MPP Development Patterns

Before writing code

Fetch live docs:

• Specification: Fetch https://paymentauth.org/ for canonical security and encoding requirements
• IETF draft: Web-search site:datatracker.ietf.org draft-ryan-httpauth-payment for the latest RFC draft
• Stripe docs: Fetch https://docs.stripe.com/payments/machine/mpp for production integration patterns
• SDK: Web-search site:npmjs.com mppx for error handling and retry APIs

Pattern: HMAC Challenge Binding

The 32-byte secretKey is the server's core security primitive:

• Every challenge ID is HMAC-bound to the secret key
• Prevents challenge forgery — attackers cannot create valid challenges without the key
• Prevents replay attacks — each challenge is unique and bound to the request context
• Generate with openssl rand -hex 32
• Rotate periodically and support key rollover

const mppx = Mppx.create({
  secretKey: process.env.MPP_SECRET_KEY,  // 32-byte hex
  methods: [/* ... */],
});

Pattern: Replay Protection

Single-use proof semantics prevent double-payment:

• Each credential can only be used once
• Server must track consumed credentials (at least until expiration)
• Atomic verification — check + consume in a single operation
• Use database transactions or atomic operations for credential tracking
• For high-throughput: use in-memory stores (Redis) with TTL matching challenge expiration

Pattern: Transport Security

MPP mandates TLS 1.2+ (TLS 1.3 recommended):

• All endpoints must serve over HTTPS (port 443)
• Cache-Control: no-store on all responses containing credentials
• Never log plaintext credentials or payment proofs
• Credentials are treated as sensitive bearer tokens

Pattern: JSON Canonicalization

RFC 8785 canonical JSON is required for deterministic encoding:

• Challenges must use canonical JSON before base64url encoding
• Keeps challenges under 8KB (server-side)
• Clients must handle minimum 4KB challenges
• Use a library for RFC 8785 compliance rather than manual JSON serialization

Pattern: Error Handling

MPP uses RFC 9457 Problem Details for error responses:

{
  "type": "https://paymentauth.org/problems/payment-required",
  "status": 402,
  "detail": "Payment is required.",
  "challengeId": "..."
}

Error types:

• payment-required — Normal challenge, no payment attempted yet
• verification-failed — Invalid proof of payment
• payment-expired — Challenge or credential expired
• malformed-credential — Cannot parse the credential

Client retry strategy:

• payment-required — Normal flow, fulfill and retry
• verification-failed — Do NOT retry with same credential (likely invalid)
• payment-expired — Request a fresh challenge
• malformed-credential — Fix credential encoding, do not retry blindly

Pattern: Receipt Validation

The Payment-Receipt header proves delivery:

• Contains payment status, method, timestamp, reference ID
• Clients should store receipts for accounting and disputes
• Servers should generate receipts atomically with resource delivery
• Receipts serve as proof in billing reconciliation

Pattern: Rate Limiting

Even with payments, implement rate limiting:

• Prevents abuse from clients with large wallets
• Protects upstream resources from overload
• Return 429 with Retry-After header when exceeded
• Client-side: honor Retry-After, add jitter

Pattern: Amount Safety

• All amounts as strings representing the smallest currency unit
• Never use floating-point for monetary calculations
• Validate amounts on both client and server
• Reject negative or zero amounts
• Verify amount matches the challenged amount in the credential

Pattern: Secret Management

• MPP_SECRET_KEY — 32-byte hex, rotate periodically
• STRIPE_SECRET_KEY — Stripe API key, never in code
• WALLET_PRIVATE_KEY — Crypto wallet key, use key management service
• Never commit secrets to version control
• Use .env files for development, secrets manager for production
• Implement key rotation without downtime

Pattern: Monitoring & Observability

• Log every payment challenge issued (without sensitive data)
• Track payment success/failure rates per method
• Monitor challenge expiration rates (too many = too short TTL)
• Alert on verification failure spikes (potential attack)
• Track revenue per endpoint for pricing optimization
• Monitor Tempo chain health and Stripe API availability
• Dashboard payment method distribution

Pattern: Testing

• Use Tempo testnet for development (not mainnet)
• Use Stripe test mode keys and test card tokens
• Test the full 402 flow end-to-end
• Test challenge expiration handling
• Test replay rejection
• Test with multiple payment methods simultaneously
• Load test to verify payment verification throughput

Pattern: Caching

• Service discovery responses (/openapi.json, /llms.txt) — cache aggressively
• Payment challenges — NEVER cache (time-bound, single-use)
• Credentials — NEVER cache (Cache-Control: no-store)
• Upstream API responses (for proxies) — cache if idempotent and appropriate

Fetch the latest MPP specification and IETF draft for current security requirements, encoding rules, and best practices before implementing.