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Custom Python Schnorr Auditor

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Custom Python Schnorr Auditor

In this final guide, we will build a Python script that demonstrates the simplicity of the Schnorr Signature verification equation. While a real BIP340 implementation requires complex elliptic curve math, we can simulate the core logic using standard integers to see why it is so much more elegant than ECDSA.

The Schnorr Auditor

import hashlib

def schnorr_verify(public_key, message, signature_r, signature_s, generator):
 """
 Simulates the Schnorr Verification Equation: sG = R + eP
 """
 print(f"--- Schnorr Signature Audit ---")
 print(f"[*] Message: {message}")
 print(f"[*] Signature: (r: {signature_r}, s: {signature_s})")

 # 1. Calculate the 'Challenge' (e)
 # In reality, this is a tagged hash of R, P, and the message
 challenge_input = f"{signature_r}{public_key}{message}"
 e = int(hashlib.sha256(challenge_input.encode()).hexdigest(), 16) % 1000 # Small for demo

 # 2. Left Side of Equation: s * G
 left_side = signature_s * generator

 # 3. Right Side of Equation: R + e * P
 # Note: In ECC, these are point additions, here we use simple math
 right_side = signature_r + (e * public_key)

 print(f"[*] Challenge (e): {e}")
 print(f"[*] Left Side (sG): {left_side}")
 print(f"[*] Right Side (R+eP): {right_side}")

 if left_side == right_side:
 print("[SUCCESS] Signature is VALID!")
 return True
 else:
 print("[ERROR] Signature is INVALID!")
 return False

# --- Simulation ---

# Dummy values for demonstration
# G = Generator, d = Private Key, P = d*G (Public Key)
G = 7
d = 123
P = d * G # 861

# To sign, we pick a random nonce k and calculate R = k*G
k = 45
R = k * G # 315

# Calculate challenge e (simplified)
msg = "Spend 1 BTC"
e_demo = 12 # Assume hash result
s = (k + e_demo * d) # The Signature equation

# Audit the generated signature
schnorr_verify(P, msg, R, s, G)

How to Run the Auditor

  1. Ensure you have Python 3 installed.

  2. Copy the code into a file named schnorr_auditor.py.

  3. Run it using python3 schnorr_auditor.py.

Technical Takeaways

  1. Algebraic Beauty: Notice how the verification equation is just simple linear algebra ($s = k + ed$). Because we multiply both sides by $G$, it becomes a test of curve points.

  2. No Modular Inverse: Unlike ECDSA, which requires calculating a modular inverse ($k^{-1}$), Schnorr only uses addition and multiplication. This is safer and faster.

  3. X-Only Advantage: In a real implementation, signature_r is just the X-coordinate. This auditor shows how that $r$ value interacts directly with the public key $P$ to reconstruct the proof of ownership.

Congratulations! You have completed the Schnorr Signatures (BIP340) module. You now understand the high-efficiency cryptography that powers the Taproot era of Bitcoin.

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