<?php
/**
 * Purpose-bound authenticated encryption for Connector V2 outbox envelopes.
 */

if (!defined('ABSPATH')) {
    exit;
}

class Akkedli_V2_Outbox_Crypto
{
    const VERSION = 'v1';
    const CIPHER = 'aes-256-gcm';
    const PURPOSE = 'akkedli:wordpress-connector:outbox-key:v1';
    const AAD = 'akkedli:wordpress-connector:outbox:v1';
    const MAX_PLAINTEXT = 5243136;

    public static function seal(array $value, $event_id, $operation)
    {
        self::validate_context($event_id, $operation);
        Akkedli_V2_Crypto::assert_available();
        $plaintext = wp_json_encode($value, JSON_UNESCAPED_SLASHES);
        if (!is_string($plaintext) || '' === $plaintext || strlen($plaintext) > self::MAX_PLAINTEXT) {
            throw new RuntimeException('Outbox encryption failed.');
        }
        try {
            $salt = random_bytes(32);
            $iv = random_bytes(12);
            $key = self::key($salt);
            $tag = '';
            $ciphertext = openssl_encrypt($plaintext, self::CIPHER, $key, OPENSSL_RAW_DATA,
                $iv, $tag, self::aad($event_id, $operation), 16);
        } catch (Throwable $error) {
            unset($error, $plaintext, $salt, $iv, $key, $tag, $ciphertext);
            throw new RuntimeException('Outbox encryption failed.');
        }
        if (!is_string($ciphertext) || '' === $ciphertext || 16 !== strlen($tag)) {
            unset($plaintext, $salt, $iv, $key, $tag, $ciphertext);
            throw new RuntimeException('Outbox encryption failed.');
        }
        $envelope = implode(':', array(self::VERSION, self::CIPHER,
            Akkedli_V2_Crypto::base64url_encode($salt), Akkedli_V2_Crypto::base64url_encode($iv),
            Akkedli_V2_Crypto::base64url_encode($ciphertext), Akkedli_V2_Crypto::base64url_encode($tag)));
        unset($plaintext, $salt, $iv, $key, $tag, $ciphertext);
        return $envelope;
    }

    public static function open($envelope, $event_id, $operation)
    {
        self::validate_context($event_id, $operation);
        Akkedli_V2_Crypto::assert_available();
        if (!is_string($envelope) || strlen($envelope) > 7500000) {
            throw new RuntimeException('Stored outbox data is invalid.');
        }
        $parts = explode(':', $envelope);
        if (6 !== count($parts) || self::VERSION !== $parts[0] || self::CIPHER !== $parts[1]) {
            throw new RuntimeException('Stored outbox data is invalid.');
        }
        try {
            $salt = Akkedli_V2_Crypto::base64url_decode($parts[2]);
            $iv = Akkedli_V2_Crypto::base64url_decode($parts[3]);
            $ciphertext = Akkedli_V2_Crypto::base64url_decode($parts[4]);
            $tag = Akkedli_V2_Crypto::base64url_decode($parts[5]);
            if (32 !== strlen($salt) || 12 !== strlen($iv) || '' === $ciphertext
                || strlen($ciphertext) > self::MAX_PLAINTEXT || 16 !== strlen($tag)) {
                throw new RuntimeException('Stored outbox data is invalid.');
            }
            $key = self::key($salt);
            $plaintext = openssl_decrypt($ciphertext, self::CIPHER, $key, OPENSSL_RAW_DATA,
                $iv, $tag, self::aad($event_id, $operation));
        } catch (Throwable $error) {
            unset($error, $salt, $iv, $ciphertext, $tag, $key, $plaintext);
            throw new RuntimeException('Stored outbox data is invalid.');
        }
        unset($salt, $iv, $ciphertext, $tag, $key);
        $value = is_string($plaintext) ? json_decode($plaintext, true) : null;
        $canonical = is_array($value) ? wp_json_encode($value, JSON_UNESCAPED_SLASHES) : false;
        if (!is_array($value) || JSON_ERROR_NONE !== json_last_error() || !is_string($canonical)
            || !hash_equals($plaintext, $canonical)) {
            unset($plaintext, $canonical, $value);
            throw new RuntimeException('Stored outbox data is invalid.');
        }
        unset($plaintext, $canonical);
        return $value;
    }

    private static function validate_context($event_id, $operation)
    {
        if (!is_string($event_id) || 1 !== preg_match('/^[0-9a-f]{64}$/D', $event_id)
            || !in_array($operation, array('order', 'status'), true)) {
            throw new RuntimeException('Outbox context is invalid.');
        }
    }

    private static function aad($event_id, $operation)
    {
        return implode(':', array(self::AAD, 'blog', (string) get_current_blog_id(),
            'event', $event_id, 'operation', $operation));
    }

    private static function key($salt)
    {
        $context = self::PURPOSE . ':blog:' . (string) get_current_blog_id();
        $key = hash_hkdf('sha256', wp_salt('auth'), 32, $context, $salt);
        if (!is_string($key) || 32 !== strlen($key)) {
            throw new RuntimeException('Outbox encryption failed.');
        }
        return $key;
    }
}