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EE 306 - Programming Assignment 3

EE 306 - Programming Assignment 3 - Department of...

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Department of Electrical and Computer Engineering The University of Texas at Austin EE 306, Fall 2007 Programming Assignment 3 Due: 28 th November, 11:59pm Tony Ambler, Instructor You may do the programming assignment in teams of up to three people. The file you submit should be an assembly language file. This is the only file you need to submit. Encrypt or Decrypt a message from the keyboard Background : Cryptography, from the Greek word kryptos, meaning "hidden", deals with the science of hiding a message from eyes you do not want to understand the contents of the message. The desire to do this has existed ever since humankind was first able to write. There are many ways to keep something secret. One way is to physically hide the document. Another way is to encrypt the text you wish to remain secret. Some people use this to keep others from understanding the contents of the files in their computer. Encrypting a file requires an input message, called the "plain text," and an encryption algorithm. An encryption algorithm transforms "plain text" into "cipher text." Just like a door needs a key to lock and unlock it, an encryption algorithm often requires a key to encrypt and decrypt a message. Just like a homeowner can selectively give the key to the front door to only those he/she wants to allow unaccompanied access, the author of a message can selectively give the encryption key to only those he/she wants to be able to read the message. In order for someone to read the encrypted message, he/she has to decrypt the cipher text, which usually requires the key. For example, suppose the plain text message is HELLO WORLD. An encryption algorithm consisting of nothing more than replacing each letter with the next letter in the alphabet would produce the cipher text IFMMP XPSME. If someone saw IFMMP XPSME, he/she would have no idea what it meant (unless, of course, he/she could figure it out, or had the key to decrypt it.)
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