Cryptography is an ancient study of secret writing. There is a wealth of literature in this field. An extremely readable book on this subject is The Code Book by Simon Singh. This is a field of study that is of particular relevance in Computer Science. Given the widespread use of computers, one of the things people are interested in is making transactions over the internet more secure.

There are two classes of ciphers - substitution ciphers and transposition ciphers. The substitution ciphers replace individual characters in a string. Transposition ciphers on the other hand scramble the characters in the string. We will look at two representative ciphers in both the classes. The transposition cipher that you will implement is called Rail Fence Cipher. For the subsitution cipher we have chosen the Vigenere Cipher.

Rail Fence Cipher: The rail fence cipher is also known as the zigzag cipher and is a form of a transposition cipher that jumbles up the order of the letters. The rail fence cipher works by writing your plain-text downwards and diagonally on successive rows. The number of rows is given by a key. Once you reach the bottom row, you traverse upwards and diagonally. After you reach the top row, the direction is changed again, leading to a zigzag manner in which the plain-text is written. After the plain-text is written in this form, the rows are combined to create the cipher-text.

For example, let us consider the plaintext helloworld with key = 3.

Plain Text: h e l l o w o r l d Key: 3 

To encrypt this message we first write over three lines as follows:

h	-	-	-	o	-	-	-	l	-
-	e	-	l	-	w	-	r	-	d
-	-	l	-	-	-	o	-	-	-

The cipher text in this case is written by reading off the top row, then the second row, then the last row from left to right.

Cipher Text: h o l e l w r d l o 

To decrypt a given cipher text, you would need to reconstruct the list. First construct a 2D list with number of rows equal to the given key and number of columns equal to the length of the cipher text. Next, traverse the 2D list in the same diagonal manner as encoding and mark the cells in this 2D list that should contain a character. Then, traverse the list row by row and fill in the characters of your ciphertext. Lastly, read the matrix in the diagonal fashion to uncover your plain-text.

In the transposition cipher you are merely scrambling all the characters in the plain text. In your assignment keep all the characters of the plain text - lower case letters, upper case letters, digits, punctuation marks and spaces. Your cipher text will have all the characters (unchanged) from the plain text.

Vigenere Cipher: Substitution ciphers can be broken by frequency analysis. To make the cipher more secure you can use two or more letters to encrypt. The Vigenere cipher uses a pass phrase to encrypt and decrypt.

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Let us say we want to encrypt helloworld and we use as our pass phrase seal. Here is how we would lay the phrases out. The pass phrase is repeated over and over again in order with the text that needs to be encrypted. The letter from the pass phrase marks the row and the letter from the plain text marks the column.

 Pass Phrase: sealsealse Plain Text: helloworld Encrypted Text: zilwgaocdh 

To decrypt, we would reverse the process. Use the same pass phrase seal repeatedly with the encrypted text. This time, the letter from the pass phrase marks the column. Within that column search for the encrypted letter. That row indicates the plain text.

 Pass Phrase: sealsealse Encrypted Text: zilwgaocdh Plain Text: helloworld 

In your implementation of the Vigenere cipher you will not use the 2-D list. You must come up with a formula that will allow you to do the transformation. Also you must write a function to filter the input string. This function will first convert the string to lower case and then remove all characters that are not lower case letters.

The file that you will be turning in will be called TestCipher.py . You must follow the template of the program given. You may NOT change the names of the functions or the names of the input parameters. This is the way we will test your program. You may write as many helper functions that you need.

import sys # Input: strng is a string of characters and key is a positive # integer 2 or greater and strictly less than the length # of strng # Output: function returns a single string that is encoded with # rail fence algorithm def rail_fence_encode ( strng, key ): return "" # placeholder for the actual return statement # Input: strng is a string of characters and key is a positive # integer 2 or greater and strictly less than the length # of strng # Output: function returns a single string that is decoded with # rail fence algorithm def rail_fence_decode ( strng, key ): return "" # placeholder for the actual return statement # Input: strng is a string of characters # Output: function converts all characters to lower case and then # removes all digits, punctuation marks, and spaces. It # returns a single string with only lower case characters def filter_string ( strng ): return "" # placeholder for the actual return statement # Input: p is a character in the pass phrase and s is a character # in the plain text # Output: function returns a single character encoded using the # Vigenere algorithm. You may not use a 2-D list def encode_character (p, s): return "" # placeholder for actual return statement # Input: p is a character in the pass phrase and s is a character # in the encrypted text # Output: function returns a single character decoded using the # Vigenere algorithm. You may not use a 2-D list def decode_character (p, s): return "" # placeholder for actual return statement # Input: strng is a string of characters and phrase is a pass phrase # Output: function returns a single string that is encoded with # Vigenere algorithm def vigenere_encode ( strng, phrase ): return "" # placeholder for the actual return statement # Input: strng is a string of characters and phrase is a pass phrase # Output: function returns a single string that is decoded with # Vigenere algorithm def vigenere_decode ( strng, phrase ): return "" # placeholder for the actual return statement def main(): # read the plain text from stdin # read the key from stdin # encrypt and print the encoded text using rail fence cipher # read encoded text from stdin # read the key from stdin # decrypt and print the plain text using rail fence cipher # read the plain text from stdin # read the pass phrase from stdin # encrypt and print the encoded text using Vigenere cipher # read the encoded text from stdin # read the pass phrase from stdin # decrypt and print the plain text using Vigenere cipher # The line above main is for grading purposes only. # DO NOT REMOVE THE LINE ABOVE MAIN if __name__ == "__main__": main() 

We will be looking for good documentation, descriptive variable names, clean logical structure, and adherence to the standard coding conventions discussed in class.

You may work with a partner on this assignment. Both of you must read the paper on Pair Programming and abide by the ground rules as stated in that paper. If you are working with a partner then only one of you will submit the code. In the header make sure that you have your name and your partner's name and both your UT EIDs. You must use the group submission feature in Gradescope. If you are working alone then remove the fields that has the partner's name and EID.

Input: Your input will be from a file cipher.in. Do not hard code the name of the file in your program. You will read the file from stdin. Here is a typical input file:

helloworld 3 holelwrdlo 3 helloworld seal zilwgaocdh seal 

The first set of input will be to encode and decode using the rail fence cipher. The second set of input will be to encode and decode using the Vignere cipher. Here are the explanations of each line in the input file. Assume that there are no errors in the input file.

plain text to be encoded using rail fence cipher

key for the rail fence cipher

encoded text to be decoded using rail fence cipher

key for the rail fence cipher

plain text to be encoded using Vignere cipher

pass phrase (no spaces) for the Vignere cipher

encoded text to be decoded using Vignere cipher

pass phrase (no spaces) for the Vignere cipher

Output: Your TestCipher program will test the encoding and decoding methods of the two Cipher algorithms. Your output will look something like this:

Rail Fence Cipher Plain Text: helloworld Key: 3 Encoded Text: holelwrdlo Encoded Text: holelwrdlo Enter Key: 3 Decoded Text: helloworld Vigenere Cipher Plain Text: helloworld Pass Phrase: seal Encoded Text: zilwgaocdh Encoded Text: zilwgaocdh Pass Phrase: seal Decoded Text: helloworld 

Use the Canvas system to submit your TestCipher.py file.