<?php
/**
 * Sealing a secret before it is stored.
 *
 * @package Adminkeep
 */

namespace Adminkeep\Service;

defined( 'ABSPATH' ) || exit;

/**
 * Authenticated encryption for one stored string.
 *
 * What this is for: a password that has to sit in the options table should not be readable in a
 * database dump or a backup. What it is not for: a server where the attacker can also read
 * wp-config.php, because the key is derived from the salts that live there.
 *
 * It is one small class so the storage can move to WordPress's own secrets handling when core
 * ships one, without touching its callers.
 */
class SecretBox {

	/**
	 * Marks a value as sealed by this class, and by which version of it.
	 */
	const PREFIX = 'ak1:';

	/**
	 * The derived key.
	 *
	 * @var string
	 */
	private $key;

	/**
	 * Build a box from any key material.
	 *
	 * @param string $material Secret material; hashed down to the key length.
	 */
	public function __construct( $material ) {
		$this->key = sodium_crypto_generichash( (string) $material, '', SODIUM_CRYPTO_SECRETBOX_KEYBYTES );
	}

	/**
	 * The box keyed by this site's salts.
	 *
	 * wp_salt() rather than the raw constants: it also answers on installs whose salts are kept
	 * in the database.
	 *
	 * @return self
	 */
	public static function from_salts() {
		return new self( wp_salt( 'auth' ) . wp_salt( 'secure_auth' ) );
	}

	/**
	 * Seal a string.
	 *
	 * @param string $plain Plain text.
	 * @return string Empty for empty input, otherwise PREFIX + base64( nonce . ciphertext ).
	 */
	public function encrypt( $plain ) {
		$plain = (string) $plain;

		if ( '' === $plain ) {
			return '';
		}

		$nonce = random_bytes( SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );

		// phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_encode -- Binary ciphertext made storable, not obfuscation.
		return self::PREFIX . base64_encode( $nonce . sodium_crypto_secretbox( $plain, $nonce, $this->key ) );
	}

	/**
	 * Open a sealed string.
	 *
	 * @param string $stored Value produced by encrypt().
	 * @return string|false The plain text, or false when it cannot be opened (wrong key,
	 *                      tampering, or a value this class did not make).
	 */
	public function decrypt( $stored ) {
		$stored = (string) $stored;

		if ( '' === $stored ) {
			return '';
		}

		if ( 0 !== strpos( $stored, self::PREFIX ) ) {
			return false;
		}

		// phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_decode -- Reverses encrypt().
		$raw = base64_decode( substr( $stored, strlen( self::PREFIX ) ), true );

		if ( false === $raw || strlen( $raw ) <= SODIUM_CRYPTO_SECRETBOX_NONCEBYTES ) {
			return false;
		}

		$nonce = substr( $raw, 0, SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );
		$plain = sodium_crypto_secretbox_open( substr( $raw, SODIUM_CRYPTO_SECRETBOX_NONCEBYTES ), $nonce, $this->key );

		return false === $plain ? false : $plain;
	}
}