payments-dashboard/src/expenseReceiver.js

import crypto from "node:crypto";
import {
  SecretsManagerClient,
  GetSecretValueCommand,
} from "@aws-sdk/client-secrets-manager";
import { LambdaClient, InvokeCommand } from "@aws-sdk/client-lambda";

const secrets = new SecretsManagerClient();
const lambda = new LambdaClient();

const EXPENSE_PROCESSOR_FN = process.env.EXPENSE_PROCESSOR_FN;
const EXPENSE_SIGNING_SECRET_NAME = process.env.EXPENSE_SIGNING_SECRET_NAME;

let cachedSigningSecret;
async function getSigningSecret() {
  if (cachedSigningSecret) return cachedSigningSecret;
  const { SecretString } = await secrets.send(
    new GetSecretValueCommand({ SecretId: EXPENSE_SIGNING_SECRET_NAME })
  );
  cachedSigningSecret = SecretString;
  return cachedSigningSecret;
}

function verifySignature(body, timestamp, signature, secret) {
  if (!timestamp || !signature) return false;
  const ts = Number(timestamp);
  if (!Number.isFinite(ts)) return false;
  if (Math.abs(Date.now() / 1000 - ts) > 300) return false;

  const base = `v0:${timestamp}:${body}`;
  const expected =
    "v0=" + crypto.createHmac("sha256", secret).update(base).digest("hex");

  return crypto.timingSafeEqual(
    Buffer.from(expected),
    Buffer.from(signature)
  );
}

export const handler = async (event) => {
  let body = event.body || "";
  if (event.isBase64Encoded) {
    body = Buffer.from(body, "base64").toString("utf-8");
  }

  const headers = Object.fromEntries(
    Object.entries(event.headers || {}).map(([k, v]) => [k.toLowerCase(), v])
  );
  const timestamp = headers["x-slack-request-timestamp"] || "";
  const signature = headers["x-slack-signature"] || "";

  const secret = await getSigningSecret();
  if (!verifySignature(body, timestamp, signature, secret)) {
    console.log("Signature verification failed");
    return { statusCode: 401, body: "unauthorized" };
  }

  const payload = JSON.parse(body);

  if (payload.type === "url_verification") {
    return {
      statusCode: 200,
      headers: { "Content-Type": "text/plain" },
      body: payload.challenge || "",
    };
  }

  if (payload.type === "event_callback") {
    await lambda.send(
      new InvokeCommand({
        FunctionName: EXPENSE_PROCESSOR_FN,
        InvocationType: "Event",
        Payload: JSON.stringify(payload.event),
      })
    );
  }

  return { statusCode: 200, body: "" };
};