Verify Anikeen ID webhooks

Webhook::constructEvent() checks the Anikeen-Signature header (HMAC-SHA256
of "t.body" with the endpoint's secret, 5-minute tolerance) and returns
the event, like Stripe's helper.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Id/Exceptions/SignatureVerificationException.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Anikeen\Id\Exceptions;
+
+use Exception;
+
+/**
+ * The webhook request was not signed by Anikeen ID with your endpoint's secret, or is too old.
+ */
+class SignatureVerificationException extends Exception
+{
+    //
+}

src/Id/Webhooks/Webhook.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Anikeen\Id\Webhooks;
+
+use Anikeen\Id\Exceptions\SignatureVerificationException;
+
+/**
+ * Verifies webhooks from Anikeen ID, like Stripe's Webhook::constructEvent().
+ *
+ * Anikeen ID signs every request with your endpoint's signing secret (whsec_…):
+ * `Anikeen-Signature: t=<unix time>,v1=<hex HMAC-SHA256 of "<t>.<raw body>">`.
+ */
+final class Webhook
+{
+    /** Requests older (or newer) than this many seconds are rejected to prevent replays. */
+    public const DEFAULT_TOLERANCE = 300;
+
+    /**
+     * Verifies the signature and returns the event: ['id', 'type', 'created', 'data' => ['object' => …]].
+     *
+     * @param  string  $payload  the raw request body (not re-encoded JSON)
+     * @param  string|null  $signatureHeader  the Anikeen-Signature header
+     *
+     * @throws SignatureVerificationException
+     */
+    public static function constructEvent(string $payload, ?string $signatureHeader, string $secret, int $tolerance = self::DEFAULT_TOLERANCE): array
+    {
+        self::verify($payload, $signatureHeader, $secret, $tolerance);
+
+        $event = json_decode($payload, true);
+
+        if (!is_array($event) || !isset($event['id'], $event['type'])) {
+            throw new SignatureVerificationException('The webhook payload is not a valid event.');
+        }
+
+        return $event;
+    }
+
+    /**
+     * @throws SignatureVerificationException
+     */
+    public static function verify(string $payload, ?string $signatureHeader, string $secret, int $tolerance = self::DEFAULT_TOLERANCE): void
+    {
+        $timestamp = null;
+        $signatures = [];
+
+        foreach (explode(',', (string) $signatureHeader) as $part) {
+            [$key, $value] = array_pad(explode('=', trim($part), 2), 2, null);
+
+            if ($key === 't' && ctype_digit((string) $value)) {
+                $timestamp = (int) $value;
+            } elseif ($key === 'v1' && $value) {
+                $signatures[] = $value;
+            }
+        }
+
+        if ($timestamp === null || $signatures === []) {
+            throw new SignatureVerificationException('The Anikeen-Signature header is missing or malformed.');
+        }
+
+        $expected = hash_hmac('sha256', "{$timestamp}.{$payload}", $secret);
+
+        if (!array_filter($signatures, fn (string $signature) => hash_equals($expected, $signature))) {
+            throw new SignatureVerificationException('The webhook signature does not match. Check the signing secret.');
+        }
+
+        if ($tolerance > 0 && abs(time() - $timestamp) > $tolerance) {
+            throw new SignatureVerificationException('The webhook timestamp is outside the tolerance.');
+        }
+    }
+}