To securely transfer critical files from one website to another, cURL was chosen due to its adaptability. It is crucial to ensure that the recipient site receives the files precisely as sent.

During the transfer process over SSL, a key is dispatched along with the files and stored exclusively on both the sending website and receiving site. The interception of files during transit is inconsequential as long as the recipient site rejects any counterfeit data. That said, what are the best tactics to secure this procedure?

To authenticate a request, a signature needs to be appended. This involves a confidential key which is shared only between the sender and receiver of the request. As an example, a sha256 signature computed as sha256(request_body + secret) can be sent via the Authorization header.

The recipient's server confirms whether the signature corresponds with the particular file. Along with the request body and key, it's also possible to include other data to produce the signature. Additionally, the hashing algorithm has the flexibility to be modified.

When dealing with cURL, the contents of the pem file should follow a similar format to this:

-----BEGIN CERTIFICATE-----
MIICoTCCAgoCAUUwDQYJKoZIhvcNAQEEBQewfsAwgaQxCzAJBgNVBAYTAlJVMQwwCgYD
-----END CERTIFICATE-----

-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: DES-EDE3-CBC,BAD99sd68972692C9D

DqcMvBR7c6podfhYPgPzLy+W4WrCR9pZZuSBxiuhIlfgOLWqFNeQfUZSJ7R4PeJrz+
-----END RSA PRIVATE KEY-----
To access a page, specify the URL of the authorization page, as well as the full path to the certificate under FILE_PEM_FORMAT and the password for the certificate, if installed, under PASSWORD.

$ch = curl_init();
$url = 'URL';
curl_setopt( $ch, CURLOPT_URL, URL);
curl_setopt ( $ch , CURLOPT_SSL_VERIFYPEER, 0 );
curl_setopt ( $ch , CURLOPT_SSL_VERIFYHOST, 0 );
curl_setopt ( $ch , CURLOPT_SSLCERT, FILE_PEM_FORMAT );
curl_setopt ( $ch , CURLOPT_SSLCERTPASSWD, PASSWORD );
echo curl_exec($ch);
Using a certificate like this can help ensure secure access to web pages. Additionally, it's important to keep the certificate and the password confidential and secure.

It's essential to use HTTPS for the transfer. This ensures that the data is encrypted during transit. The use of SSL certificates makes it much harder for any third parties to intercept or read the files. You should also ensure that the SSL certificates are up-to-date and properly configured, as misconfigured certificates can lead to vulnerabilities.

Second, implement authentication mechanisms. Before the file transfer occurs, both websites should verify each other's identities. This could be achieved through API keys or tokens. Additionally, the shared key for encrypting the files should be stored in a secure manner, such as using environment variables or a secrets management system.

Third, consider using a hashing algorithm to generate a checksum for the files being transferred. After the transfer, the receiving site should compute the checksum again and compare it to the original. This ensures that the files have not been altered during the transfer. Secure hashing algorithms such as SHA-256 should be employed for this task as they are considered secure.

Fourth, limit the access to the transfer points. This means that only authorized users or systems should be able to initiate a file transfer. You might want to implement IP whitelisting or basic authentication to restrict access.

Fifth, logging and monitoring should be integrated into the transfer process. Keeping a record of when files are sent, who sent them, and any errors or issues encountered can help to troubleshoot problems and also indicate any potential unauthorized attempts to access the files.

Sixth, use a secure method for sending the file itself, such as cURL with the correct flags. For instance, using the -T flag to transfer files and making sure to validate the certificate with the --cacert option. Furthermore, you can use the --cert and --key options for client certificates to enhance authentication.

Finally, perform regular security audits and updates to both the sending and receiving sites. Keeping software and libraries up-to-date minimizes the risk of exploits. Implementing a Content Security Policy (CSP) and regularly reviewing access logs could also be beneficial in identifying any odd behavior.

Below is a simple example using cURL in PHP to securely transfer a file from one server to another over HTTPS. The example includes file integrity checks with a hash, as well as basic authentication.

<?php
// Variables for the transfer
$sourceFile = 'path/to/your/local/file.txt';
$destinationUrl = 'https://recipient-website.com/upload.php';
$apiKey = 'YOUR_API_KEY'; // For authentication
$privateKey = 'path/to/your/private.key'; // Client certificate
$publicCert = 'path/to/your/public.cert'; // Public certificate

// Calculate the hash of the file before sending
$fileHash = hash_file('sha256', $sourceFile);

// Initialize cURL session
$ch = curl_init();

// Set cURL options
curl_setopt($ch, CURLOPT_URL, $destinationUrl);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, [
    'file' => new CURLFile($sourceFile), // Using CURLFile to upload
    'hash' => $fileHash, // Sending the hash for verification
]);
curl_setopt($ch, CURLOPT_HTTPHEADER, [
    'Authorization: Bearer ' . $apiKey, // Bearer token for API key
]);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, true);
curl_setopt($ch, CURLOPT_CAINFO, $publicCert); // Certificate for SSL
curl_setopt($ch, CURLOPT_SSLCERT, $publicCert);
curl_setopt($ch, CURLOPT_SSLKEY, $privateKey);

// Execute the transfer
$response = curl_exec($ch);

// Error handling
if (curl_errno($ch)) {
    echo 'Error:' . curl_error($ch);
} else {
    // If response is OK, you might want to handle the success
    echo 'Response from server: ' . $response;
}

// Close the cURL session
curl_close($ch);
?>

On the recipient side (in `upload.php`), you would receive the file and verify its hash as follows:

<?php
if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['file']) && isset($_POST['hash'])) {
    $uploadedFile = $_FILES['file']['tmp_name'];
    $receivedHash = $_POST['hash'];

    // Calculate hash of the received file
    $fileHash = hash_file('sha256', $uploadedFile);

    // Verify the hash
    if ($fileHash === $receivedHash) {
        // Move uploaded file to desired location
        move_uploaded_file($uploadedFile, 'path/to/save/' . $_FILES['file']['name']);
        echo 'File uploaded successfully!';
    } else {
        echo 'File integrity check failed. Possible tampering detected.';
    }
} else {
    echo 'Invalid request.';
}
?>

In this example, the sender computes a SHA-256 hash of the file before transferring it. The receiver computes the hash of the uploaded file and compares it with the hash sent in the request. This way, you can ensure that the file has not been tampered with during transfer. Additionally, the use of API key and SSL certificates enhances the security of the process.