The buyer gate

The buyer gate is the buyer-side check that compares the served bytes with H before anything is signed. It is the one step that makes paying mean agreeing: the buyer signs only a payment bound to a record it has fetched, hashed and kept.

sequenceDiagram
  participant B as Buyer
  participant S as Seller
  B->>S: request the resource
  S-->>B: 402 challenge carrying H and the link
  B->>S: fetch the link
  S-->>B: the ATR's exact bytes
  Note over B: SHA-256 over the bytes received, compared with H
  alt the hashes differ, or the fetch fails
    Note over B: decline, and sign nothing
  else the hashes are equal
    Note over B: build the payment with H in its place, then sign
    B->>S: the payment, carrying H
  end

The stepsCopy Anchor Link

1. Read. The pairing's read(doc) gives H, the link, and the options this pairing can pay. A document without a well-formed hash and https link is refused before anything is fetched.
2. Fetch. Fetch the link and keep the bytes exactly as received. Bound the fetch: this package's shared vectors fix a limit of 1 MiB on the body, a 10-second deadline, and no redirects.
3. Compare. hashEquals(await hash(bytes), h). On any difference, stop: nothing is signed.
4. Build and sign. The pairing's build(choice, h) returns what the signer signs, with H in its place. Pass the H you compared, never a value read again from the document.
5. Finish. complete(signature) gives the payment. Before sending it, bound(payment) reads H back from what was signed; it must equal the H you compared.
6. Keep the bytes. The buyer's copy of the ATR is its record of what it agreed to.

One changed byteCopy Anchor Link

The comparison is over bytes, not meaning. Re-serialising the same JSON, or changing one byte, gives another hash:

import { hash, hashEquals } from "@integraledger/lcp";

const utf8 = new TextEncoder();
const served = utf8.encode('{"atrVersion":"1","id":"0f8fad5b-d9cb-469f-a165-70867728950e","terms":"10000 units"}');
const h = await hash(served);

const reformatted = utf8.encode(JSON.stringify(JSON.parse(new TextDecoder().decode(served)), null, 1));
const edited = served.slice();
edited[served.length - 3] = 0x31;

console.log(hashEquals(await hash(served), h));
console.log(hashEquals(await hash(reformatted), h));
console.log(hashEquals(await hash(edited), h));

true
false
false

The gate as a packageCopy Anchor Link

This package gives the gate's pieces: read, hash, hashEquals, build and bound. The buyer packages in integra-agentic-terms assemble them into one gate with a bounded fetch and named declines, for TypeScript, Python and MCP clients. They follow the rows of vectors/buyer.json, which this package ships:

The agreement URLCopy Anchor Link

Where a pairing's payment carries H in nothing public, the seller may advertise an agreement URL (legalContextAgreementUrl) beside the link. The buyer first pays that URL, whose payment carries H publicly, and pays the full payment only after its 200 receipt names the same H. Where the chosen pairing's payment is itself a public proof of H (pattern.publicProof), the buyer does not pay the agreement URL. The buyer vector rows BA1 to BA7 fix this order.

NextCopy Anchor Link

• Buyer: the gate in code, step by step.
• Vectors: running the shared rows.