH command with strict
   keychecking, which could not be faked.  Or IPsec could be used for
   the ping packets, making them secure.

* Security Summary
  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.

* Copyright

    Copyright © 2008 Teddy Hogeborn
                2008 Björn Påhlsson
  
** License:
   
   This program 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.

   This program 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 this program.  If not, see
   <http://www.gnu.org/licenses/>.