The ATR hash

The ATR hash (H) is SHA-256 over the ATR's exact bytes: the bytes the seller serves, as they arrive. Nothing is parsed, trimmed or re-encoded before hashing. hash(bytes) computes it, and assemble returns it beside the bytes it wrote.

Every ATR carries its own random id, so two ATRs with the same terms still have different hashes. H is unpredictable before the seller issues it and unique to one transaction.

The forms of HCopy Anchor Link

The same 32 bytes are written in four forms. The package writes every hash in lowercase and reads either case.

The structured form puts the link to the seller's copy beside the hash. toLegalContext(h, url, "snake") writes the link as legal_context_url for protocols that use snake case, and fromLegalContext reads either spelling.

import { fromLcpString, hash, hashEquals, toLcpString, toLegalContext, toRawBytes } from "@integraledger/lcp";

const h = await hash(new TextEncoder().encode("abc"));
console.log(h);
console.log(toLcpString(h));
console.log(JSON.stringify(toLegalContext(h, "https://atr.seller.example/abc")));
console.log(toRawBytes(h).length);

const upper = `0x${h.slice(2).toUpperCase()}`;
console.log(hashEquals(h, upper), fromLcpString(toLcpString(h)) === h);

0xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad
lcp:sha256:0xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad
{"legalContext":{"type":"sha256","value":"0xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad","legalContextUrl":"https://atr.seller.example/abc"}}
32
true true

Comparing two hashesCopy Anchor Link

hashEquals(a, b) is the one comparison (LCP §2.5). It is true only when both strings are 0x followed by 64 hex digits, in either case, and they decode to the same 32 bytes. Anything else, including a hash in another form, is false. It compares every byte, whatever the first difference.

Compare with hashEquals, never with ===: a hash that arrives in upper case is the same hash.

The linkCopy Anchor Link

The link beside H is an https URL that serves the ATR's exact bytes. isHttpsLink is the one rule every pairing applies to it:

• the string holds no whitespace, control character or backslash;
• it parses as an absolute URL whose scheme is https, in either case;
• its authority has a host, a DNS name or an IP literal, and no user information.

A pairing refuses a link of another scheme as <surface>/link-not-https, and any other link that fails the rule as <surface>/legal-context-malformed. A link is at most 2048 characters.

NextCopy Anchor Link

• Binding: where H rides in each payment.
• The buyer gate: comparing the served bytes with H before signing.