<?php
/**
 * Encryption of the connection's secrets at rest.
 *
 * @package ShowBits
 */

namespace ShowBits\WordPress;

defined( 'ABSPATH' ) || exit;

/**
 * The token and the push secret live in the options table, which backups, exports and SQL injections reach: they are
 * stored encrypted (libsodium secretbox, XSalsa20-Poly1305) with a key that lives in wp-config.php, either its own
 * (SHOWBITS_ENCRYPTION_KEY) or one derived from the site's salts. New salts make the stored secrets unreadable: then
 * the site connects again. A site whose wp-config.php has no salts of its own gets them from wp_salt(), which keeps
 * them in the options table next to the secrets: the settings page says so (key_in_config()).
 */
final class Crypto {
	private const PREFIX = 'sb1:';
	/** The salts wp_salt() compares for duplicates, and the four the key is derived from. */
	private const SALTS   = array( 'AUTH_KEY', 'AUTH_SALT', 'SECURE_AUTH_KEY', 'SECURE_AUTH_SALT', 'LOGGED_IN_KEY', 'LOGGED_IN_SALT', 'NONCE_KEY', 'NONCE_SALT', 'SECRET_KEY', 'SECRET_SALT' );
	private const DERIVED = array( 'AUTH_KEY', 'AUTH_SALT', 'SECURE_AUTH_KEY', 'SECURE_AUTH_SALT' );

	/** Whether the key comes from wp-config.php: SHOWBITS_ENCRYPTION_KEY, or salts that wp_salt() takes from there. */
	public static function key_in_config(): bool {
		if ( defined( 'SHOWBITS_ENCRYPTION_KEY' ) && is_string( SHOWBITS_ENCRYPTION_KEY ) && '' !== SHOWBITS_ENCRYPTION_KEY ) {
			return true;
		}
		$defined = array();
		foreach ( self::SALTS as $name ) {
			if ( defined( $name ) ) {
				$defined[ $name ] = (string) constant( $name );
			}
		}
		return self::salts_usable( $defined );
	}

	/**
	 * As wp_salt() decides: the four salts the key comes from are set, not the sample's phrase, and none of the site's
	 * salts has the same value as another (else wp_salt() makes one up and stores it in the database).
	 *
	 * @param array<string, string> $defined The salts wp-config.php defines, by name.
	 */
	public static function salts_usable( array $defined ): bool {
		$counts = array_count_values( array_values( $defined ) );
		foreach ( self::DERIVED as $name ) {
			$value = $defined[ $name ] ?? '';
			if ( '' === $value || 'put your unique phrase here' === $value || $counts[ $value ] > 1 ) {
				return false;
			}
		}
		return true;
	}

	/** The key: from SHOWBITS_ENCRYPTION_KEY if defined, else from AUTH_KEY and SECURE_AUTH_KEY with their salts. */
	private static function key(): string {
		$material = defined( 'SHOWBITS_ENCRYPTION_KEY' ) && is_string( SHOWBITS_ENCRYPTION_KEY ) && '' !== SHOWBITS_ENCRYPTION_KEY
			? SHOWBITS_ENCRYPTION_KEY
			: wp_salt( 'auth' ) . wp_salt( 'secure_auth' );
		return hash_hkdf( 'sha256', $material, SODIUM_CRYPTO_SECRETBOX_KEYBYTES, 'showbits-credentials' );
	}

	/** Seals a secret: a version mark, then nonce and ciphertext in base64. */
	public static function seal( string $plain ): string {
		$nonce = random_bytes( SODIUM_CRYPTO_SECRETBOX_NONCEBYTES );
		return self::PREFIX . base64_encode( $nonce . sodium_crypto_secretbox( $plain, $nonce, self::key() ) ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_encode
	}

	/** Opens a sealed secret; null when it is not one or the key changed. */
	public static function open( string $sealed ): ?string {
		if ( ! str_starts_with( $sealed, self::PREFIX ) ) {
			return null;
		}
		$raw = base64_decode( substr( $sealed, strlen( self::PREFIX ) ), true ); // phpcs:ignore WordPress.PHP.DiscouragedPHPFunctions.obfuscation_base64_decode
		if ( false === $raw || strlen( $raw ) <= SODIUM_CRYPTO_SECRETBOX_NONCEBYTES ) {
			return null;
		}
		$plain = sodium_crypto_secretbox_open( substr( $raw, SODIUM_CRYPTO_SECRETBOX_NONCEBYTES ), substr( $raw, 0, SODIUM_CRYPTO_SECRETBOX_NONCEBYTES ), self::key() );
		return false === $plain ? null : $plain;
	}
}