MPP

MPP, the Machine Payments Protocol, carries payment in HTTP authentication, as the Payment HTTP authentication scheme defines it: the seller answers with one or more WWW-Authenticate: Payment challenges, each naming a method (such as evm, tempo, solana or stripe) and an intent (charge, session or subscription), and the buyer answers with a credential that echoes the challenge it pays. Import the MPP pieces from @integraledger/lcp/mpp.

Where H ridesCopy Anchor Link

On every MPP pairing, H rides in the challenge itself, in two places:

• The challenge id is the base64url (no padding) of H's 32 bytes, then . and the challenge's position in the 402 response. A Tempo subscription challenge's id is the bare base64url of H. challengeId(h, index) writes it and challengeH(id) reads H back.
• The challenge's opaque map carries legalContext, H's LCP string, and legalContextUrl, the link. Where the pairing's payment carries H in nothing public, it also carries legalContextAgreementUrl.

MPP binds the credential to the challenge it echoes, so whatever the payer signs over the challenge id, or over a value derived from it, commits to H. Some methods also carry H in a request member the payer signs; CARRIER names that member for each intent and method (for example externalId for solana charges, and methodDetails.invoiceId for xrpl).

Placing and readingCopy Anchor Link

place(doc, h, link, option) is the seller's placement over a list of challenges: the challenge whose bound members equal option gets the id derived from H and an opaque holding the seller's own map plus the LCP members. read(doc) is the buyer's reading: the challenges whose opaque carries an LCP hash and an https link and whose id derives from that hash. pairingsOf(challenge) names the pairings a challenge offers, and tie(challenges) is the ATR's binding slot: every challenge's bound members, as issued.

import { hash } from "@integraledger/lcp";
import { challengeH, pairingsOf, place, read, type MppChallenge } from "@integraledger/lcp/mpp";

const request = Buffer.from(
  JSON.stringify({
    amount: "10000",
    currency: "0x036CbD53842c5426634e7929541eC2318f3dCF7e",
    methodDetails: {
      chainId: 84532,
      credentialTypes: ["permit2", "authorization"],
      permit2Address: "0x000000000022D473030F116dDEE9F6B43aC78BA3",
    },
    recipient: "0x209693Bc6afc0C5328bA36FaF03C514EF312287C",
  }),
).toString("base64url");
const issued: MppChallenge = {
  id: "issued-by-the-seller",
  realm: "api.seller.example",
  method: "evm",
  intent: "charge",
  request,
  expires: "2026-09-21T14:14:20Z",
};
console.log(pairingsOf(issued));

const h = await hash(new TextEncoder().encode("abc"));
const placed = place([issued], h, `https://atr.seller.example/${h}`, issued);
if ("refused" in placed) throw new Error(placed.code);
const id = placed[0]!.id!;
console.log(id, challengeH(id) === h);
console.log(Buffer.from(placed[0]!.opaque!, "base64url").toString());

const offer = read(placed);
if ("refused" in offer) throw new Error(offer.code);
console.log(offer.h === h, offer.offer.challenges.length);

[ 'mpp/charge/evm/permit2', 'mpp/charge/evm/authorization' ]
ungWv48Bz-pBQUDeXa4iI7ADYaOWF3qctBD_YfIAFa0.0 true
{"legalContext":"lcp:sha256:0xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad","legalContextUrl":"https://atr.seller.example/0xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"}
true 1

parseChallenges(fieldValues) parses WWW-Authenticate field values into challenges, and problem(code) gives the Payment scheme's problem type and HTTP status for a refusal code.

The MPP pairingsCopy Anchor Link

Each pairing's build(choice, h) takes the chosen challenge (MppChoice) and returns what the buyer signs. Its bound(credential) reads H from the echoed challenge and checks that what was signed carries it; reference and status read the settlement where the method lands on a chain.

The LCP profile mpp/charge states these rules in full.