@ar.io/anchor-s3

Anchor as you store. Wrap your S3 client once, and every putObject also:

1. stores your object exactly as asked,
2. anchors a tamper-evident provenance record to ar.io (your bytes are hashed locally — they never leave your infrastructure), and
3. writes that record beside the object as <key>.provenance.json, so the bucket carries its own offline-verifiable audit trail.

npm install @ar.io/anchor-s3 @ar.io/anchor @aws-sdk/client-s3

import { S3Client } from "@aws-sdk/client-s3";
import { createAnchorer } from "@ar.io/anchor";
import { anchoredS3 } from "@ar.io/anchor-s3";

const s3 = anchoredS3(new S3Client({}), createAnchorer()); // dev mode: zero config

const { receipt, provenanceKey } = await s3.putObject({
  Bucket: "models",
  Key: "prod/scorer.pkl",
  Body: modelBytes,
});

receipt.txId;        // permanent ar.io anchor for these exact bytes
provenanceKey;       // "prod/scorer.pkl.provenance.json" — verify offline, anytime

Body must be a string or Uint8Array (it is hashed in-process; streams aren't accepted here).

Verifying a sidecar

The sidecar is JSON: { txId, gatewayUrl, envelope, record, contentHash, environment }. Anyone with the object and its sidecar can verify offline with the read-only @ar.io/proof — no ar.io service in the trust path:

import { ed25519Verify, jcs, sha256Hex, utf8 } from "@ar.io/proof";

const sidecar = JSON.parse(sidecarJson);

// 1. The object bytes are what was anchored.
(await sha256Hex(objectBytes)) === sidecar.record.event.content_hash;

// 2. The record is what the envelope committed to.
(await sha256Hex(utf8(jcs(sidecar.record)))) === sidecar.envelope.payload_hash;

// 3. The envelope is authentically signed.
const { signature, ...pre } = sidecar.envelope;
await ed25519Verify(signature, utf8(jcs(pre)), sidecar.envelope.public_key);

Cross-check against the chain by fetching txId from any ar.io gateway (https://<gateway>/raw/<txId> — e.g. turbo-gateway.com; browse the network at gateways.ar.io) and comparing it to sidecar.envelope.

Semantics

• If anchoring fails, the object is already stored but no sidecar is written — the put itself is never rolled back.
• For production credentials (explicit signing key, funded wallet) and the typed-error table: see @ar.io/anchor.