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Secure file upload with asymmetric encryption.

0.2.0 2016-07-25 17:05 UTC

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Last update: 2022-01-08 05:07:47 UTC


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This library is intended to help protect the contents of uploaded documents against an attacker who might gain file-system read-access to a PHP web application.

Using this library involves three main components:

  1. A public/private key pair.
  2. A web server for receiving uploaded documents. Uploaded documents are encrypted using the public key immediately upon upload, and then the unencrypted document is immediately destroyed.
  3. A file server from which your authorized users can retrieve uploaded documents. You will provide a process which retrieves encrypted documents from the web server and decrypts these documents onto the file server using the private key. Of course, you are responsible for maintaining a file server which you trust to host these unencrypted documents.

Note that the private key does not live on web server. If an attacker were to gain read-access to your web server while there were documents waiting to be moved to your file server, then the attacker would only see a set of encrypted documents and they would not be able to retrieve the private key which would give them the ability to decrypt these documents.


For this example, we assume that:

  1. Your web server is running Apache, *nix, and of course PHP.
  2. Your file server may be either *nix or Windows.
  3. We use (Composer)[https://getcomposer.org/] for package management, but you can modify the example to work without Composer.

This example does not answer how to move the encrypted files from the web server to the file server. On *nix, you might choose to move them with an rsync --delete ... command. On Windows, you could use WinSCP. Using an authorized_keys file, it's possible to create an automated job on either *nix or Windows which could move these files over automatically.

Create a Private/Public Key Pair

To create a private, public key pair in *nix:

  openssl genrsa -out my_key_name.pem 4096
  openssl rsa -in my_key_name.pem -pubout > my_key_name.pub

You should put the public copy of your key (my_key_name.pub) onto your web server. But you should not put the private copy of your key (my_key_name.pem) onto your web server. The private copy of your key will need to be on your file server.

Sample Web Application

Web application structure:

├── composer.json
├── index.php
├── cert
    ├── .htaccess
    └── my_key_name.pub
└── uploads
    └── .htaccess

The uploads directory must be writable to your Apache user. For example, you might use chmod o+w uploads.

The composer.json specifies the uwdoem/secure-upload package as a requirement. You'll need to run composer install to install this package and the vendor directory.


    "require": {
        "uwdoem/secure-upload": "^0.2.0"

We place .htaccess files that block visitor access to the cert and uploads directories.


deny from all


deny from all

The index.php is our primary page.



require_once(__DIR__ . '/vendor/autoload.php');

use UWDOEM\SecureUploads\Cipher;

// Turn on error reporting, but only for troubleshooting and development
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);

if ($_SERVER['REQUEST_METHOD'] === 'GET') { ?>

<form method="POST" action="." enctype="multipart/form-data">
    <input type="file" name="myFile">
    <input type="submit" value="Submit">

<?php } elseif ($_SERVER['REQUEST_METHOD'] === 'POST') {

    $thereWereUploadErrors = $_FILES['myFile']['error'] || $_FILES['myFile']['size'] === 0;
    if ($thereWereUploadErrors) { echo "Error uploading document"; die(); }

    // The path to your public key
    $publicKeyLocation = __DIR__ . '/cert/my_key_name.pub';

    // The path of the file, as uploaded by Apache
    $fileLocation = $_FILES['myFile']['tmp_name'];

    // The path to your uploads directory, where encrypted files shall be
    // stored.
    $destination = __DIR__ . '/uploads/';

    // The name of the uploaded file, as chosen by the submitter
    $oldFilename = $_FILES['myFile']['name'];

    // Here we determine what the name of the file will be when it is
    // decrypted onto the file server. The `Cipher` class provides a static
    // method for "scrubbing" the file name, but you could also choose to
    // prepend the file name with some identifying information, such as the
    // visitor's UWNetID, if you're forcing NetID authentication.
    $newFilename = Cipher::cleanFilename($oldFilename);

    Cipher::encrypt($newFilename, $fileLocation, $destination, $publicKeyLocation);

<p>Your file has been uploaded.</p>


For each file that is uploaded, four encryption files will be created. For example 719b5e92a27aefb858982131e8d3be56.data, 719b5e92a27aefb858982131e8d3be56.data.key, 719b5e92a27aefb858982131e8d3be56.info, and 719b5e92a27aefb858982131e8d3be56.info.key. Each uploaded file will have its own unique hash, prefixing the .data, .data.key, .info, and .info.key files.

All four of these files must be moved to your decryption script in order to decrypt the uploaded file.

Sample Decryption Script

In the tree below, I have uploaded two documents using the web application above and moved the resulting files into my decryption scrypt:

├── cert
│   └── my_key_name.pem
├── composer.json
├── decrypt.php
├── in
│   ├── 719b5e92a27aefb858982131e8d3be56.data
│   ├── 719b5e92a27aefb858982131e8d3be56.data.key
│   ├── 719b5e92a27aefb858982131e8d3be56.info
│   ├── 719b5e92a27aefb858982131e8d3be56.info.key
│   ├── 7a3f189af2fc309128e144f2fc3d419e.data
│   ├── 7a3f189af2fc309128e144f2fc3d419e.data.key
│   ├── 7a3f189af2fc309128e144f2fc3d419e.info
│   └── 7a3f189af2fc309128e144f2fc3d419e.info.key
├── out
└── processed

The composer.json specifies the uwdoem/secure-upload package as a requirement. You'll need to run composer install to install this package and the vendor directory.


    "require": {
        "uwdoem/secure-upload": "^0.2.0"

Here is the script which performs the decryption. Having placed your encrypted data files into the in directory, you would invoke the script using php decrypt.php.



require_once dirname(__FILE__) . '/vendor/autoload.php';

use UWDOEM\SecureUploads\Cipher;

// The directory where the encrypted files are
$in = __DIR__ . '/in/';

// The directory that files shall be decrypted to
$out = __DIR__ . '/out/';

// A directory to put encrypted files after they have been decrypted
$processed = __DIR__ . '/processed/';

// The path to your pirvate key
$privateKeyLocation = __DIR__ . '/cert/my_key_name.pem';

// Scan through all of the files in the input directory...
$files = scandir($in);
foreach ($files as $file) {
    // If this is a data file...
    if (pathinfo($file, PATHINFO_EXTENSION) === 'data') {

        // Then identify the hash...
        $hash = strtok($file, '.');

        // Decrypt the file to the $out directory...
        Cipher::decrypt($in . $file, $out, $privateKeyLocation);

        // And move all of the encrypted/key/data files for this upload into
        // the $processed directory.
        foreach (["data", "data.key", "info", "info.key"] as $suffix) {
            rename("$in//$hash.$suffix", "$processed//$hash.$suffix");