Information Security

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Assignment 4: Authenticating Voters

For this assignment, you will work in a group.


Diebold voting machines use a smartcard, called a "voter card", to control who can vote. On arriving at the polling place, the voter checks in and states his name and address. Election workers verify the voter's identity (we'll assume they can do this flawlessly), and verify that the voter is registered to vote and hasn't voted yet. Assuming that everything is in order, they enable a voter card (a special device the size of a credit card) and give it to the voter.

The voter casts his vote on a voting machine. Before allowing him to vote, the machine insists that he insert the voter card. The machine interacts with the voter card, and if it likes the interaction, it disables the voter card and allows the voter to cast a vote.

After the voter is done voting, the machine ejects the now-disabled voter card, and the voter gives the voter card back to the election workers at the front desk before leaving the polling place. (The voter card may later be re-enabled for use by another voter.)

The idea of this scheme is that an enabled voter card allows one vote to be cast. Unfortunately, Diebold's implementation of this scheme is insecure, allowing an adversary to re-enable his own voter card as often as he likes, and thereby to cast as many votes as he likes. Your job is to fix this.

Encoder, Voting Machine, and Voter Card

There are three kinds of devices. The encoder is used by election workers at the front desk when they want to enable a voter card. Assume that it plugs into the wall and is the size of a deck of cards, with a slot into which a voter card can be inserted, and a single button marked "enable voter card". To enable a voter card, an election worker will insert that card into the encoder and press the "enable voter card" button.

The voting machine is a computer. It plugs into the wall and looks like an oversized laptop. It has a slot into which a voter card can be inserted. When a voter card has been inserted, the voting machine interacts with the card and then decides whether to allow a vote to be cast. If necessary, it interacts with the voter card to tell the voter card to disable itself.

The voter card is a smartcard -- a little computing device the size and thickness of a credit card that has a processor and some persistent memory. The voter card doesn't have its own power source; when it is inserted into another device (e.g. an encoder or voting machine) it gets power from that device and is able to run code. But when it is removed from the other device it stops running and loses its program counter, registers, and so on. The persistent memory is the only part of the card's state that is preserved when it is removed from its power source.

When the voter card is inserted into another device, the voter card can communicate with that device using a bidirectional serial channel. For example, the voter card can send a byte to the other device, and the other device can receive that byte; or vice versa. This is the only way the devices can interact.

It is important to recognize that the voter card is a computer in its own right. Other devices, such as the encoder and voting machine, can never read or write the voter card's memory directly. All they can do is communicate with the voter card and rely on it to change its state.

The Software

You need to create four programs. We are giving you initial versions of the programs, which work but are insecure. Your job is to make them secure.

The createCard program creates a new smartcard. Its job is to initialize the persistent memory of the new smartcard. When you invoke createCard, you give it a filename; createCard creates a new file with that name, and stores the smartcard's persistent memory in that file. You can have as many smartcards as you want -- there will be one file for each card, to hold the contents of that card's persistent memory.

The encoder program is invoked whenever an election worker pushes the "enable voter card" button on the encoder device. encoder takes one commandline argument, which is the name of a file containing (the persistent memory) of a smartcard; this smartcard is assumed to be inserted into the encoder device when the button is pressed.

The encoder program does some initial setup steps in its main function, which you are not allowed to change. One of these steps is to launch a separate program that represents the inserted smartcard. These two programs run simultaneously, communicating via a serial connection, just as the real encoder would communicate with a real smartcard that is inserted into it.

Having set everything up correctly, encoder's main function then calls the activateEncoder function, which does the main work of the encoder. This is what you will change.

The voting machine program, called machine, works similarly to the encoder. It takes one commandline argument (the name of a file containing a smartcard's persistent memory). It does some setup, which includes launching a program to represent the smartcard. Then it calls a function called activateMachine, which does the main work of the voting machine and which you can modify.

The final program is the smartcard program, which is invoked whenever a smartcard is inserted into a device. This program does some initial setup steps, which we have provided and you may not modify. Then it calls the activateCard procedure, which you may modify. This procedure is invoked whenever the smartcard is inserted into another device. Read the comments in activateCard.c for more information.

Normally, you will not launch smartcard directly. Instead, it will be launched automatically when you run encoder or machine.

We have provided working (but insecure) code for all of these programs (and for another program, dumpCard which you may find useful but you are not required to use). It's probably a good idea to read the code for these programs, and to run them after adding "hello world" code to the activate* procedures, to get a better handle on how the pieces fit together.

The encoder and machine programs we have given you do not use any persistent storage. You can give them persistent storage if you want. If you do this, the persistent storage should be kept in a file, and of course one program's persistent storage should never by accessed by another program.


Assume that:


Your solution should allow voter cards to be enabled by inserting them into the encoder, and insertion of an enabled voter card into the voting machine should allow a vote to be cast and should disable the voter card. A disabled voter card should be able to be re-enabled by putting it into the encoder again, and so on.

Even if the adversary makes his own smartcards and inserts them into your encoder and/or your voting machine, he should never be able to cast a vote without having used the encoder to enable a card first.

Your Report

Your report should describe your solution, and justify why it prevents the adversary from achieving any of his goals. Your report should be concise but should be as convincing as you can make it. The quality of your report will be a very important component of your grade, so pay at least as much attention to your report as to your code.


Your solution should be a zip-file containing your source code (including any provided files that you did not modify), a Makefile, and a report that describes what you did and why. The report should be an HTML file named index.html. (It may contain links to other files, if you include those files in your submission.)

When we unzip your submission into a directory and type "Make" there, it should build programs that we can execute by typing "createCard", "encoder", "machine", and "smartcard", which operate as described above.

You must work in a group on this assignment. You may not collaborate with anyone outside your group.

Copyright 2000-2008, Edward W. Felten.