/mandos/release

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152 by Teddy Hogeborn
* README: New file.
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-*- org -*-
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* Mandos
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  - Have your cake and eat it too!
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  You know how it is.  You've heard of it happening.  The Man comes
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  and takes away your servers, your friends' servers, the servers of
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  everybody in the same hosting facility. The servers of their
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  neighbors, and their neighbors' friends.  The servers of people who
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  owe them money.  And like *that*, they're gone.  And you doubt
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  you'll ever see them again.
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  That is why your servers have encrypted root file systems.  However,
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  there's a downside.  There's no going around it: rebooting is a
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  pain.  Dragging out that rarely-used keyboard and screen and
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  unraveling cables behind your servers to plug them in to type in
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  that password is messy, especially if you have many servers.  There
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  are some people who do clever things like using serial line consoles
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  and daisy-chain it to the next server, and keep all the servers
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  connected in a ring with serial cables, which will work, if your
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  servers are physically close enough.  There are also other
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  out-of-band management solutions, but with *all* these, you still
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  have to be on hand and manually type in the password at boot time.
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  Otherwise the server just sits there, waiting for a password.
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  Wouldn't it be great if you could have the security of encrypted
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  root file systems and still have servers that could boot up
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  automatically if there was a short power outage while you were
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  asleep?  That you could reboot at will, without having someone run
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  over to the server to type in the password?
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  Well, with Mandos, you (almost) can!  The gain in convenience will
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  only be offset by a small loss in security.  The setup is as
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  follows:
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  The server will still have its encrypted root file system.  The
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  password to this file system will be stored on another computer
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  (henceforth known as the Mandos server) on the same local network.
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  The password will *not* be stored in plaintext, but encrypted with
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  OpenPGP.  To decrypt this password, a key is needed.  This key (the
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  Mandos client key) will not be stored there, but back on the
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  original server (henceforth known as the Mandos client) in the
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  initial RAM disk image.  Oh, and all network Mandos client/server
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  communications will be encrypted, using TLS (SSL).
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  So, at boot time, the Mandos client will ask for its encrypted data
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  over the network, decrypt it to get the password, use it to decrypt
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  the root file, and continue booting.
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  Now, of course the initial RAM disk image is not on the encrypted
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  root file system, so anyone who had physical access could take the
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  server offline and read the disk with their own tools to get the
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  authentication keys used by a client.  *But*, by then the Mandos
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  server should notice that the original server has been offline for
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  too long, and will no longer give out the encrypted key.  The timing
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  here is the only real weak point, and the method, frequency and
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  timeout of the server’s checking can be adjusted to any desired
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  level of paranoia
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  (The encrypted keys on the Mandos server is on its normal file
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  system, so those are safe, provided the root file system of *that*
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  server is encrypted.)
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* FAQ - couldn’t the security be defeated by...
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** Grabbing the Mandos client key from the initrd *really quickly*?
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   This, as mentioned above, is the only real weak point.  But if you
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   set the timing values tight enough, this will be really difficult
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   to do.  An attacker would have to physically disassemble the client
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   computer, extract the key from the initial RAM disk image, and then
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   connect to a *still online* Mandos server to get the encrypted key,
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   all *before* the Mandos server timeout kicks in and the Mandos
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   server refuses to give out the key to anyone.
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   Now, as the typical SOP seems to be to barge in and turn off and
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   grab *all* computers, to maybe look at them months later, this is
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   not likely.  If someone does that, the whole system *will* lock
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   itself up completely, since Mandos servers are no longer running.
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   For sophisticated attackers who *could* do such a thing, *and* had
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   physical access to the server for enough time, it would be simpler
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   to get a key for an encrypted file system by using hardware memory
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   scanners and reading it right off the memory bus.
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** Replay attacks?
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   Nope, the network stuff is all done over TLS, which provides
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   protection against that.
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** Man-in-the-middle?
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   No.  The server only gives out the passwords to clients which have
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   *in the TLS handshake* proven that they do indeed hold the OpenPGP
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   private key corresponding to that client.
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** Physically grabbing the Mandos server computer?
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   You could protect *that* computer the old-fashioned way, with a
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   must-type-in-the-password-at-boot method.  Or you could have two
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   computers be the Mandos server for each other.
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   Multiple Mandos servers can coexist on a network without any
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   trouble.  They do not clash, and clients will try all available
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   servers.  This means that if just one reboots then the other can
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   bring it back up, but if both reboots at the same time they will
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   stay down until someone types in the password on one of them.
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** Faking ping replies?
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   The default for the server is to use "fping", the replies to which
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   could be faked to eliminate the timeout.  But this could easily be
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   changed to any shell command, with any security measures you like.
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   It could, for instance, be changed to an SSH command with strict
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   keychecking, which could not be faked.  Or IPsec could be used for
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   the ping packets, making them secure.
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* Security Summary
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  So, in summary:  The only weakness in the Mandos system is from
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  people who have:
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  1. The power to come in and physically take your servers, *and*
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  2. The cunning and patience to do it carefully, one at a time, and
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     *quickly*, faking Mandos client/server responses for each one
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     before the timeout.
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  While there are some who may be threatened by people who have *both*
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  these attributes, they do not, probably, constitute the majority.
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  If you *do* face such opponents, you must figure that they could
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  just as well open your servers and read the file system keys right
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  off the memory by running wires to the memory bus.
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  What this system is designed to protect against is *not* such
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  determined, focused, and competent attacks, but against the early
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  morning knock on your door and the sudden absence of all the servers
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  in your server room.  Which it does nicely.