<?php
/**
 * Backup encryption (AES-256-GCM via OpenSSL, or XChaCha20-Poly1305 via Sodium).
 *
 * @package Spinix_Migration
 */

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

class Spinix_Crypto {

	const CIPHER_AES_GCM  = 1;
	const CIPHER_XCHACHA  = 2;
	const KDF_ITERATIONS  = 100000;

	/** @var string */
	private $key;
	/** @var int */
	private $cipher;

	/**
	 * @param string $password Password.
	 * @param string $salt     16-byte salt.
	 * @param int    $cipher   Cipher id.
	 * @throws Spinix_Error When no crypto backend exists.
	 */
	public function __construct( $password, $salt, $cipher ) {
		$this->cipher = (int) $cipher;
		$this->key    = hash_pbkdf2( 'sha256', (string) $password, $salt, self::KDF_ITERATIONS, 32, true );
		if ( ! self::supports( $this->cipher ) ) {
			throw Spinix_Error::make( 'php_extension_missing', array( 'ext' => esc_html( self::CIPHER_AES_GCM === $this->cipher ? 'openssl' : 'sodium' ) ) );
		}
	}

	/**
	 * Best cipher available on this server, or 0.
	 *
	 * @return int
	 */
	public static function best_cipher() {
		if ( self::supports( self::CIPHER_AES_GCM ) ) {
			return self::CIPHER_AES_GCM;
		}
		if ( self::supports( self::CIPHER_XCHACHA ) ) {
			return self::CIPHER_XCHACHA;
		}
		return 0;
	}

	/**
	 * @param int $cipher Cipher.
	 * @return bool
	 */
	public static function supports( $cipher ) {
		if ( self::CIPHER_AES_GCM === (int) $cipher ) {
			return function_exists( 'openssl_encrypt' ) && in_array( 'aes-256-gcm', array_map( 'strtolower', openssl_get_cipher_methods() ), true );
		}
		if ( self::CIPHER_XCHACHA === (int) $cipher ) {
			return function_exists( 'sodium_crypto_aead_xchacha20poly1305_ietf_encrypt' );
		}
		return false;
	}

	/**
	 * Value stored in the header to check the password without decrypting data.
	 *
	 * @return string 32 bytes.
	 */
	public function key_check() {
		return hash_hmac( 'sha256', 'spinix-key-check-v1', $this->key, true );
	}

	/**
	 * Fixed per-block overhead in bytes.
	 *
	 * @return int
	 */
	public function overhead() {
		return self::CIPHER_AES_GCM === $this->cipher ? 28 : 40;
	}

	/**
	 * Encrypt a block.
	 *
	 * @param string $plain Data.
	 * @return string nonce . tag . ciphertext
	 */
	public function encrypt( $plain ) {
		if ( self::CIPHER_AES_GCM === $this->cipher ) {
			$iv  = random_bytes( 12 );
			$tag = '';
			$ct  = openssl_encrypt( $plain, 'aes-256-gcm', $this->key, OPENSSL_RAW_DATA, $iv, $tag, '', 16 );
			return $iv . $tag . $ct;
		}
		$nonce = random_bytes( 24 );
		$ct    = sodium_crypto_aead_xchacha20poly1305_ietf_encrypt( $plain, '', $nonce, $this->key );
		return $nonce . $ct;
	}

	/**
	 * Decrypt a block.
	 *
	 * @param string $data Data.
	 * @return string|false
	 */
	public function decrypt( $data ) {
		if ( self::CIPHER_AES_GCM === $this->cipher ) {
			$iv  = substr( $data, 0, 12 );
			$tag = substr( $data, 12, 16 );
			$ct  = (string) substr( $data, 28 );
			return openssl_decrypt( $ct, 'aes-256-gcm', $this->key, OPENSSL_RAW_DATA, $iv, $tag );
		}
		$nonce = substr( $data, 0, 24 );
		try {
			return sodium_crypto_aead_xchacha20poly1305_ietf_decrypt( (string) substr( $data, 24 ), '', $nonce, $this->key );
		} catch ( Throwable $e ) {
			return false;
		}
	}
}