<?php
/**
 * Encrypted form token.
 *
 * @package ALPEG\Forms
 */

namespace ALPEG\Forms;

defined( 'ABSPATH' ) || exit;

/**
 * The form configuration (fields, recipient address, texts) travels with the
 * form, encrypted and tamper-proof. The server therefore knows exactly which
 * rules apply on submission, no matter whether the form sits in a page, a
 * template, a pattern or a widget. Visitors can neither read the
 * configuration (e.g. the recipient address) nor change it.
 *
 * Uses libsodium (part of PHP since 7.2; WordPress also ships sodium_compat).
 */
class Token {

	/**
	 * Derive the key from the WordPress salts.
	 *
	 * @return string 32 bytes.
	 */
	private static function key() {
		return sodium_crypto_generichash( wp_salt( 'auth' ) . '|alpeg-forms|token', '', SODIUM_CRYPTO_SECRETBOX_KEYBYTES );
	}

	/**
	 * Encrypt data.
	 *
	 * @param array $payload Data.
	 * @return string URL-safe base64.
	 */
	public static function encode( array $payload ) {
		$nonce  = random_bytes( SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );
		$json   = wp_json_encode( $payload );
		$cipher = sodium_crypto_secretbox( (string) $json, $nonce, self::key() );
		return rtrim( strtr( base64_encode( $nonce . $cipher ), '+/', '-_' ), '=' ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_encode
	}

	/**
	 * Decrypt and verify a token.
	 *
	 * @param string $token Token.
	 * @return array|null Data, or null if tampered with.
	 */
	public static function decode( $token ) {
		if ( ! is_string( $token ) || strlen( $token ) > 60000 ) {
			return null;
		}
		$raw = base64_decode( strtr( $token, '-_', '+/' ), true ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_decode
		if ( false === $raw || strlen( $raw ) <= SODIUM_CRYPTO_SECRETBOX_NONCEBYTES + SODIUM_CRYPTO_SECRETBOX_MACBYTES ) {
			return null;
		}
		$nonce  = substr( $raw, 0, SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );
		$cipher = substr( $raw, SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );
		try {
			$json = sodium_crypto_secretbox_open( $cipher, $nonce, self::key() );
		} catch ( \Throwable $e ) {
			return null;
		}
		if ( false === $json ) {
			return null;
		}
		$data = json_decode( $json, true );
		return is_array( $data ) ? $data : null;
	}
}