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.