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crypto/challenges/04_Classic_Vigenere_Cipher.md

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+Certainly! Here are three additional cryptographic challenges suitable for your class, ranging in difficulty:
+
+### Challenge 4: Classic Vigenère Cipher
+
+**Level:** Beginner
+
+**Description:**
+Crack a message encrypted using the Vigenère cipher with a known keyword.
+
+**Challenge Text:**
+```
+Encrypted Message: "XBGXLTVJZTFKTRDCXWPNCRTGDHDDJQKFTZR"
+Keyword: "KEYWORD"
+```
+
+**Instructions:**
+1. Utilize the given keyword to decrypt the Vigenère cipher.
+2. Provide the original plaintext.
+
+### Challenge 5: Implement Diffie-Hellman Key Exchange
+
+**Level:** Intermediate
+
+**Description:**
+Simulate the Diffie-Hellman key exchange algorithm to securely share a symmetric key between two parties.
+
+**Challenge 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.