I am having this issue, I am making a demo client-server application that include end-to-end encryption for word document. I can send word document without images properly but I can’t with files that contain images. I don’t think it is due to encryptor/decryptor because I can still encrypt and decrypt files that don’t contain images and it’s maybe due to sending and receiving function itself, here my code:

Sending files function:

private void SendFileButton_Click(object sender, RoutedEventArgs e)
        {
            Microsoft.Win32.OpenFileDialog openFileDialog = new Microsoft.Win32.OpenFileDialog
            {
                Filter = "Word Documents|*.docx"
            };

            if (openFileDialog.ShowDialog() == true)
            {
                string filePath = openFileDialog.FileName;
        
                byte[] encryptedData = EncryptData(filePath);

                stream.Write(encryptedData, 0, encryptedData.Length);
                MessageBox.Show("File sent!");
            }
        }`

Receiving files function:

private void ReceiveFileButton_Click(object sender, RoutedEventArgs e)
        {
           
              byte[] buffer = new byte[client.ReceiveBufferSize];
              int bytesRead = stream.Read(buffer, 0, buffer.Length);
             byte[] encryptedData = new byte[bytesRead];
              Array.Copy(buffer, encryptedData, bytesRead);
         
            byte[] decryptedData = DecryptData(encryptedData);
             
            Microsoft.Win32.SaveFileDialog saveFileDialog = new Microsoft.Win32.SaveFileDialog
            {
              Filter = "Word Documents|*.docx"
           };
         
            if (saveFileDialog.ShowDialog() == true)
            {
                string filePath = saveFileDialog.FileName;
                File.WriteAllBytes(filePath, decryptedData);
               MessageBox.Show("File received and saved!");
           }
        }`

I tried to check all over the encryptor, decryptor. I want to send encrypted word documents that contains images in order to decrypt it. When I tried to send them, and I click receive button, it say “padding is not invalid and cannot be removed”.
If you still want encryptor, decryptor, here they are:

       private byte[] EncryptData(string inputFilePath)
       {
           byte[] documentBytes;
           using (MemoryStream memoryStream = new MemoryStream())
           {
               using (WordprocessingDocument wordDoc = WordprocessingDocument.Open(inputFilePath, false))
               {
                   wordDoc.Clone(memoryStream);
               }
               documentBytes= memoryStream.ToArray();
           }

           byte[] encryptedData;
           using (Aes aes = Aes.Create())
           {
               aes.Key = aesKey;
               aes.GenerateIV();
               using (MemoryStream memoryStream = new MemoryStream())
               {
                   memoryStream.Write(aes.IV, 0, aes.IV.Length);
                   using (CryptoStream cryptoStream = new CryptoStream(memoryStream, aes.CreateEncryptor(), CryptoStreamMode.Write))
                   {
                       cryptoStream.Write(documentBytes, 0, documentBytes.Length);
                   }
                   encryptedData = memoryStream.ToArray();
                   return encryptedData;
               }
           }
         
       }

        private byte[] DecryptData(byte[] data)
        {
            byte[] decryptedData;
            using (Aes aes = Aes.Create())
            {
                aes.Key = aesKey;
                byte[] iv = new byte[aes.BlockSize / 8];
                Array.Copy(data, 0, iv, 0, iv.Length); 
                aes.IV = iv;

                using (MemoryStream memoryStream = new MemoryStream(data, iv.Length, data.Length - iv.Length))
                using (CryptoStream cryptoStream = new CryptoStream(memoryStream, aes.CreateDecryptor(), CryptoStreamMode.Read))
                using (MemoryStream decryptedStream = new MemoryStream())
                {
                    cryptoStream.CopyTo(decryptedStream);
                    decryptedData = decryptedStream.ToArray();
                    return decryptedData;
                }
            }
         }