Inspiration
As AI becomes more capable, payment attacks can become faster, more convincing, and harder to detect. We wanted to build a payment protection system that focuses on securing transactions rather than making payments smarter.
That idea became Quantum Shield PayPal: a merchant-side security layer designed to validate payment operations, enforce authorization boundaries, and produce verifiable security evidence.
What it does
Quantum Shield PayPal adds a security layer around PayPal Sandbox payment workflows. It focuses on transaction integrity through cryptographic protection, payment validation, explicit authorization controls, replay prevention, and tamper-evident audit evidence.
The system is designed to detect and defend against threats such as unauthorized capture attempts, forged webhook signatures, replayed requests, and payment-data manipulation.
It also provides an operator dashboard for monitoring payment activity and security events.
How we built it
We built Quantum Shield PayPal using Rust, with a dedicated web dashboard and PayPal Sandbox integration.
The security architecture incorporates AES-256-GCM authenticated encryption, ML-KEM-768 post-quantum key encapsulation, and ML-DSA-65 digital signatures, alongside transaction validation, replay protection, encrypted configuration, key-management controls, and audit evidence.
We also developed a dedicated security testing environment to exercise attack scenarios and evaluate the system's defensive controls.
An important design decision was keeping payment authorization and capture under explicit application controls rather than giving an AI system authority over financial operations. AI-assisted tools can help conduct adversarial testing, but they are not the payment authority.
Challenges we ran into
One of our biggest challenges was combining advanced cryptographic mechanisms with practical payment workflows while maintaining reliable transaction handling.
We had to address authorization boundaries, duplicate events, invalid signatures, replay attempts, key management, secure configuration, and failure handling.
PayPal Sandbox compatibility also presented challenges. In particular, checkout encountered an UNSUPPORTED_PAYEE_CURRENCY error, preventing us from demonstrating a completed end-to-end payment. We chose to document this limitation rather than present an incomplete transaction as a successful payment.
Accomplishments we're proud of
We built a Rust-based payment-security application with a dedicated operator dashboard, encrypted configuration, transaction validation, explicit capture controls, audit evidence, and a security testing environment.
Our latest recorded automated Rust test run passed 284 tests, with 5 tests ignored. The payment security lab also reported 12 out of 12 scenarios passing.
We are particularly proud of building the security controls around the payment workflow rather than relying on encryption alone.
What we learned
We learned that payment security requires more than cryptography. Every transaction needs reliable authorization, strict validation, replay prevention, careful key management, and verifiable evidence.
We also learned that integrating cryptographic techniques into payment infrastructure requires careful separation between what our application controls and what remains dependent on external payment providers.
Most importantly, a successful automated security test is not the same as a successful live payment. Testing, integration, and production readiness must be evaluated separately.
What's next for Quantum Shield PayPal
Our next goals are to expand adversarial security testing, strengthen webhook verification, improve key lifecycle management, and complete more extensive end-to-end PayPal Sandbox testing.
We also want to evaluate the system under broader real-world conditions before considering any production deployment.
Quantum Shield PayPal is currently a Sandbox-focused security project, not production-ready payment infrastructure. It does not claim to make PayPal, TLS, or external payment infrastructure post-quantum secure, and it has not received production certification or compliance approval.
