/mandos/trunk

To get this branch, use:
bzr branch http://bzr.recompile.se/loggerhead/mandos/trunk
152 by Teddy Hogeborn
* README: New file.
1
-*- org -*-
2
3
* Mandos
4
  - Have your cake and eat it too!
5
  
6
  You know how it is.  You've heard of it happening.  The Man comes
7
  and takes away your servers, your friends' servers, the servers of
8
  everybody in the same hosting facility. The servers of their
9
  neighbors, and their neighbors' friends.  The servers of people who
10
  owe them money.  And like *that*, they're gone.  And you doubt
11
  you'll ever see them again.
12
  
13
  That is why your servers have encrypted root file systems.  However,
14
  there's a downside.  There's no going around it: rebooting is a
15
  pain.  Dragging out that rarely-used keyboard and screen and
16
  unraveling cables behind your servers to plug them in to type in
17
  that password is messy, especially if you have many servers.  There
18
  are some people who do clever things like using serial line consoles
19
  and daisy-chain it to the next server, and keep all the servers
20
  connected in a ring with serial cables, which will work, if your
21
  servers are physically close enough.  There are also other
22
  out-of-band management solutions, but with *all* these, you still
23
  have to be on hand and manually type in the password at boot time.
24
  Otherwise the server just sits there, waiting for a password.
25
  
26
  Wouldn't it be great if you could have the security of encrypted
27
  root file systems and still have servers that could boot up
28
  automatically if there was a short power outage while you were
29
  asleep?  That you could reboot at will, without having someone run
30
  over to the server to type in the password?
31
  
32
  Well, with Mandos, you (almost) can!  The gain in convenience will
33
  only be offset by a small loss in security.  The setup is as
34
  follows:
35
  
36
  The server will still have its encrypted root file system.  The
37
  password to this file system will be stored on another computer
38
  (henceforth known as the Mandos server) on the same local network.
39
  The password will *not* be stored in plaintext, but encrypted with
40
  OpenPGP.  To decrypt this password, a key is needed.  This key (the
41
  Mandos client key) will not be stored there, but back on the
42
  original server (henceforth known as the Mandos client) in the
43
  initial RAM disk image.  Oh, and all network Mandos client/server
44
  communications will be encrypted, using TLS (SSL).
45
  
46
  So, at boot time, the Mandos client will ask for its encrypted data
47
  over the network, decrypt it to get the password, use it to decrypt
48
  the root file, and continue booting.
49
  
50
  Now, of course the initial RAM disk image is not on the encrypted
154 by Teddy Hogeborn
* README: Improved spelling.
51
  root file system, so anyone who had physical access could take the
153 by Teddy Hogeborn
* README: Improved wording.
52
  server offline and read the disk with their own tools to get the
53
  authentication keys used by a client.  *But*, by then the Mandos
54
  server should notice that the original server has been offline for
55
  too long, and will no longer give out the encrypted key.  The timing
56
  here is the only real weak point, and the method, frequency and
57
  timeout of the server’s checking can be adjusted to any desired
58
  level of paranoia
152 by Teddy Hogeborn
* README: New file.
59
  
60
  (The encrypted keys on the Mandos server is on its normal file
153 by Teddy Hogeborn
* README: Improved wording.
61
  system, so those are safe, provided the root file system of *that*
152 by Teddy Hogeborn
* README: New file.
62
  server is encrypted.)
63
64
* FAQ - couldn’t the security be defeated by...
65
66
** Grabbing the Mandos client key from the initrd *really quickly*?
67
   This, as mentioned above, is the only real weak point.  But if you
68
   set the timing values tight enough, this will be really difficult
69
   to do.  An attacker would have to physically disassemble the client
70
   computer, extract the key from the initial RAM disk image, and then
71
   connect to a *still online* Mandos server to get the encrypted key,
72
   all *before* the Mandos server timeout kicks in and the Mandos
73
   server refuses to give out the key to anyone.
74
   
153 by Teddy Hogeborn
* README: Improved wording.
75
   Now, as the typical SOP seems to be to barge in and turn off and
76
   grab *all* computers, to maybe look at them months later, this is
77
   not likely.  If someone does that, the whole system *will* lock
78
   itself up completely, since Mandos servers are no longer running.
79
   
80
   For sophisticated attackers who *could* do such a thing, *and* had
81
   physical access to the server for enough time, it would be simpler
82
   to get a key for an encrypted file system by using hardware memory
83
   scanners and reading it right off the memory bus.
152 by Teddy Hogeborn
* README: New file.
84
85
** Replay attacks?
86
   Nope, the network stuff is all done over TLS, which provides
87
   protection against that.
88
89
** Man-in-the-middle?
90
   No.  The server only gives out the passwords to clients which have
91
   *in the TLS handshake* proven that they do indeed hold the OpenPGP
92
   private key corresponding to that client.
93
94
** Physically grabbing the Mandos server computer?
95
   You could protect *that* computer the old-fashioned way, with a
96
   must-type-in-the-password-at-boot method.  Or you could have two
153 by Teddy Hogeborn
* README: Improved wording.
97
   computers be the Mandos server for each other.
98
   
99
   Multiple Mandos servers can coexist on a network without any
100
   trouble.  They do not clash, and clients will try all available
101
   servers.  This means that if just one reboots then the other can
102
   bring it back up, but if both reboots at the same time they will
103
   stay down until someone types in the password on one of them.
152 by Teddy Hogeborn
* README: New file.
104
105
** Faking ping replies?
106
   The default for the server is to use "fping", the replies to which
107
   could be faked to eliminate the timeout.  But this could easily be
108
   changed to any shell command, with any security measures you like.
109
   It could, for instance, be changed to an SSH command with strict
110
   keychecking, which could not be faked.  Or IPsec could be used for
111
   the ping packets, making them secure.
112
113
* Security Summary
114
  So, in summary:  The only weakness in the Mandos system is from
115
  people who have:
153 by Teddy Hogeborn
* README: Improved wording.
116
  1. The power to come in and physically take your servers, *and*
152 by Teddy Hogeborn
* README: New file.
117
  2. The cunning and patience to do it carefully, one at a time, and
118
     *quickly*, faking Mandos client/server responses for each one
119
     before the timeout.
120
  
121
  While there are some who may be threatened by people who have *both*
122
  these attributes, they do not, probably, constitute the majority.
123
  
124
  If you *do* face such opponents, you must figure that they could
153 by Teddy Hogeborn
* README: Improved wording.
125
  just as well open your servers and read the file system keys right
126
  off the memory by running wires to the memory bus.
152 by Teddy Hogeborn
* README: New file.
127
  
128
  What this system is designed to protect against is *not* such
129
  determined, focused, and competent attacks, but against the early
153 by Teddy Hogeborn
* README: Improved wording.
130
  morning knock on your door and the sudden absence of all the servers
131
  in your server room.  Which it does nicely.