Added PHP script that can decrypt data generated by Consturct 3's Cryptography plugin

Author: fodi

php/c3-cryptography-plugin-decrypt.php

@@ -0,0 +1,63 @@
+<?php
+
+// This script attempts to decrypt an urlencoded & base64 encoded string that
+// was generated using Construct 3's Cryptography plugin using password-based 
+// AES 256 encryption.
+// The decrypt_c3_aes_gcm function takes a base64 encoded string as an input.
+// The script expects the payload in the "data" GET parameter; since base64
+// strings are not URL safe, the payload must be urlencoded as well.
+
+define('PASSWORD', 'hackerman'); // enter the same password you specified in C3
+
+function decrypt_c3_aes_gcm($encrypted_base64, $password) {
+
+    // Base64 decode the input
+    $encrypted_data = base64_decode($encrypted_base64);
+
+    if ($encrypted_data === false) {
+        return false; // Return false if Base64 decoding fails
+    }
+
+    // Extract parts from the binary data based on the format described here:
+    // https://www.construct.net/en/make-games/manuals/construct-3/plugin-reference/cryptography#internalH1Link4
+
+    $reserved = ord($encrypted_data[0]);      // Reserved byte (should be 0)
+    $salt = substr($encrypted_data, 1, 16);   // Salt (16 bytes)
+    $iv = substr($encrypted_data, 17, 12);    // Initialization Vector (12 bytes)
+    $iterations = unpack('N', substr($encrypted_data, 29, 4))[1]; // Iterations (4 bytes, uint32)
+    $ciphertext = substr($encrypted_data, 33); // Ciphertext starts from byte 33 onwards
+    // Validate the reserved byte is zero
+    if ($reserved !== 0) {
+        return false; // Invalid format (reserved byte is not 0)
+    }
+
+    // Derive the encryption key using PBKDF2 with the provided password, salt, and iterations
+    $key = hash_pbkdf2('sha256', $password, $salt, $iterations, 32, true); // 256-bit (32-byte) key
+    // Separate the encrypted payload and the GCM authentication tag
+    $tag = substr($ciphertext, -16); // GCM authentication tag (16 bytes at the end)
+    $encrypted_payload = substr($ciphertext, 0, -16); // Encrypted data (without tag)
+    // Decrypt the payload
+    $decrypted_data = openssl_decrypt(
+            $encrypted_payload, // Ciphertext
+            'aes-256-gcm', // Cipher method
+            $key, // Key derived from password
+            OPENSSL_RAW_DATA, // Output raw data
+            $iv, // Initialization vector (IV)
+            $tag // GCM tag
+    );
+
+    // Return the decrypted text if successful, or false if decryption failed
+    return $decrypted_data !== false ? $decrypted_data : false;
+}
+
+// Get the encrypted data from the "data" GET parameter and urldecode it
+$encrypted_data_base64 = urldecode(filter_input(INPUT_GET, 'data'));
+
+// Try to decrypt the data
+$decrypted_text = decrypt_c3_aes_gcm($encrypted_data_base64, PASSWORD);
+
+if ($decrypted_text !== false) {
+    echo 'Decrypted Text: ' . $decrypted_text;
+} else {
+    echo 'Decryption failed!';
+}