Vuvuzela is a dead-drop system, in which one user leaves a message for another at a predefined location — in this case, a memory address on an Internet-connected server — and the other user retrieves it. But it adds several layers of obfuscation to cover the users’ trails.
To illustrate how the system works, Lazar describes a simplified scenario in which it has only three users, named, by cryptographic convention, Alice, Bob, and Charlie. Alice and Bob wish to exchange text messages, but they don’t want anyone to be able to infer that they’ve been in touch.
If Alice and Bob send messages to the dead-drop server, and Charlie doesn’t, then an observer would conclude that Alice and Bob are communicating. So the system’s first requirement is that all users send regular messages to the server, whether they contain any information or not.
If an adversary has infiltrated the server, however, he or she can see which users are accessing which memory addresses. If Charlie’s message is routed to one address, but both Alice’s and Bob’s messages are routed to another, the adversary, again, knows who’s been talking.
So instead of using a single server, Vuvuzela uses three. Corresponding to the three servers, every message sent through the system is wrapped in three layers of encryption. The first server peels off the first layer of encryption before passing messages on to the second server. But it also randomly permutes their order. So if, for example, Alice’s message arrived at the first server before Bob’s, and Bob’s arrived before Charlie’s, the first server will pass them to the second in the order Bob, Alice, Charlie, or Charlie, Bob, Alice, or the like.
The second server peels off the second layer of encryption and permutes the message order yet again. Only the third server sees which messages are bound for which memory addresses. But even if it’s been infiltrated, and even if the adversary observed the order in which the messages arrived at the first server, he or she can’t tell whose message ended up where.
The adversary does, however, know that two users whose messages reached the first server within some window of time have been talking. And even that is more information than Vuvuzela’s designers want to give away.
Here’s where the noise comes in: When the first server passes on the messages it’s received, it also manufactures a slew of dummy messages, with their own encrypted destinations. The second server does the same. So statistically, it’s almost impossible for the adversary to determine even whether any of the messages arriving within the same time window ended up at the same destination.