ledgerflow-backend/lambdas/authorizer/index.js

// lambdas/authorizer/index.js
// Lambda Authorizer — validates Google ID tokens sent as Bearer tokens
// Attached to every protected API Gateway route

const { OAuth2Client } = require("google-auth-library");
const { DynamoDBClient } = require("@aws-sdk/client-dynamodb");
const { DynamoDBDocumentClient, GetCommand, PutCommand } = require("@aws-sdk/lib-dynamodb");

const GOOGLE_CLIENT_ID = process.env.GOOGLE_CLIENT_ID;
const ALLOWED_DOMAINS  = (process.env.ALLOWED_DOMAINS || "").split(",").filter(Boolean); // e.g. "yourcompany.com"
const SESSIONS_TABLE   = process.env.SESSIONS_TABLE || "ledgerflow-sessions";

const googleClient = new OAuth2Client(GOOGLE_CLIENT_ID);

const dynamo = DynamoDBDocumentClient.from(
  new DynamoDBClient({ region: process.env.AWS_REGION || "us-east-1" }),
  { marshallOptions: { removeUndefinedValues: true } }
);

// ─── Main Handler ────────────────────────────────────────────────────────────

exports.handler = async (event) => {
  // Allow CORS preflight through without auth
  const method = event.requestContext?.http?.method || event.httpMethod;
  if (method === "OPTIONS") {
    return allowPolicy("preflight", {});
  }

  const token = extractToken(event);

  if (!token) {
    console.warn("[AUTH] No token provided");
    return denyPolicy("anonymous", "No token provided");
  }

  try {
    const payload = await verifyGoogleToken(token);

    // Optional: restrict to specific Google Workspace domains
    if (ALLOWED_DOMAINS.length > 0) {
      const domain = payload.email?.split("@")[1];
      if (!ALLOWED_DOMAINS.includes(domain)) {
        console.warn(`[AUTH] Domain not allowed: ${domain}`);
        return denyPolicy(payload.sub, "Domain not authorized");
      }
    }

    // Persist/update session record for audit trail
    await upsertSession(payload);

    console.info(`[AUTH] Granted: ${payload.email}`);

    return allowPolicy(payload.sub, {
      userId:  payload.sub,
      email:   payload.email,
      name:    payload.name,
      picture: payload.picture,
    });

  } catch (err) {
    console.error("[AUTH] Token verification failed:", err.message);
    return denyPolicy("unknown", err.message);
  }
};

// ─── Token Extraction ────────────────────────────────────────────────────────

function extractToken(event) {
  // API Gateway v2 (HTTP API) passes headers differently
  const authHeader =
    event.headers?.authorization ||
    event.headers?.Authorization ||
    "";
  if (authHeader.startsWith("Bearer ")) {
    return authHeader.slice(7);
  }
  // Also accept token in query string for WebSocket / SSE use cases
  return event.queryStringParameters?.token || null;
}

// ─── Google Token Verification ───────────────────────────────────────────────

async function verifyGoogleToken(idToken) {
  const ticket = await googleClient.verifyIdToken({
    idToken,
    audience: GOOGLE_CLIENT_ID,
  });
  const payload = ticket.getPayload();

  if (!payload) throw new Error("Empty token payload");
  if (!payload.email_verified) throw new Error("Email not verified by Google");

  return payload;
}

// ─── Session Persistence ─────────────────────────────────────────────────────

async function upsertSession(payload) {
  try {
    await dynamo.send(new PutCommand({
      TableName: SESSIONS_TABLE,
      Item: {
        userId:    payload.sub,
        email:     payload.email,
        name:      payload.name,
        picture:   payload.picture,
        lastSeen:  new Date().toISOString(),
        // TTL: auto-expire session record after 7 days of inactivity
        ttl: Math.floor(Date.now() / 1000) + (7 * 24 * 60 * 60),
      },
    }));
  } catch (err) {
    // Non-fatal — don't block auth if session write fails
    console.warn("[AUTH] Session upsert failed:", err.message);
  }
}

// ─── IAM Policy Builders ─────────────────────────────────────────────────────

function allowPolicy(principalId, context = {}) {
  return {
    isAuthorized: true,
    context,       // passed to downstream Lambdas via event.requestContext.authorizer.lambda
  };
}

function denyPolicy(principalId, reason = "") {
  return {
    isAuthorized: false,
    context: { reason },
  };
}