CSE 114 - HW 3

CSE 114 - HW 3 - CSE 114 - HW 3 CSE 114 - Spring 2010...

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CSE 114 - HW 3 http://www.cs.sunysb.edu/~cse114/hw/HW3.html[4/13/2010 5:18:20 PM] CSE 114 - Spring 2010 Computer Science II Home Syllabus Schedule HW 3 Note that this HW assignment has only one part, the Monoalphabetic Cipher. Work for this HW assignment should be done using the Unix environment. Random Monoalphabetic Cipher using Objects An important function in the world of Computer Security is encryption. In the encryption process, an original readable message, referred to as plaintext , is converted into apparently random nonsense, referred to as ciphertext . The purpose of such an operation is to be able to send messages that include sensitive information like passwords, credit card numbers, etc. .., from one location to another without snoopers intercepting and reading the contents. The encryption process requires 2 components: an encryption algorithm and a key. There are many different algorithms, and for each, a different key will produce different ciphertext. Once a message is transformed into ciphertext, it can be transmitted to its destination. Upon receipt of a ciphertext message, that message can be decrypted into its original plaintext message using a decryption algorithm and key. Some encryption/decryption algorithm pairs will both use the same key, while other one-way algorithms will use two keys (typically called public and private keys). A substitution encrption technique is one of the simplest, and is one in which the letters of plaintext are replaced by other letters or by numbers or symbols. The earliest known use of a substitution cipher was by Julius Caesar. The Caesar cipher involves replacing each letter of the alphabet with the letter standing K places further down the alphabet, where K would be the Key value. In such an algorithm, the key ultimately determines which letter to substitute a given letter with. For example, the following plain/cipher text pairs would result from a key of 3: plain: MEET ME AFTER THE TOGA PARTY cipher: PHHW PH DIWHU WKH WRJD SDUWB Note that the alphabet is wrapped around, so that the letter following Z is A. We can define the transformation by listing all possibilities, as follows: plain: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z cipher: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C Since adding characters and numbers is not convenient, Caesar would assign a numeric value to each character (no I don't mean 5 for V). In the above alphabet, A would be 0, B would be 1, etc. .. until Z is 25. Then,
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CSE 114 - HW 3 http://www.cs.sunysb.edu/~cse114/hw/HW3.html[4/13/2010 5:18:20 PM] the ciphertext may be determined character by character using a simple mathematical formula: It would not be difficult to crack passwords encrypted using the Caesar cipher. Using the ASCII characters (32 -
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CSE 114 - HW 3 - CSE 114 - HW 3 CSE 114 - Spring 2010...

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