Let's break down the question and solve it step by step: ### Step 1: Understanding the Algorithm The question asks for an algorithm similar to the **Vigenère cipher**. - **Vigenère cipher** is a method of encrypting alphabetic text using a simple form of polyalphabetic substitution, where a keyword is repeated to match the length of the plaintext. ### Step 2: Solution using Vigenère Cipher #### Plaintext: ``` message ``` #### Keyword: ``` monarchy ``` ### Step 3: Repeat the Keyword Repeat or trim the keyword to match the length of the plaintext: - Plaintext: m e s s a g e - Keyword: m o n a r c h ### Step 4: Assign Alphabet Positions - A = 0, B = 1, ..., Z = 25 Let's write the corresponding numbers: | Letter | m | e | s | s | a | g | e | |--------|---|---|---|---|---|---|---| | Value |12 | 4 |18 |18 | 0 | 6 | 4 | | Keyword | m | o | n | a | r | c | h | |---------|---|---|---|---|---|---|---| | Value |12 |14 |13 | 0 |17 | 2 | 7 | ### Step 5: Encrypt Using Vigenère Formula Ciphertext letter = (Plaintext letter + Keyword letter) mod 26 | Step | Calculation | Value | Letter | |------|---------------------|-------|--------| | 1 | (12 + 12) mod 26 | 24 | Y | | 2 | (4 + 14) mod 26 | 18 | S | | 3 | (18 + 13) mod 26 | 5 | F | | 4 | (18 + 0) mod 26 | 18 | S | | 5 | (0 + 17) mod 26 | 17 | R | | 6 | (6 + 2) mod 26 | 8 | I | | 7 | (4 + 7) mod 26 | 11 | L | ### Step 6: Final Ciphertext **Encrypted Message:** ``` YSFSRIL ``` --- ### **Summary Table** | Plaintext | m | e | s | s | a | g | e | |-----------|---|---|---|---|---|---|---| | Keyword | m | o | n | a | r | c | h | | Encrypted | Y | S | F | S | R | I | L | --- ### **Final Answer:** - **Algorithm to use:** Vigenère Cipher (or any polyalphabetic cipher similar to it) - **Ciphertext:** `YSFSRIL` If you have any more questions or want to see the decryption process, let me know!

Let's provide a detailed step-by-step explanation with calculations for encrypting the plaintext "message" using the keyword "monarchy" with the Vigenère cipher. --- ### Step 1: Understanding the Vigenère Cipher The Vigenère cipher is a method of encrypting alphabetic text by applying a series of Caesar ciphers based on the letters of a keyword. - Each letter of the plaintext is shifted forward in the alphabet by a number of positions equal to the corresponding letter in the keyword. - The alphabet is considered circular, so after 'Z', it wraps around to 'A'. --- ### Step 2: Write the plaintext and keyword - Plaintext: **m e s s a g e** - Keyword: **m o n a r c h** Since the keyword is shorter than the plaintext, it is repeated or truncated to match the plaintext length. Here, the keyword is exactly the same length as the plaintext. --- ### Step 3: Convert letters to numerical positions Assign each letter a number from 0 to 25: - A = 0, B = 1, ..., Z = 25 Convert each letter: **Plaintext:** - m → 12 - e → 4 - s → 18 - s → 18 - a → 0 - g → 6 - e → 4 **Keyword:** - m → 12 - o → 14 - n → 13 - a → 0 - r → 17 - c → 2 - h → 7 --- ### Step 4: Apply the Vigenère encryption formula The encryption formula for each character is: **Ciphertext letter = (Plaintext letter + Keyword letter) mod 26** Calculate each: 1. **First letter:** - Plaintext: 12 (m) - Keyword: 12 (m) - Sum: 12 + 12 = 24 - 24 mod 26 = 24 - Corresponds to 'Y' (since 24 → Y) 2. **Second letter:** - Plaintext: 4 (e) - Keyword: 14 (o) - Sum: 4 + 14 = 18 - 18 mod 26 = 18 - Corresponds to 'S' (18 → S) 3. **Third letter:** - Plaintext: 18 (s) - Keyword: 13 (n) - Sum: 18 + 13 = 31 - 31 mod 26 = 5 - Corresponds to 'F' (5 → F) 4. **Fourth letter:** - Plaintext: 18 (s) - Keyword: 0 (a) - Sum: 18 + 0 = 18 - 18 mod 26 = 18 - Corresponds to 'S' (18 → S) 5. **Fifth letter:** - Plaintext: 0 (a) - Keyword: 17 (r) - Sum: 0 + 17 = 17 - 17 mod 26 = 17 - Corresponds to 'R' (17 → R) 6. **Sixth letter:** - Plaintext: 6 (g) - Keyword: 2 (c) - Sum: 6 + 2 = 8 - 8 mod 26 = 8 - Corresponds to 'I' (8 → I) 7. **Seventh letter:** - Plaintext: 4 (e) - Keyword: 7 (h) - Sum: 4 + 7 = 11 - 11 mod 26 = 11 - Corresponds to 'L' (11 → L) --- ### **Final encrypted message:** **"YSFSRIL"** --- ### **Summary:** - The algorithm used is **Vigenère cipher**. - The encrypted ciphertext for the plaintext "message" with the keyword "monarchy" is **"YSFSRIL"**. If you need further clarification or decryption steps, feel free to ask!