/mandos/trunk : revision 152

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

51

root file system, so anyone who would come and take the the whole

52

computer would have the Mandos client key when they took the server

53

offline and read the disk with their own tools. *But*, by then the

54

Mandos server will have detected that the original server is no

55

longer online and will no longer give out the encrypted key. The

56

timing here is the only real weak point, and the method, frequency

57

and timeout of checking can be adjusted to any desired level of

58

paranoia.

59

60

(The encrypted keys on the Mandos server is on its normal file

61

system, so those are safe, provided the root file system of that

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

75

As the typical SOP seems to be to barge in and turn off and grab

76

*all* computers to maybe look at them months later, this is not

77

likely.

78

79

** Replay attacks?

80

Nope, the network stuff is all done over TLS, which provides

81

protection against that.

82

83

** Man-in-the-middle?

84

No. The server only gives out the passwords to clients which have

85

*in the TLS handshake* proven that they do indeed hold the OpenPGP

86

private key corresponding to that client.

87

88

** Physically grabbing the Mandos server computer?

89

You could protect *that* computer the old-fashioned way, with a

90

must-type-in-the-password-at-boot method. Or you could have two

91

computers be the Mandos server for each other. (Multiple Mandos

92

servers can coexist on a network without any trouble. They do not

93

clash, and clients will try all available servers.)

94

95

** Faking ping replies?

96

The default for the server is to use "fping", the replies to which

97

could be faked to eliminate the timeout. But this could easily be

98

changed to any shell command, with any security measures you like.

99

It could, for instance, be changed to an SSH command with strict

100

keychecking, which could not be faked. Or IPsec could be used for

101

the ping packets, making them secure.

102

103

* Security Summary

104

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

105

people who have:

106

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

107

2. The cunning and patience to do it carefully, one at a time, and

108

*quickly*, faking Mandos client/server responses for each one

109

before the timeout.

110

111

While there are some who may be threatened by people who have *both*

112

these attributes, they do not, probably, constitute the majority.

113

114

If you *do* face such opponents, you must figure that they could

115

just as well open your servers and read the keys right off the

116

memory by running wires to the memory bus.

117

118

What this system is designed to protect against is *not* such

119

determined, focused, and competent attacks, but against the early

120

morning knock on your door and the sudden absence of all servers in

121

your server room.