Assignment No. 03
Semester: Fall 2019
CS205: Information Security Total Marks: 25

Due Date: 22/01/2020

Instructions:
Please read the following instructions carefully before submitting assignment:
You need to use MS word document to prepare and submit the assignment on VU-LMS.
It should be clear that your assignment will not get any credit if:

 The assignment is submitted after due date.
 The assignment is not in the required format (doc or docx)
 The submitted assignment does not open or file is corrupt.
 Assignment is copied (partial or full) from any source (websites, forums, students, etc)

Objectives:

To enhance the learning capabilities of the students about cryptography and crypto algorithms.

Assignment

Question 1:
You are required to encrypt the string “Security Professional” and decrypt the string “MdgfjVhjfbteut” using following tables with “Substitution Cipher Method”.

Question 2:
You are required to encrypt the plain text “ACCOUNTABILITY” using Vigenere Square Cipher method? Suppose that keyword is “SILENT”.

Question 3:
You are required to encrypt the plain text “wearesecurityprofessionalsinuk” into cipher text using columnar transposition method with a Key: “231645”.

Best of luck

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Here are the step-by-step solutions for your cryptography questions.

Question 1: Substitution Cipher

To solve this, we map each letter of the plaintext to its corresponding character in the provided substitution table. (Note: Since you didn’t provide the specific mapping tables, I have demonstrated the method using a standard Caesar-style shift or a placeholder logic. Please apply your specific table values to this process).

• Encryption (“Security Professional”):

1. Take each letter (S-e-c-u-r-i-t-y…).
2. Look up ‘S’ in the “Plaintext” row of your table.
3. Write down the character directly below it in the “Cipher” row.

• Decryption (“MdgfjVhjfbteut”):

1. Look up ‘M’ in the “Cipher” row.
2. Write down the character directly above it in the “Plaintext” row.

Question 2: Vigenere Square Cipher

Plaintext: ACCOUNTABILITY

Keyword: SILENT

Step 1: Align the Key

Repeat the keyword until it matches the length of the plaintext:
Plaintext: A C C O U N T A B I L I T Y
Keyword: S I L E N T S I L E N T S I

Step 2: Intersection Logic

Using a Vigenere Tableau, find the intersection of the Plaintext letter (column) and Keyword letter (row).

Final Ciphertext: SKNSHGLIMMYBLG

Question 3: Columnar Transposition

Plaintext: wearesecurityprofessionalsinuk

Key: 231645

Step 1: Arrange into Grid

The key has 6 digits, so we arrange the text in rows of 6.

Step 2: Read Columns by Key Order

Read the columns based on the numerical order of the key (1, 2, 3, 4, 5, 6).

1. Column 1 (Key 1): AURII
2. Column 2 (Key 2): WEYSL
3. Column 3 (Key 3): ECPSS
4. Column 4 (Key 4): EIFNU
5. Column 5 (Key 5): STEAK
6. Column 6 (Key 6): RROON

Final Ciphertext: AURIW EY SLE CPSS EIFN UST EAK RROON
(Spaces added for readability)