misc

server/contact.mjs

@@ -1,14 +1,20 @@
 /**
  * The contact form handler. nginx proxies POST /contact here; everything else on the site stays static.
  *
- * The form posts first_name, last_name, email, message, lang and the honeypot website. A browser
- * with scripting asks for JSON and shows the result in place; without scripting the answer is a
- * redirect to /contact/sent or /contact/error (under /de for German).
+ * The form posts first_name, last_name, email, message, lang, the honeypot website and altcha, the
+ * answer to a proof-of-work challenge from GET /contact/challenge (ALTCHA, solved in the browser
+ * while the visitor types). The browser asks for JSON and shows the result in place; a post that
+ * asks for HTML is answered with a redirect to /contact/sent or /contact/error (under /de for German).
+ * A post without a solved challenge is refused, so the form needs scripting; the address next
+ * to it does not.
  *
  * Configuration comes from the environment only, see README.md.
  */
 import { createServer } from 'node:http';
+import { randomBytes } from 'node:crypto';
 import nodemailer from 'nodemailer';
+import { createChallenge, randomInt, verifySolution } from 'altcha-lib';
+import { deriveKey } from 'altcha-lib/algorithms/pbkdf2';
 
 const env = process.env;
 const PORT = Number(env.CONTACT_PORT || 3000);
@@ -22,6 +28,11 @@ if (!env.SMTP_HOST || !FROM) {
   process.exit(78); // EX_CONFIG: the entrypoint does not restart on this one.
 }
 
+/* Challenge secrets. Without them set, each start makes its own; challenges then only outlive a
+   restart if the visitor fetched a new one, which the form does after every send. */
+const ALTCHA_SECRET = env.ALTCHA_HMAC_SECRET || randomBytes(32).toString('hex');
+const ALTCHA_KEY_SECRET = env.ALTCHA_HMAC_KEY_SECRET || randomBytes(32).toString('hex');
+
 const transport = nodemailer.createTransport({
   host: env.SMTP_HOST,
   port: SMTP_PORT,
@@ -38,6 +49,64 @@ const MAX = { first_name: 100, last_name: 100, email: 254, message: 5000 };
 const PER_IP = { count: 5, windowMs: 10 * 60 * 1000 };
 const GLOBAL = { count: 60, windowMs: 60 * 60 * 1000 };
 
+/* Proof of work. About half a second of a laptop's time and one to two of a phone's, spent while
+   the visitor is still typing; a script pays it for every message. The challenge is signed, so
+   the server keeps nothing until a solution comes back. */
+const POW = { algorithm: 'PBKDF2/SHA-256', cost: 1000, counterMin: 2000, counterMax: 5000 };
+const CHALLENGE_TTL_MS = 30 * 60 * 1000;
+// Nobody writes a message in three seconds from first touching the form; a script does.
+const MIN_FILL_MS = 3000;
+// Solved challenges, kept until they expire, so one solution sends one message.
+const spent = new Map();
+setInterval(() => {
+  const now = Date.now();
+  for (const [sig, expires] of spent) if (expires < now) spent.delete(sig);
+}, 60 * 1000).unref();
+
+function issueChallenge() {
+  const now = Date.now();
+  return createChallenge({
+    algorithm: POW.algorithm,
+    cost: POW.cost,
+    counter: randomInt(POW.counterMin, POW.counterMax),
+    deriveKey,
+    data: { issued: now },
+    expiresAt: new Date(now + CHALLENGE_TTL_MS),
+    hmacSignatureSecret: ALTCHA_SECRET,
+    hmacKeySignatureSecret: ALTCHA_KEY_SECRET,
+  });
+}
+
+/** The posted altcha field: base64 JSON of { challenge, solution }. */
+async function solved(field, now = Date.now()) {
+  let payload;
+  try {
+    payload = JSON.parse(Buffer.from(field || '', 'base64').toString('utf8'));
+  } catch {
+    return false;
+  }
+  const { challenge, solution } = payload || {};
+  if (!challenge?.signature || spent.has(challenge.signature)) return false;
+  let result;
+  try {
+    result = await verifySolution({
+      challenge,
+      solution,
+      deriveKey,
+      hmacSignatureSecret: ALTCHA_SECRET,
+      hmacKeySignatureSecret: ALTCHA_KEY_SECRET,
+    });
+  } catch {
+    return false;
+  }
+  if (!result.verified) return false;
+  // Signed with the challenge, so it is the time the server issued it.
+  const issued = Number(challenge.parameters.data?.issued);
+  if (!(now - issued >= MIN_FILL_MS)) return false;
+  spent.set(challenge.signature, challenge.parameters.expiresAt * 1000);
+  return true;
+}
+
 const hits = new Map();
 const globalHits = [];
 function limited(ip, now = Date.now()) {
@@ -85,7 +154,7 @@ function readBody(req) {
 }
 
 function reply(req, res, lang, outcome) {
-  const status = { sent: 200, invalid: 400, limited: 429, failed: 502 }[outcome];
+  const status = { sent: 200, invalid: 400, limited: 429, unverified: 403, failed: 502 }[outcome];
   if ((req.headers.accept || '').includes('application/json')) {
     res.writeHead(status, { 'Content-Type': 'application/json', 'Cache-Control': 'no-store' });
     res.end(JSON.stringify({ ok: outcome === 'sent', outcome }));
@@ -99,6 +168,11 @@ function reply(req, res, lang, outcome) {
 const server = createServer(async (req, res) => {
   const path = (req.url || '').split('?')[0];
   if (path === '/contact/healthz') { res.writeHead(200).end('ok\n'); return; }
+  if (path === '/contact/challenge' && req.method === 'GET') {
+    res.writeHead(200, { 'Content-Type': 'application/json', 'Cache-Control': 'no-store' });
+    res.end(JSON.stringify(await issueChallenge()));
+    return;
+  }
   if (req.method !== 'POST' || path !== '/contact') { res.writeHead(404).end(); return; }
 
   // A cross-site post is not a visitor using the form.
@@ -121,6 +195,10 @@ const server = createServer(async (req, res) => {
   const data = validate(form);
   if (!data) { reply(req, res, lang, 'invalid'); return; }
 
+  // No solved challenge: a script, or a browser without scripting. Checked before the rate
+  // limits, so unsolved posts cannot use up the budget meant for visitors.
+  if (!(await solved(form.get('altcha')))) { reply(req, res, lang, 'unverified'); return; }
+
   // nginx sets X-Real-IP from the proxy chain; fall back to the socket for local runs.
   const ip = req.headers['x-real-ip'] || req.socket.remoteAddress || 'unknown';
   if (limited(String(ip))) { reply(req, res, lang, 'limited'); return; }