fitness/backend/AuthenticationHandler.php

<?php
/**
 * Authentication & Security Handler
 * HMAC-based request validation for API endpoints
 */

class AuthenticationHandler {
    private $db;
    private const SIGNATURE_ALGORITHM = 'sha256';
    private const TIMESTAMP_TOLERANCE = 300; // 5 minutes in seconds

    public function __construct() {
        $this->db = Database::getInstance();
    }

    /**
     * Validate HMAC signature of incoming request
     * 
     * @param string $api_key The API key from request header
     * @param string $signature The HMAC signature from request header
     * @param string $payload The raw request body
     * @param string $timestamp The request timestamp
     * @return array ['valid' => bool, 'user_id' => int|null, 'error' => string|null]
     */
    public function validateHmacSignature($api_key, $signature, $payload, $timestamp) {
        // Validate timestamp to prevent replay attacks
        if (!$this->isValidTimestamp($timestamp)) {
            return [
                'valid' => false,
                'user_id' => null,
                'error' => 'Request timestamp is invalid or expired'
            ];
        }

        // Get user by API key
        $user = $this->getUserByApiKey($api_key);
        if (!$user) {
            // Log suspicious activity
            $this->logSecurityEvent('INVALID_API_KEY', $api_key);
            return [
                'valid' => false,
                'user_id' => null,
                'error' => 'Invalid API key'
            ];
        }

        // Generate expected signature
        $expectedSignature = $this->generateSignature(
            $payload,
            $user['api_secret'],
            $timestamp,
            $api_key
        );

        // Compare signatures using timing-safe comparison
        if (!hash_equals($expectedSignature, $signature)) {
            // Log failed authentication attempt
            $this->logSecurityEvent('INVALID_SIGNATURE', $api_key, $user['id']);
            return [
                'valid' => false,
                'user_id' => null,
                'error' => 'Invalid signature'
            ];
        }

        // Check if user is active
        if (!$user['is_active']) {
            return [
                'valid' => false,
                'user_id' => null,
                'error' => 'User account is inactive'
            ];
        }

        return [
            'valid' => true,
            'user_id' => $user['id'],
            'error' => null
        ];
    }

    /**
     * Generate HMAC signature
     * 
     * Signature format: HMAC-SHA256(timestamp|payload, api_secret)
     */
    private function generateSignature($payload, $api_secret, $timestamp, $api_key) {
        $data = $timestamp . '|' . $api_key . '|' . $payload;
        return hash_hmac(self::SIGNATURE_ALGORITHM, $data, $api_secret);
    }

    /**
     * Verify timestamp is within acceptable range
     */
    private function isValidTimestamp($timestamp) {
        $current_time = time();
        $request_time = (int)$timestamp;
        $time_diff = abs($current_time - $request_time);

        return $time_diff <= self::TIMESTAMP_TOLERANCE;
    }

    /**
     * Get user by API key
     */
    private function getUserByApiKey($api_key) {
        $stmt = $this->db->prepare('
            SELECT id, api_secret, is_active, uuid
            FROM users
            WHERE api_key = ?
            LIMIT 1
        ');

        $stmt->bind_param('s', $api_key);
        $stmt->execute();
        $result = $stmt->get_result();

        if ($result->num_rows === 0) {
            return null;
        }

        return $result->fetch_assoc();
    }

    /**
     * Log security events for audit trail
     */
    private function logSecurityEvent($event_type, $api_key, $user_id = null) {
        $ip_address = $this->getClientIpAddress();
        $user_agent = $_SERVER['HTTP_USER_AGENT'] ?? 'Unknown';

        $stmt = $this->db->prepare('
            INSERT INTO api_logs (user_id, endpoint, method, status_code, ip_address, user_agent, error_message, created_at)
            VALUES (?, ?, ?, ?, ?, ?, ?, NOW())
        ');

        $endpoint = $event_type;
        $method = $_SERVER['REQUEST_METHOD'];
        $status_code = 401;
        $error_msg = $event_type;

        $stmt->bind_param(
            'issssss',
            $user_id,
            $endpoint,
            $method,
            $status_code,
            $ip_address,
            $user_agent,
            $error_msg
        );

        $stmt->execute();
        $stmt->close();
    }

    /**
     * Get client IP address (handles proxies)
     */
    private function getClientIpAddress() {
        if (!empty($_SERVER['HTTP_CLIENT_IP'])) {
            return $_SERVER['HTTP_CLIENT_IP'];
        } elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
            $ips = explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']);
            return trim($ips[0]);
        } else {
            return $_SERVER['REMOTE_ADDR'] ?? 'Unknown';
        }
    }

    /**
     * Generate API key and secret for new user
     */
    public static function generateApiCredentials() {
        return [
            'api_key' => bin2hex(random_bytes(32)),
            'api_secret' => bin2hex(random_bytes(32))
        ];
    }

    /**
     * Hash password using bcrypt
     */
    public static function hashPassword($password) {
        return password_hash($password, PASSWORD_BCRYPT, ['cost' => 12]);
    }

    /**
     * Verify password
     */
    public static function verifyPassword($password, $hash) {
        return password_verify($password, $hash);
    }
}
?>