Update 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
# Challenge 4: Classic Vigenère Cipher
**Level:** Beginner
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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.