Elixir implementation of the Machine Payments Protocol (MPP).

MPP enables machine-to-machine payments via HTTP 402. This library provides Plug middleware that adds pay-per-call billing to any Phoenix or Plug application.

How it works

1. Client requests a protected resource
2. Server responds with 402 Payment Required and a payment challenge
3. Client fulfills payment (Stripe, stablecoins, or on-chain)
4. Client retries with payment credential in Authorization: Payment header
5. Server verifies payment and returns the resource with a Payment-Receipt header

No user accounts. No API keys. No signup flow. Payment is authentication.

Protocol

Built on the MPP specification — an IETF draft co-developed by Stripe and Tempo Labs. Also supports x402 for on-chain payments.

Modules

• MPP.Plug — Plug middleware implementing the 402 payment handshake
• MPP.Challenge — HMAC-SHA256 bound challenge creation and verification
• MPP.Credential — Payment credential encoding/decoding
• MPP.Receipt — Proof-of-payment receipt serialization
• MPP.Headers — WWW-Authenticate, Authorization, Payment-Receipt header formatting
• MPP.Errors — RFC 9457 Problem Detail error types
• MPP.Method — Behaviour for pluggable payment methods
• MPP.Methods.Stripe — Stripe SPT payment verification
• MPP.Methods.Tempo — Tempo on-chain TIP-20 payment verification
• MPP.Methods.EVM — Generic EVM on-chain payment verification (any EVM chain)
• MPP.Intents.Charge — Charge intent request schema
• MPP.BodyDigest — SHA-256 body digest computation and verification
• MPP.Amount — Amount/decimals helpers (parse_units, dollar parsing)
• MPP.Expires — Expiration timestamp helpers (duration offsets, assert!)
• MPP.DID — DID helpers for EVM credential sources
• MPP.JCS — RFC 8785 JSON Canonicalization Scheme for HMAC interop
• MPP.Verifier — Transport-neutral payment credential verification
• MPP.Mcp — MCP (JSON-RPC) transport constants and helpers
• MPP.Client.PaymentProvider — Behaviour for client-side payment providers
• MPP.Client.MultiProvider — Multi-provider dispatch (first-match routing)
• MPP.Client.AcceptPolicy — Gate Accept-Payment header injection by URL

Discovery

Use MPP.describe/0-2 for progressive API discovery:

MPP.describe()                          # Level 1: all modules
MPP.describe(:challenge)                # Level 2: functions in Challenge
MPP.describe(:challenge, :create)       # Level 3: full contract for create/2