~ ASSEMBLY LANGUAGE USING NASM ~

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The Vigenre cipher, invented in 1553, uses multiple Caesar ciphers so that the frequency distribution of encoded letters is spread over several different encodings. Some versions were based on something like sliding tables or concentric letter wheels. However for military use in the field it is better not to need special equipment and to have the cipher be based on something easy to remember such as a keyword.

For better frequency characteristics the keyword should not have any repeated letters. Also, if it contains the letter A the encrypted letter will be the same as the plaintext, although this is not necessarily a bad thing.

To implement this algorithm with a pencil and paper, many descriptions ask you to build a Vigenre Square. However this is not really necessary when you are using a computer to do the encoding and decoding.

Essentially the keyword is written repeatedly over and over above the plaintext. Suppose the keyword is CRYPTOGRAM.

CRYPTOGRAMCRYPTOGRAMCRYPTOGRAMCRYPTOGRAMCRYPTOGRAMCRYPTOGRAMCRYPTOGR WEHAVEBEENBETRAYEDALLISDISCOVEREDFLYATONCEMEETUSBYTHEOLDTREEATNINEPM

Consider that the letters are numbered 0 to 25. The letter on the top determines which Caesar-cypher to use for the letter below. Thus C means shift the alphabet by 2, A means shift by 0, and so on. In mathematical terms, we are adding the two letters together modulo 26. (The square was used because the concept of modular arithmetic was not generally understood by soldiers in 1553.)

To decrypt the message, the same operation is performed in reverse. That is, the value of the keyword letter is subtracted rather than added.

Another example (Andrew Pennycook, Codes and Ciphers, MacKay, New York, 1980):

Key: SUFFERINGCATSSUFFERINGCA Plaintext: BETHEREATMIDNIGHTTONIGHT Ciphertext: TYYMIIMNZOIWFAAMYXFVVMST

Notice that the word "night" appears twice in the message. The first time it is encoded as FAAMY and the second time as VVMJT. There is nothing to show that they came from the same word in the original.

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~~~ What your code should do ~~~~~ 1. Your code should use STDIN and STDOUT for input and output. (This is the default.) Use redirection on the command line to read from a file and write to a file. 2. Your code should open a file, read it character by character and save it into an array. Use C I/O functions. 3. When you get to the end of the file you should encode the contents of the array with a Vigenre cipher using the keyword CRYPTOGRAM, then print it out. 4. Maintain the distinction between upper-case and lower-case letters, and do not modify non-alphabetic characters. This is not very good for the security of your message, but the result will look neater. 5. This program should use glibc functions. In addition to printf(), you may need getchar() and putchar(). 6. Assume that the input file contains just ASCII text Don't worry about what happens with non-text files.