How to protect an already existing PDF with a password?

Question

How to set password for an existing PDF?

Answer 1

Answer for @Alberto Question: How do u encrypt a pdf if only you have the byte array as input - and need another byte array as output, Used the previous answer.

I have a method called addPassword(byte[] templateByte) which accepts byte array of PDF file as a parameter and returns the encrypted byte array as a response.

public byte[] addPassword(byte[] templateByte)
    {

        String USER_PASS = "Hello123";
        String OWNER_PASS = "Deva123";
        PdfReader pdfReader = null;

        ByteArrayOutputStream byteArrayOutputStream = ByteArrayOutputStream(templateByte.length);

        byteArrayOutputStream.write(templateByte, 0, templateByte.length); 

        try
        {

            pdfReader = new PdfReader(templateByte);
            PdfStamper stamper = new PdfStamper(pdfReader, byteArrayOutputStream);
            stamper.setEncryption(USER_PASS.getBytes(), OWNER_PASS.getBytes(),
                    PdfWriter.ALLOW_PRINTING,
                    PdfWriter.ENCRYPTION_AES_128 | PdfWriter.DO_NOT_ENCRYPT_METADATA);
            stamper.close();
            pdfReader.close();

            return byteArrayOutputStream.toByteArray();

        }
        catch (Exception e)
        {

            e.printStackTrace();
        }

        return byteArrayOutputStream.toByteArray();
    }