Untraceable Communication is Here
Spies have been using physical "Dead Drops" to exchange information securely for decades. This method of communication has become possible online with recent cryptographic advances.
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Have you ever watched a spy movie where the hero casually walks to a bench in a park and nonchalantly retrieves some papers from under it, or a microfilm roll, or in a more modern spy movie an sd-card?
That is what spycraft calls a “Dead Drop”. During a “Dead Drop” the two operatives exchanging information don’t need to meet, which enhances operational security. Even if third parties are actively monitoring one or even both operatives, the two will never be at the same place at the same time, therefore no useful information can be gained by watching them. If they are careful enough, that is.
“Dead Drops” are real-life things, although often using more sophisticated methods, like retrieving things from places that are less visible than a park bench, like from inside a hotel room or digitally transferring information by radio to some unseen device hidden in a wall, tree or rock.
Could a Dead Drop Even Work on the Internet?
If you’re a somewhat internet-savvy person, you might be thinking there’s no way a Dead Drop would work online. On the internet individual data packets are sent from one device to another, with both addresses present in these data packets in the clear, letting everyone who is willing to listen know who is communicating with whom. This is the case even if packets take the “scenic” route over multiple routers, devices, networks, etc. In essence, two devices must meet on the internet to communicate. This is at best a Live Drop, not a Dead Drop.
Indeed it turns out that even on networks like Tor, which try to obfuscate who is talking to whom by introducing multiple indirections and statistical noise, long-term and large-scale monitoring of the network can lead to uncovering of the social nets of individuals. This is not because of a bug, mind you, but because of how it is designed. By letting devices communicate interactively a whole treasure-trove of statistical information is unleashed onto the internet for everyone to see. One can try to mask it, obfuscate it, introduce noise, which makes analysis more difficult, but it can not be made perfectly untraceable this way.
So, how do you send a message to someone over the internet and still stay untraceable?
Threat Model on the Internet
A Threat Model is a list of capabilities you assign to your adversaries and a list of countermeasures you deploy against those capabilities. Can your adversary follow you around all day? Can they bug your hotel room? Can they follow all your potential contacts? Do they have access to credit card histories? Etc.
In the physical world, monitoring even one person is expensive. It needs a whole team of people to do, cooperation of other parties, like hotels, dispatch centers, not even mentioning lots of hardware like vehicles, gadgets, etc.
Monitoring on the internet, as most of us are probably aware already, is much much cheaper. It is not just cheaper, but can be done and is done on a large scale. Even worse, you can monitor someone retroactively, looking back at what they were up to several days or even weeks before, using stored data.
What capabilities can we realistically assign to our potential adversaries then? Let’s take a look in detail.
Let’s Assume All Data on the Internet Is Seen and Collected
Even if some piece of data is on the internet for a fraction of a second, we should assume that it is captured and stored. This assumption may seem trivial, but has a lot of consequences.
While encrypting the data between peers protects somewhat (see later points) against seeing the contents of the data, what about the connections themselves? The existence of the connection between two devices can already help to trace the people on both sides of the communication. Also, if one of them is already known, like a known journalist, a specialist doctor, or a political group, the person communicating already potentially incriminates herself or himself through affiliation alone.
You may have seen some VPN providers advertising that they “delete logs”, or don’t keep logs at all. This has several problems (see next points), but the problem under this assumption is, that the connection information was already out on the internet. It is not a secret. Sure, packets are going from you to the VPN provider, and from the VPN provider to your destination, and the two are not directly linked, but they can be linked by statistical information easily for someone who has access to connection information on both sides.
For example, an attacker seeing your traffic to be 10KB up / 100KB down just has to look which connection from the VPN provider matches this traffic profile to connect you to your destination. If a single instant is not enough, changes of this profile over time make this attack even easier.
This is not to say that VPNs are useless. VPNs operate under a different threat model for a different use-case, which they do well, but for our purposes, it is not enough.
For untraceable communication, there can not be any statistical data that leaks to the internet. In other words, the physical act of sending and receiving data needs to be completely independent from user action. If it is not, some information will eventually leak and compromise the user.
So how can the physical act of sending and receiving data be independent of user action? Quite easily actually. Let’s just make all devices send and receive at predefined intervals, regardless of whether there are messages to be sent or received. To avoid traffic profiles, let’s also make all messages the same size. Although this introduces overhead, it solves all of the problems above.
Let’s Assume Companies Can Be Forced To Divulge Encryption Keys
Laws are different for different jurisdictions of course, but with all we know this is not even an assumption, more a fact for most. Even in jurisdictions where this is not the case, laws can easily change for the worse.
Keys can obviously not be in the hands of the companies providing the service. The answer is end-to-end encryption. I.e. the users themselves have keys, the secret parts of which are never disclosed. Ever. Not even to the provider of the communication service.
Most “encrypted” communication apps already use end-to-end encryption this way.
Let’s Assume Companies Can Be Forced To Divulge Any and All Information They Have, Even if Just for a Split Second
All current communication apps require some form of routing. That is, moving the incoming message, even if it is end-to-end encrypted, to its intended receiver. To do this the service needs to know, at least for a fraction of a second, who is talking to whom.
Even if the service tries its best not to record any of this information, it is still available. The company can conceivably be forced by law to collect, log and/or provide this data upon request.
Another way to force this information out of the server is to crack (break into) the server either digitally or even physically. This is why some companies try really hard to physically secure their servers, or try to dissociate the user from their devices as well as possible.
But even if there is no law and the servers are physically secured, what about just paying an employee to reveal some data? What about a disgruntled employee taking and revealing some data? Any information the server knows the provider and its employees could potentially know as well. It is therefore at risk.
The only way to avoid all these problems is to not route. Without the routing, the server doesn’t need to know anything at all, it is just a technical component that sits somewhere. Even if all the data that is available on it is stolen, leaked, or collected, there is nothing there. Not even for a millisecond. No connections between devices, no contents, no contact lists, nothing.
The server must be oblivious to what’s happening on it. But, how do we send a message to somebody without actually telling the server who it is for?
Private Information Retrieval
A quick and easy solution for the server would be to just deliver every message to every user. In essence creating a broadcast protocol, in which everybody sees everything. Because the messages are end-to-end encrypted, arrive at pre-determined regular intervals, and are the same size (see previous points), the server would not know what’s going on at all. Also because of the end-to-end encryption users would only be able to actually read the message which was written explicitly for them, so it would still be perfectly secure.
This protocol would work, but would be extremely difficult to scale. If we assume a handheld device with somewhat limited connectivity, receiving all messages from all the users worldwide would quickly run into problems.
Luckily there’s a new-ish field of cryptography called Private Information Retrieval. Surprisingly, it is possible to query the server for a message without the server finding out which message was queried and in a way that uses significantly less bandwidth than downloading all messages.
So PIR is a way to query the server for a message without the server finding out what message, if any, was returned. The server is therefore at no time in possession of any information that would reveal the existence of a communication, let alone the identities of parties involved, because it is unable to make the connection between sender and receiver. Because the messages are encrypted and would look like random noise to the server, it can not even tell whether a single message is being exchanged or not.
There are some trade-offs involved in using PIR however. It is computationally expensive relative to other services that route messages. A server that routes messages can directly answer a query by accessing the required message (or inbox). Every single PIR query however has to calculate a cryptographic function over all messages. This is the only way the server can stay oblivious to what message is selected by the query.
This makes a PIR-based server costly to operate and more difficult to scale. This is the price for untraceable communication.
Putting in All Together
Let’s look at what tools we identified to implement a Dead Drop digitally:
- Dissociate user action from network traffic to avoid leaking statistical information. Send / receive at predefined intervals, using uniform message sizes.
- End-to-end encryption with secret keys only on the user’s device. This prevents access by third parties anywhere on the internet, including the service provider.
- Using PIR to retrieve a message, leaving the server oblivious to what is happening, instead of letting the server route messages and with it revealing the parties involved.
Published at DZone with permission of Robert Brautigam. See the original article here.
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