<?php
/**
 * Authenticated encryption for secrets stored in the database (client secret, OAuth tokens).
 *
 * Uses libsodium's secretbox (XSalsa20-Poly1305). WordPress ships a sodium_compat polyfill,
 * so this works even when the PHP sodium extension is missing.
 *
 * @package Tidelist
 */

namespace Tidelist\Security;

use InvalidArgumentException;

/**
 * Authenticated encryption for secrets stored in the database (client secret, OAuth tokens).
 */
final class SecretBox {

	const PREFIX = 'sb1:';

	/**
	 * 32-byte key.
	 *
	 * @var string
	 */
	private $key;

	/**
	 * Constructor.
	 *
	 * @param string $key Raw 32-byte key (see derive_key()).
	 * @throws InvalidArgumentException When the key has the wrong length.
	 */
	public function __construct( string $key ) {
		if ( SODIUM_CRYPTO_SECRETBOX_KEYBYTES !== strlen( $key ) ) {
			throw new InvalidArgumentException( 'SecretBox key must be 32 bytes.' );
		}
		$this->key = $key;
	}

	/**
	 * Derives a 32-byte key from arbitrary key material (e.g. WordPress salts).
	 *
	 * @param string $material Key material.
	 */
	public static function derive_key( string $material ): string {
		return sodium_crypto_generichash( 'tidelist|' . $material, '', SODIUM_CRYPTO_SECRETBOX_KEYBYTES );
	}

	/**
	 * Encrypts a string.
	 *
	 * @param string $plain Plain text.
	 * @return string Prefixed base64 string, safe to store in an option.
	 */
	public function encrypt( string $plain ): string {
		$nonce = random_bytes( SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );
		// phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_encode -- encoding ciphertext for storage.
		return self::PREFIX . base64_encode( $nonce . sodium_crypto_secretbox( $plain, $nonce, $this->key ) );
	}

	/**
	 * Decrypts a string produced by encrypt().
	 *
	 * @param string $sealed Value from encrypt().
	 * @return string|null Plain text, or null if the value is malformed, tampered with or the key changed.
	 */
	public function decrypt( string $sealed ): ?string {
		if ( 0 !== strpos( $sealed, self::PREFIX ) ) {
			return null;
		}
		// phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_decode -- decoding stored ciphertext.
		$raw = base64_decode( substr( $sealed, strlen( self::PREFIX ) ), true );
		if ( false === $raw || strlen( $raw ) <= SODIUM_CRYPTO_SECRETBOX_NONCEBYTES ) {
			return null;
		}
		$nonce  = substr( $raw, 0, SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );
		$cipher = substr( $raw, SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );
		$plain  = sodium_crypto_secretbox_open( $cipher, $nonce, $this->key );
		return false === $plain ? null : $plain;
	}
}