Multi-Tier SaaS Billing and Per-Tenant Quota Enforcement for Quantum-Safe Security-as-a-Service Platforms

As quantum-safe security transitions from research tooling to production security-as-a-service (SECaaS), platforms must implement commercially viable billing and usage enforcement without compromising the stateless, zero-dependency architecture that enables rapid deployment. We present a multi-tier billing and quota enforcement system for QCrypton comprising: (1) a Stripe-integrated subscription engine with four plan tiers (Trial at 10,000 queries/month, Large at 200,000 queries/month, Enterprise at unlimited, and unregistered at 1,000 queries/month), (2) per-tenant quota enforcement middleware that tracks monthly API usage via tenant key resolution (tenantId, API key, or IP fallback) and returns standard rate-limit headers (X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Used), (3) a 14-day free trial mechanism with automatic Stripe customer provisioning, and (4) webhook-driven subscription lifecycle management covering checkout completion, invoice payment, payment failure, and cancellation events. The system maintains QCrypton's stateless design through lazy Stripe initialization and in-memory quota tracking (Redis-backed in production), and integrates with the existing audit logging infrastructure for compliance. We evaluate the billing pipeline across all subscription lifecycle events and demonstrate quota enforcement overhead of under 0.05ms per request.