Information Assurance Homework 4: Vigenere Cipher and Encryption Techniques, Assignments of Computer Science

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Information Assurance: Homework 4

Due October 6, 2005 Submit electronic files via compass. See newsgroups for details of submitting via compass.

1. A stream of encrypted traffic has been captured, and you are tasked with breaking it. We have reason to believe the stream is encrypted using a vignere cipher, but we don't know the period or the key. We do know that the text is in English. You are each assigned a portion of the text. Last name starts with Attack file A, L http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig1-enc.txt B, M http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig2-enc.txt C, N http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig3-enc.txt D, O http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig4-enc.txt E, P http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig5-enc.txt F, R http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig6-enc.txt G, S http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig7-enc.txt H, T http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig8-enc.txt J, V http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig9-enc.txt K, W http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/vig10-enc.txt a. For your text, calculate the index of coincidence (IC) and look for repetitions to make an initial guess of the period length. Submit your initial guess and this evidence. b. Go ahead and use the techniques discussed in class to find the key characters and decipher the message. You may used automated tools such as the java applet shown in class http://math.ucsd.edu/~crypto/java/EARLYCIPHERS/Vigenere.html. Submit the deciphered text and the key.
2. This portion of the homework involves working with AES and DES encryption. Pull the AES reference library from http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/aes- files.zip. This library includes a test AES program which you can augment as necessary. It also includes a makeKey program which creates a key of the specified length using the rand() pseudo-random function. Compile it for your target system. I have tested this program on Windows XP last year and have tested it on Linux this year. Makefiles and project files are included.

If you don't have access to a Linux or Windows system with a C compiler, let us now as soon as possible. a. Fetch an encrypted file and a key file from http://www.cs.uiuc.edu/class/fa06/cs498sh/hw4/hw4-enc.bin and http://www.cs.uiuc.edu/class/fa06/cs498sh/hw3/key06-128.bin. Decrypt the file using ECB mode. It should result in a plain English file. Submit the resulting plaintext. b. Select a file to encrypt using a key in CBC mode. Submit the encrypted file, key file, and initialization vector file. c. Try encrypting your file in ECB mode using different key lengths using 128 bit, 192 bit, and 256 bit keys. AES operations are very fast. You will want to use a high resolution timer such as the gettimeofday or clock_gettime system calls. In addition, you will probably need to perform your target measured operation multiple times to have something that can be measured by the clock granularity. Submit a table of the key length and the associated average encrypt time. d. Measure the encryption using DES. On Linux the function ebc_crypt and des_parity should do the trick. On Windows the base provider for the Crypto API should provide a DES encryption operation. I will verify that over the weekend and post details in the newsgroup. Add a row to your table from part c showing the average time for the DES encryption operation.