<?php
/**
 * Form Submission Token.
 *
 * Generates and verifies time-windowed HMAC tokens used to authenticate
 * public form submissions. Unlike WordPress nonces (12–24 h expiry), these
 * tokens are keyed on half-day windows and remain valid across multiple
 * windows, making them safe to embed in cached pages without any client-side
 * refresh logic.
 *
 * @package SureForms
 * @since   2.6.0
 */

namespace SRFM\Inc;

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

/**
 * Submit_Token
 *
 * Security properties:
 *  - HMAC-SHA256 keyed on the site's WordPress auth salt (unique per install).
 *  - Tokens are form-specific: a token issued for form 5 is invalid for form 6.
 *  - Each token encodes a half-day time window (43 200 s). Verification accepts
 *    the current window plus the previous SRFM_SUBMIT_TOKEN_ACCEPTED_WINDOWS − 1
 *    windows, giving up to 48 hours of validity. This covers even aggressive CDN
 *    or full-page-cache TTLs without any refresh round-trip from the browser.
 *  - Constant-time comparison (`hash_equals`) prevents timing-based oracle attacks.
 *
 * @since 2.6.0
 */
class Submit_Token {
	/**
	 * Length of a single time window in seconds (12 hours).
	 *
	 * Rotating every 12 hours keeps the attack window short while staying
	 * far below typical cache TTLs.
	 *
	 * @since 2.6.0
	 */
	public const WINDOW_SECONDS = 43200;

	/**
	 * Number of consecutive windows that are considered valid.
	 *
	 * 4 windows × 12 hours = 48 hours maximum token lifetime.
	 * Raise via the `srfm_submit_token_accepted_windows` filter if your
	 * deployment uses unusually long cache TTLs.
	 *
	 * @since 2.6.0
	 */
	public const DEFAULT_ACCEPTED_WINDOWS = 4;

	/**
	 * Generate a submission token for a given form.
	 *
	 * The token encodes the form ID and the current half-day window, signed
	 * with the site's auth salt. It is safe to embed in cached HTML because
	 * `verify()` accepts several consecutive past windows.
	 *
	 * @since  2.6.0
	 * @param  int $form_id The form post ID.
	 * @return string 64-character lowercase hex HMAC-SHA256 token.
	 */
	public static function generate( int $form_id ): string {
		return self::sign( $form_id, self::current_window() );
	}

	/**
	 * Verify a token submitted with a form.
	 *
	 * Checks the token against every accepted window, from newest to oldest,
	 * using constant-time comparison throughout.
	 *
	 * @since  2.6.0
	 * @param  string $token   Token value received from the client.
	 * @param  int    $form_id Form post ID extracted from the request body.
	 * @return bool True if the token is valid for the given form, false otherwise.
	 */
	public static function verify( string $token, int $form_id ): bool {
		if ( '' === $token || $form_id <= 0 ) {
			return false;
		}

		// Clamp to [1, 14]: must accept at least one window; cap at 14 (7 days)
		// to prevent a misconfigured filter from making tokens effectively permanent.
		$accepted = max( 1, min( 14, (int) apply_filters( 'srfm_submit_token_accepted_windows', self::DEFAULT_ACCEPTED_WINDOWS ) ) );

		// Walk backwards through accepted windows; current window first.
		for ( $offset = 0; $offset < $accepted; $offset++ ) {
			if ( hash_equals( self::sign( $form_id, self::current_window() - $offset ), $token ) ) {
				return true;
			}
		}

		return false;
	}

	/**
	 * Return the index of the current half-day window.
	 *
	 * Dividing Unix time by WINDOW_SECONDS yields an integer that increments
	 * once every 12 hours, regardless of the server's local timezone.
	 *
	 * @since  2.6.0
	 * @return int
	 */
	private static function current_window(): int {
		return (int) floor( time() / self::WINDOW_SECONDS );
	}

	/**
	 * Build an HMAC-SHA256 signature for a specific form / window pair.
	 *
	 * The payload deliberately combines a namespace prefix, the form ID, and
	 * the window index so that tokens cannot be repurposed across forms or
	 * replayed across time windows.
	 *
	 * @since  2.6.0
	 * @param  int $form_id Post ID of the form.
	 * @param  int $window  Half-day window index.
	 * @return string 64-character lowercase hex digest.
	 */
	private static function sign( int $form_id, int $window ): string {
		// Derive a plugin-specific sub-key from the site's auth salt so this
		// system has an independent key surface from WordPress session cookies.
		// Rotating wp-config.php secrets invalidates all outstanding tokens, which
		// is intentional — a cache purge should follow any secret key rotation.
		$signing_key = hash_hmac( 'sha256', 'srfm-submit-token-v1', wp_salt( 'auth' ) );
		$payload     = implode( '|', [ 'srfm_submit', $form_id, $window ] );
		return hash_hmac( 'sha256', $payload, $signing_key );
	}
}