CAPTCHA なしでフォームスパムを防ぐ方法

const encoder = new TextEncoder();

async function key(secret: string) {
  return crypto.subtle.importKey("raw", encoder.encode(secret), { name: "HMAC", hash: "SHA-256" }, false, [
    "sign",
    "verify",
  ]);
}

function toHex(bytes: ArrayBuffer) {
  return [...new Uint8Array(bytes)].map((byte) => byte.toString(16).padStart(2, "0")).join("");
}

function fromHex(hex: string) {
  return new Uint8Array((hex.match(/../g) ?? []).map((pair) => Number.parseInt(pair, 16)));
}

export async function signRender(secret: string, now = Date.now()) {
  const signature = await crypto.subtle.sign("HMAC", await key(secret), encoder.encode(`render.${now}`));
  return `${now}.${toHex(signature)}`;
}

export type RenderCheck = "ok" | "too_fast" | "invalid";

export async function checkRender(secret: string, token: string, minSeconds = 3, now = Date.now()): Promise<RenderCheck> {
  const [time = "", signature = ""] = token.split(".");
  if (!/^\d{13}$/.test(time) || !/^[0-9a-f]{64}$/.test(signature)) return "invalid";
  const valid = await crypto.subtle.verify("HMAC", await key(secret), fromHex(signature), encoder.encode(`render.${time}`));
  const age = now - Number(time);
  if (!valid || age < 0 || age > 86_400_000) return "invalid";
  return age < minSeconds * 1000 ? "too_fast" : "ok";
}

---
import { signRender } from "../lib/render-token";

export const prerender = false;

const rendered = await signRender(import.meta.env.RENDER_SECRET);
---

<form method="POST" action="/api/contact">
  <input type="hidden" name="rendered" value={rendered} />
  <input name="_gotcha" type="text" tabindex="-1" autocomplete="off" hidden />
  <label for="name">Name</label>
  <input id="name" name="name" required />
  <label for="email">Email</label>
  <input id="email" name="email" type="email" required />
  <label for="message">Message</label>
  <textarea id="message" name="message" required></textarea>
  <button>Send</button>
</form>

const WEIGHTS = { honeypot: 5, too_fast: 3, invalid_render: 3, link_in_name: 3, many_links: 2, phrase: 2 };
const THRESHOLD = 5;
const LINK = /https?:\/\/|www\./gi;
const PHRASES = /\b(seo services|guest post|backlinks|first page of google|crypto investment)\b/i;

type Reason = keyof typeof WEIGHTS;

export function scoreContact(input: {
  name: string;
  message: string;
  trap: string;
  render: "ok" | "too_fast" | "invalid";
}) {
  const reasons: Reason[] = [];
  if (input.trap.trim()) reasons.push("honeypot");
  if (input.render === "too_fast") reasons.push("too_fast");
  if (input.render === "invalid") reasons.push("invalid_render");
  if (input.name.match(LINK)) reasons.push("link_in_name");
  if ((`${input.name} ${input.message}`.match(LINK)?.length ?? 0) >= 3) reasons.push("many_links");
  if (PHRASES.test(`${input.name} ${input.message}`)) reasons.push("phrase");
  const score = reasons.reduce((total, reason) => total + WEIGHTS[reason], 0);
  return { score, reasons, spam: score >= THRESHOLD };
}

import type { APIRoute } from "astro";
import { checkRender } from "../../lib/render-token";
import { scoreContact } from "../../lib/score";

export const POST: APIRoute = async ({ request, redirect }) => {
  const form = await request.formData();
  const render = await checkRender(import.meta.env.RENDER_SECRET, String(form.get("rendered") ?? ""));
  const verdict = scoreContact({
    name: String(form.get("name") ?? ""),
    message: String(form.get("message") ?? ""),
    trap: String(form.get("_gotcha") ?? ""),
    render,
  });
  await saveMessage(Object.fromEntries(form), verdict.spam ? "spam" : "received", verdict.reasons);
  return redirect("/thanks", 303);
};

import { getSchema, submit } from "@ikiform/sdk";

const FORM_ID = "frm_xxxxxxxxxxxxxxxx";
const form = document.querySelector("form");
const schema = await getSchema(FORM_ID);
const rendered = schema.ok ? schema.schema.ts : null;

form?.addEventListener("submit", async (event) => {
  event.preventDefault();
  const data = new FormData(form);
  if (rendered) data.set("_ikiform_ts", rendered);
  const result = await submit(FORM_ID, data);
  form.replaceWith(result.ok ? "Thanks, we'll be in touch." : result.error.message);
});