Encryption and Decryption using Vigenere with Cipher Block Chaining: Up to 50 dollars will be given

Programming Assignment 1: Encryption and Decryption using Vigenere with Cipher Block Chaining (CBC)

1. Introduction

In this programming assignment, you will implement Vigenere encryption and decryption with Cipher Block

Chaining (CBC). The original form of Vigenere cipher has been introduced in “L6: Symmetric Encryption” (slide

No. 15-18). CBC mode has been introduced in “L8: Modes of Operation” (slide No. 15-19). In the lecture of

“Modes of Operation”, we used Vigenere block cipher as an example to show how to apply the modes of

operation to ciphers. The description of Vigenere block cipher is in slide No. 7 of “L8: Modes of Operation”.

Comparing with original form of Vigenere cipher, the Vigenere block cipher defines a block size that is equal to

length of Vigenere key. According to the slide No. 18 of “L8: Modes of Operation”, for CBC mode an IV (Initial

Vector) is also needed as input. So, the inputs to the encryption algorithm include Vigenere key, IV and plaintext

and the inputs to the decryption algorithm include Vigenere key, IV and cyphertext. In the lecture slides, we have

focused on the encryption part. But in this assignment, you will implement both encryption and decryption

algorithms. Therefore, you will create two programs or source files: one for the encryption (or encipher) and one

for the decryption (or decipher). The encipher program will be run first. When encrypting, the encipher program

will take inputs including plaintext, key and IV. The encipher encrypts the plaintext with the key & IV, and

generate a file to store the ciphertext. Then when the decipher program is run, the ciphertext file will be used as

input along with key & IV. The decipher program decrypts the ciphertext to recover the plaintext.

2. Basic Encryption and Decryption Schemes

In the slides of “L8: Modes of Operation”, we have mentioned that Vigenere block cipher is a character cipher.

Instead of using the XOR defined in CBC mode, we used plus (+) in Vigenere with CBC (slide No. 18 of “L8:

Modes of Operation”). Therefore, the encryption of Vigenere with CBC can be expressed as

𝐶0 = 𝐸(𝑃0 + 𝐼𝑉) = (𝑃0 + 𝐼𝑉 + 𝑘𝑒𝑦𝑤𝑜𝑟𝑑) 𝑚𝑜𝑑 26

𝐶𝑗 = 𝐸(𝑃𝑗 + 𝐶𝑗−1) = (𝑃𝑗 + 𝐶𝑗−1 + 𝑘𝑒𝑦𝑤𝑜𝑟𝑑) 𝑚𝑜𝑑 26

Since we have modified the CBC operation for Vigenere character cipher, we have to adapt the CBC decryption

too. Once one already establishes the encryption algorithm (or encipher). The general rule for the decryption

algorithm (or decipher) is to run the encryption in reverse. Following this rule, we can express the decryption of

Vigenere with CBC as:

𝑃0 = (𝐶0 − 𝐼𝑉 − 𝑘𝑒𝑦𝑤𝑜𝑟𝑑) 𝑚𝑜𝑑 26

𝑃𝑗 = (𝐶𝑗 − 𝐶𝑗−1 − 𝑘𝑒𝑦𝑤𝑜𝑟𝑑) 𝑚𝑜𝑑 26

Note: the result of (𝐶0 − 𝐼𝑉 − 𝑘𝑒𝑦𝑤𝑜𝑟𝑑) 𝑜𝑟 (𝐶𝑗 − 𝐶𝑗−1 − 𝑘𝑒𝑦𝑤𝑜𝑟𝑑) can be negative but modular value should

not be negative. Since we have adapted both encryption and decryption of original CBC. The mathematical

notations shown above may not match the CBC diagram (Slide No. 16 of “L8: Modes of Operation”), which uses

XOR operation.

3. Implementation

3.1. Preprocessing

The plaintext input to the Vigenere CBC encipher should come from a text file. You may use any text file that is

less than 4 Kilobytes in size. You can generate your text file by simply copying and pasting any text content.

However, the normal text file contains upper case and lower case alphabetic characters, numbers, punctuations

and other special characters including space and line feed etc.. These are way more than 26 letters defined in

Vigenere cipher. So you have to preprocess the text file to eliminate all the special characters, numbers, white

space and punctuations, as well as convert all upper case letters into lower case letters. The result of preprocessing

should be a string of lower case letters. (as shown Figure 1) Then the encipher will use these letter sequences as the plaintext input.

Figure 1: Plaintext after Preprocessing

3.2. Inputs In addition to the letter sequence you generated from the preprocessing. The Vigenere CBC encipher needs a key

and an IV. For this assignment, you would allow the user to enter a key (key length ranges from 2 to 10) and an IV

with the same length as key. You may want to mandate that the length of IV should be the same as the key. Inputs

to the Vigenere CBC decipher should be the same key and IV, as well as the ciphertext. The ciphertext sequence

should be generated by the Vigenere CBC encipher.

The easy way to allow the user’s input is using the command line arguments. For example, assuming the executable

of your Vigenere CBC encipher is called V_encipher, plaintext file is called sample.txt, the key is rock and the IV

is dark. The Unix command line to run the encipher would look like this:

./V_encipher sample.txt rock dark

Assuming your encipher would save the ciphertext to a file ciphertext.txt and your decipher executable is called

V_decipher (the key and the IV are same as the above), the Unix command line to run the decipher would look like

this:

./V_decipher ciphertext.txt rock dark

3.3. Outputs As mentioned above, you would create two programs: one for the encipher (including the preprocessing) and one

for the decipher. The outputs of the encipher program should include the following:

Original text file name

ireturnedfromthecityaboutthreeoclockonthatmayafternoonprettyw

elldisgustedwithlifeihadbeenthreemonthsintheoldcountryandwasfe

dupwithitifanyonehadtoldmeayearagothatiwouldhavebeenfeelingli

kethatishouldhavelaughedathimbuttherewasthefacttheweathermad

emeliverishthetalkoftheordinaryenglishmanmademesickicouldntge

tenoughexerciseandtheamusementsoflondonseemedasflatassodaw

aterthathasbeenstandinginthesunrichardhannayikepttellingmyselfy

ouhavegotintothewrongditchmyfriendandyouhadbetterclimbout

Key

IV

Block size

Plaintext (after preprocessing) printed on the screen

Number of characters in plaintext (before padding)

Ciphertext printed on the screen

Name of ciphertext file (ciphertext saved to a file)

Number of padding characters

The ciphertext file will be used as one of the inputs to the decipher. The outputs of the decipher program should

include the following:

Ciphertext file name

Number of characters in ciphertext

Recovered plaintext printed on the screen

3.4. Other Implementation details Since your original text file could be any size less than 4 kilobytes, the padding may be needed when the plaintext

is divided into blocks. According to No. 18 of “L8: Modes of Operation”, Letter X should be used for padding.

4. Submission

When you submit your assignment, you should submit your source codes. The names of your source files should

clearly state encipher and decipher. You should also your text file (although TA may use his text file to test your

programs). You will also generate a report to describe your programs briefly and explain how to compile/run your

program on Eustis. Your report should include the screenshots to show the output results. Please zip all your files

and submit through Webcourses. As discussed before, you may submit multiple versions before deadline. So

submit early. Only the last version before the deadline would be graded. The late submission will receive penalty

of 1 point per minute past the deadline.

5. Grading

Please complete this assignment on your own. Do not copy or share codes. When TA grades your submission, the

following rubrics will be followed.

Criteria Ratings Pts

The assignment report provides description of the programs and explanation of the codes. The codes are compiled successfully. Full pts: 20 No pts: 0 20

Program accepts command line argument, correctly read/write the file and show it on screen Full pts: 20 No pts: 0 20

Program successfully and correctly perform Vigenere encryption on the first block, using the correct plaintext, keyword and IV Full pts:15 No pts: 0 15

Program successfully and correctly perform CBC encryption on subsequent block(s) Full pts:15 No pts: 0 15

Program successfully and correctly perform vigenere decryption on the first block, using the correct ciphertext, keyword and IV Full pts:15 No pts: 0 15

Program successfully and correctly perform CBC decryption on subsequent block(s) Full pts:15 No pts: 0 15

100