/mandos/trunk : revision 24.1.87

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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 would then have pyshical access,

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could try to take the server offline and read the disk with their

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own tools so to steal the authentication keys used by a client

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*But*, by then the Mandos server should have detected that the

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original server is no longer online and will no longer give out the

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encrypted key. The timing here is the only real weak point, and the

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method, frequency and timeout of checking can be adjusted to any

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

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

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likely. It is just simplier to steal a password from a encrypted

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system by hardware memory scanners if one have this amount of time

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of physical access to the server.

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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. (Multiple Mandos

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

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clash, and clients will try all available servers.). That mean if

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just one is down then the other can bring it back up, but if bouth

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is out then they stay down until a

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must-type-in-the-password-at-boot have happend.

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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 keys right off the

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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 servers in

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your server room.