Midterm Programming Project Encryption for IM Client

Transcription

1 1 CS 334 Computer Security Fall 2008 Midterm Programming Project Encryption for IM Client Assigned: October 24 Due: Monday, November 17, 11:59:59pm 1 Introduction In this project, you will take a functioning IMClient, and extend it so that the text of conversations is transferred encrypted using standard DES (i.e., using a 64-bit key). The DES key for a given conversation is to be generated by using the Diffie-Hellman key exchange algorithm. To be clear, you need to implement the code that performs the DES encryption-decryption as well as the code that performs the Diffie-Hellman exchange. The DES mode of operation you should use is CBC with initialization vector set to zero. 2 The IMClient Code You are given as a starting point, Java code that implements an IM client. It is capable of communicating via either AIM or ichat, as long as both parties are using AIM addresses (as opposed to, say,.mac addresses). For testing purposes, you will want to have more than one AIM address. The code requires two special libraries, squint and TOCtools, which are contained in the zip file on the course web site. You will need to place the jar files in the appropriate directories on your machine (as determined by the value of the CLASSPATH environment variable). 3 Getting Started It is likely that some of you have never coded using the graphical user interface components of Java (e.g., the Swing classes). The code that you have been given actually uses a somewhat simplified API, which is provided in the squint library. In particular, squint provides a GUIManager class that simplifies some of the functionality typically provided by Swing. Documentation for squint can be found at on the page tom/weavingcs/materials/, which contains a link to the API. Though the code you are provided does use Swing, you need only learn a small amount to do what is necessary for this project. I recommend looking through the code before trying to learn any Swing in almost all cases it is clear what the various classes are doing. If you are having trouble

2 making sense of the code, I am happy to explain it to you. (And actually, I am happy to explain it even if you DO understand it, though I don t really see the need for that.) 4 Implementing DES and Diffie-Hellman 4.1 Generating a DES Session Key The Diffie-Helman exchange allows each of two communicating parties to establish a secret number. There are several ways that this number could be converted into a DES key. Since you will be working with the Java BigIntegers class(see below), the easiest way to do this is for this project is to use the tobytearray() method in the BigInteger class, and use the last seven bytes of this array as the key. Remember that though DES is said to use a 64 bit key (and much of the literature describing it details how things are done with a 64 bit key), it is really a 56 bit key with one parity byte attached, so that there are only 56 meaningful bits. 4.2 Getting at the Bits The data that is transmitted via IM is ASCII text. Your IM client code receives it as a String from the textfield in which it is entered. You should use the getbytes() method, and specifically the version of getbytes() that takes a string charset as input parameter (you must use the charset US-ASCII), to convert that string to a byte array (in this case to it s 7 bit ASCII value). You will need to write your own class to get at the individual bits within each byte (which amounts to performing bitwise operations of your byte with bitmasks). 4.3 Arithemetic with Big Integers Appropriately enough, you ll want to use the BigInteger class that is included in the Java standard library. In addition to providing methods for performing arithmetic (including exponentiation) on large integers, it provides methods for modular arithmetic, and for finding large primes (well large numbers that are very likely prime). 5 Client or Server? Note that your code must act both as a client and a server. By this, I mean that your code should both be able to initiate enrypted communication, and decrypt transmissions that have been encrypted. In order to do this, your code needs to have a means of communicating to a chat client that it wishes to perform a Diffie-Hellman exchange. After a sucessful exchange, all communication

3 3 is expected to be encrypted in both directions until one side indicates that it no longer wishes to communicate in an encrypted mode. In order to accomplish this, the TOC protocol must be extended to include control signals that indicate the start/end of encryption. Since TOC messages pass through AIM servers which do not have encryption capabilities, these signals must be passed in-band (gasp). That is, the signals will take the form of a normal IM message, but with special content in the payload (data). Although when communicating with an AIM or ichat client your clients will receive text encoded in HTML, when you run two instances of your code against each other, you will simply received text or the form IM IN2:ProfSzajda:F:F:here is the message body! We will augment the TOC protocol to handle our additional signal by adding one more field delimited by colons. This field, which I call the encryption status field, will consist of a single uppercase letter, the values of which are P APpacket indicates that a client wishes to initiate a Diffie-Hellman exchange. APpacket has the form (in addition to the usual IM IN2 fields):p: : , where the base-10 number immediately following the :P: represents the the prime p in the Diffie-Helman exchange, and the number following the next colon represents the number g in the exchange. Though it is numbers that are being communicated, as data types you are sending Strings, so what is being sent is a string representing the base-10 value of these numbers. T ATpacket is used when a client is exchanging it s T value with its communication partner. The number should again be transmitted as a string representing the base-10 representation of the number. E An E packet indicates that the data following is encrypted. N An N packet indicates that the data following is not encrypted. 6 Transmitting Information There is a bit of a problem with exactly what gets transmitted from client to client once the Diffie- Hellman has completed, because all traffic passes through an AOL server. The server expects that

4 messages have a specific format, and that format expects that the body of the packet is a string enclosed in double quotes. This means that all messages need to be transmitted as strings. There are lots of ways to do this. For the sake of interoperability, I have selected one. That is, the bits you transmit should be transmitted as a string. As an example, if you wish to transmit the bit pattern (which is the ASCII bit pattern for uppercase A) this should be transmitted as the string Transmitting it this way is obviously not the most efficient way to do things, since instead of transmitting a byte, you re actually transmitting 16 bytes (Java encodes text using the unicode format, which uses two bytes for every character). But it has the advantage that the string will be understood and passed without modification by the AOL server. 7 The Graphical User Interface You should modify the GUI supplied in the code by adding two JButtons to ChatWindow displays. These should have the labelsencryt anddecrypt. These buttons, as you may have guessed, turn on and off the encryption capability for the particular conversation represented by that ChatWindow. The new ChatWindow GUI should also include a JLabel that indicates whether or not communication is in encrypted mode, and another JButton that displays the DES key (in hexidecimal notation) that is created by the Diffie-Hellman exchange. When the conversation is not being encrypted, the label should simply be blank. 8 Project Zipfile Contents The contents of the zip file are: Two jar files, squint.jar and TOCtools.jar Four Java source code files, IMControl.java, ChatWindow.java, TOCServerPacket.java, and SBox.java An pdf file containing excerpts from the the text Network Security: Private Communication in a Public World (2nd Ed.) by Kaufmann, Perlman, and Speciner, describing detials of DES and of Diffie-Hellman. You will also likely want to consult FIPS Pub 46 (found at which describes DES, and FIPS Pub 81 (found at which describes DES modes of operation. The pdf files for Labs 4 and 5 of my CS 150 class. These are necessary so that you understand the TOC protocol that is being used by the code I have provided.

5 9 Interoperability Like all network code, a client written by you should be able to converse with a client written by me, provided we both code to the protocol. So, in testing your code, I will be running one of your clients against one of mine. 10 Deliverables You will need to submit to me all of the source code files required for your project (including those that I have provided to you, since you may want to make some changes to them). You do not need to include the squint and TOCtools jar files. I will eventually provide you with a working client against which you can test. You should submit to me only a single jar file containing all of the material you are submitting. The name of the jar file should be XXXIMClient.jar, where XXX is the first three letters of your last name.