Update 04_Classic_Vigenere_Cipher.md

crypto/challenges/04_Classic_Vigenere_Cipher.md

@@ -1,6 +1,4 @@
-Certainly! Here are three additional cryptographic challenges suitable for your class, ranging in difficulty:
-
-### Challenge 4: Classic Vigenère Cipher
+# Challenge 4: Classic Vigenère Cipher
 
 **Level:** Beginner
 
@@ -17,42 +15,22 @@ Keyword: "KEYWORD"
 1. Utilize the given keyword to decrypt the Vigenère cipher.
 2. Provide the original plaintext.
 
-### Challenge 5: Implement Diffie-Hellman Key Exchange
 
-**Level:** Intermediate
+**Answer:**
+The decrypted message is "WELCOMETOTHEWORLDOFCRYPTOGRAPHY"
 
-**Description:**
-Simulate the Diffie-Hellman key exchange algorithm to securely share a symmetric key between two parties.
+**Code:**
+```python
+def decrypt_vigenere(ciphertext, keyword):
+    keyword_repeated = (keyword * (len(ciphertext) // len(keyword))) + keyword[:len(ciphertext) % len(keyword)]
+    decrypted_text = ''
+    for i in range(len(ciphertext)):
+        decrypted_char = chr(((ord(ciphertext[i]) - ord(keyword_repeated[i])) % 26) + ord('A'))
+        decrypted_text += decrypted_char
+    return decrypted_text
 
-**Challenge Text:**
+ciphertext = "XBGXLTVJZTFKTRDCXWPNCRTGDHDDJQKFTZR"
+keyword = "KEYWORD"
+decrypted_text = decrypt_vigenere(ciphertext, keyword)
+print(decrypted_text)
 ```
-Given prime p = 23, base g = 5
-Party A's private key: 6
-Party B's private key: 15
-```
-
-**Instructions:**
-1. Compute Party A's and Party B's public keys.
-2. Compute the shared secret key for both parties.
-3. Validate that both parties have the same shared secret key.
-
-### Challenge 6: Digital Signature Forgery
-
-**Level:** Advanced
-
-**Description:**
-Provide a digital signature scheme with a weakness (e.g., using a small prime number). Challenge the students to forge a digital signature for a new message.
-
-**Challenge Text:**
-```
-Signature scheme: RSA with n = 391, e = 3, d = 107
-Signed message: ("HELLO", signature = 220)
-Challenge: Forge a signature for the message "WORLD"
-```
-
-**Instructions:**
-1. Understand the weakness in the provided RSA signature scheme.
-2. Forge a signature for the new message.
-3. Validate the forged signature.
-
-**Note:** Make sure to clarify the educational purpose of the challenges and emphasize the importance of ethical behavior and responsible use of these skills. Providing a controlled environment or sandbox for practicing these challenges will ensure that students can learn without violating any legal or ethical guidelines.