1 files changed, 88 insertions, 0 deletions
diff --git a/lib/models/CaptchaChallenge.php b/lib/models/CaptchaChallenge.php
new file mode 100644
index 0000000..446985d
--- /dev/null
+++ b/lib/models/CaptchaChallenge.php
@@ -0,0 +1,88 @@
+<?php
+final class CaptchaChallenge extends SimpleORMap
+{
+    public const ALGORITHM = 'SHA-256';
+    public const CHALLENGE_EXPIRATION = 5 * 60;
+
+    protected static function configure($config = [])
+    {
+        $config['db_table'] = 'captcha_challenges';
+
+        parent::configure($config);
+    }
+
+    protected static function getKey(): string
+    {
+        $key = Config::get()->CAPTCHA_KEY;
+        if ($key === '') {
+            $key = bin2hex(random_bytes(32));
+            Config::get()->store('CAPTCHA_KEY', $key);
+        }
+        return $key;
+    }
+
+    public static function createChallenge(string $salt, int $number): array
+    {
+        $algorithm = 'sha256';
+        $challenge = hash($algorithm, $salt . $number);
+        $signature = hash_hmac($algorithm, $challenge, self::getKey());
+
+        return [
+            'algorithm' => self::ALGORITHM,
+            'challenge' => $challenge,
+            'salt'      => $salt,
+            'signature' => $signature,
+        ];
+    }
+
+    public static function createNewChallenge(): array
+    {
+        do {
+            $salt = time() . '-' . bin2hex(random_bytes(12));
+            $number = random_int(1e3, 1e5);
+        } while (self::countBySql('salt = ? AND number = ?', [$salt, $number]) > 0);
+
+        return self::createChallenge($salt, $number);
+    }
+
+    public static function decodePayload(string $payload): array|null
+    {
+        return json_decode(base64_decode($payload), true);
+    }
+
+    public static function validatePayload(string $payload): string|bool
+    {
+        $json = self::decodePayload($payload);
+
+        if ($json === null) {
+            return _('Sie haben nicht bestätigt, dass Sie kein Roboter sind');
+        }
+
+        $time = explode('-', $json['salt'])[0];
+        if ($time < time() - self::CHALLENGE_EXPIRATION) {
+            return _('Die Challenge ist abgelaufen');
+        }
+
+        // Replay?
+        if (\CaptchaChallenge::countBySql('salt = ? AND number = ?', [$json['salt'], $json['number']]) > 0) {
+            return _('Nicht schummeln!');
+        }
+
+        $check = self::createChallenge($json['salt'], $json['number']);
+
+        if (
+            $json['algorithm'] !== $check['algorithm']
+            || $json['challenge'] !== $check['challenge']
+            || $json['signature'] !== $check['signature']
+        ) {
+            return _('Sie sind scheinbar ein Roboter...');
+        }
+
+        return true;
+    }
+
+    public static function gc(): void
+    {
+        self::deleteBySQL("mkdate < UNIX_TIMESTAMP() - ?", [self::CHALLENGE_EXPIRATION]);
+    }
+}