feat: Implement Formal Verification and Harden Flash Loan Logic (#126)

[nuhumusamagaji]

🚀 Overview

This PR addresses issue #126 by hardening the Flash Loan provider contract against arithmetic vulnerabilities and introducing formal verification proofs using the Kani model checker. The implementation ensures that the flash loan logic is mathematically sound, safe from overflows, and correctly enforces atomic repayment.

🛡️ Security Improvements

• Checked Arithmetic: Replaced standard arithmetic operators with checked_mul, checked_div, and checked_add in src/flash_loan/lib.rs. This prevents potential overflow/underflow attacks that could occur with very large loan amounts or balances.
• Reentrancy Protection: Performed an architectural audit of the flash_loan call. Since the provider contract is stateless (no storage used during the execution) and relies on local stack variables for balance checks, it is inherently resistant to reentrancy attacks in the Soroban host environment.
• Explicit Failure Modes: Added descriptive panic messages for arithmetic failures to improve debuggability and ensure transaction atomicity.

🧪 Formal Verification (Kani Proofs)

Introduced a new verification suite in src/flash_loan/verification.rs that includes:

• Overflow/Underflow Safety Proof: Proves that the fee calculation is safe for the entire valid range of i128 inputs.
• Atomic Repayment Proof: Mathematically verifies that the loan can only succeed if the final balance is exactly initial_balance + fee, preventing any logic leaks.
• Invariants: Verified that the calculated fee is always non-negative and proportional to the loan amount.

📝 Changes

• Modified: src/flash_loan/lib.rs (Hardened logic & documentation)
• Created: src/flash_loan/verification.rs (Formal proofs)
• Created: implementation_plan.md (Detailed verification strategy)

✅ Verification Results

• Unit Tests: All existing tests in src/flash_loan/tests.rs passed.
• Kani Proofs: Verified that the contract logic remains safe under non-deterministic symbolic execution.

🔗 Related Issues

Fixes #126

[drips-wave]

@nuhumusamagaji Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[nuhumusamagaji]

Hello Maintainer, please review the PR as i have resolved the issue so i can get my point