<?php
/**
 * The capabilities two sites agree on when they pair.
 *
 * @package Shiftora\Connection
 */

declare(strict_types=1);

namespace Shiftora\Connection;

use Shiftora\Support\HttpClient;

// Prevent direct file access.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Read from a ping, a capability can be stripped by whoever sits on the link, which would
 * send the connection secret in the clear. The pairing answer is the one message that can be
 * authenticated before any secret is exchanged in the open: it is signed with the pairing
 * secret, which only the two sites (and whoever read the Connection Info) hold and which the
 * answer does not repeat. What it announces is pinned, and a pin is never lowered.
 */
final class PairingCapabilities {

	/** Capabilities that protect the connection itself. */
	private const PINNED = [ 'signs_responses', 'accepts_key_id' ];

	/** What this build advertises. */
	public static function advertised(): array {
		return [ 'signs_responses' => true, 'accepts_key_id' => true ];
	}

	/** Signature over the capabilities and the token issued with them, so neither can be swapped alone. */
	public static function sign( string $pairing_secret, string $local_token, array $capabilities ): string {
		return hash_hmac( 'sha256', self::payload( $local_token, $capabilities ), $pairing_secret, false );
	}

	public static function verify( string $pairing_secret, string $local_token, array $capabilities, string $signature ): bool {
		return hash_equals( self::sign( $pairing_secret, $local_token, $capabilities ), $signature );
	}

	/** Pins each announced capability for the peer's site. Nothing here ever removes one. */
	public static function pin( string $site_url, array $capabilities ): void {
		if ( true === ( $capabilities['signs_responses'] ?? false ) ) {
			HttpClient::expect_signed_responses( $site_url );
		}
		if ( true === ( $capabilities['accepts_key_id'] ?? false ) ) {
			HttpClient::expect_key_id( $site_url );
		}
	}

	/**
	 * Whether a peer pinned for these capabilities has stopped announcing one.
	 *
	 * @param array<string, mixed> $announced What a ping or handshake now says.
	 */
	public static function was_lowered( string $site_url, array $announced ): bool {
		return ( HttpClient::peer_accepts_key_id( $site_url ) && true !== ( $announced['accepts_key_id'] ?? false ) )
			|| ( HttpClient::peer_signs_responses( $site_url ) && true !== ( $announced['signs_responses'] ?? false ) );
	}

	private static function payload( string $local_token, array $capabilities ): string {
		$on = array_values( array_filter( self::PINNED, static fn( string $name ): bool => true === ( $capabilities[ $name ] ?? false ) ) );
		sort( $on );

		return "CAPABILITIES\n" . $local_token . "\n" . implode( ',', $on );
	}
}