Create 06_Digital_Signature_Forgery_Advanced.md

crypto/challenges/06_Digital_Signature_Forgery_Advanced.md

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+# Challenge 6: Digital Signature Forgery
+
+**Level:** Advanced
+
+**Description:**
+Provide a digital signature scheme with a weakness (e.g., using a small prime number). 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.
+
+
+**Answer:**
+For the message "WORLD," a forged signature could be 115.
+
+**Code:**
+```python
+n = 391
+e = 3
+message = "WORLD"
+
+# Compute numeric representation of message
+message_numeric = sum([ord(c) * (256 ** i) for i, c in enumerate(message[::-1])])
+
+# Compute forged signature
+forged_signature = message_numeric ** e % n
+
+print("Forged signature:", forged_signature)
+```
+