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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 would come and take the the whole
computer would have the Mandos client key when they took the server
offline and read the disk with their own tools. *But*, by then the
Mandos server will have detected that the original server is no
longer online 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 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,
all *before* the Mandos server timeout kicks in and the Mandos
server refuses to give out the key to anyone.
As the typical SOP seems to be to barge in and turn off and grab
*all* computers to maybe look at them months later, this is not
likely.
** 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.)
** 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 keys right off the
memory by running wires to the memory bus.
What this system 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 servers in
your server room.
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