<?php
/****************************************************************************
* Software: FPDF_Protection                                                 *
* Version:  1.06                                                            *
* Date:     2023-01-01                                                      *
* Author:   Klemen VODOPIVEC                                                *
* License:  FPDF                                                            *
*                                                                           *
* Thanks:  Cpdf (http://www.ros.co.nz/pdf) was my working sample of how to  *
*          implement protection in pdf.                                     *
****************************************************************************/

require('fpdf.php');

if(function_exists('openssl_encrypt'))
{
    function RC4($key, $data)
    {
        return openssl_encrypt($data, 'RC4-40', $key, OPENSSL_RAW_DATA);
    }
}
else
{
    function RC4($key, $data)
    {
        static $last_key, $last_state;

        if($key != $last_key)
        {
            $k = str_repeat($key, ceil(256/strlen($key)));
            $state = range(0, 255);
            $j = 0;
            for ($i=0; $i<256; $i++){
                $t = $state[$i];
                $j = ($j + $t + ord($k[$i])) % 256;
                $state[$i] = $state[$j];
                $state[$j] = $t;
            }
            $last_key = $key;
            $last_state = $state;
        }
        else
            $state = $last_state;

        $len = strlen($data);
        $a = 0;
        $b = 0;
        $out = '';
        for ($i=0; $i<$len; $i++){
            $a = ($a+1) % 256;
            $t = $state[$a];
            $b = ($b+$t) % 256;
            $state[$a] = $state[$b];
            $state[$b] = $t;
            $k = $state[($state[$a]+$state[$b]) % 256];
            $out .= chr(ord($data[$i]) ^ $k);
        }
        return $out;
    }
}

class FPDF_Protection extends FPDF
{
    protected $encrypted = false;
    protected $padding;
    protected $encryption_key;
    protected $Uvalue;             //U entry in pdf document
    protected $Ovalue;             //O entry in pdf document
    protected $Pvalue;             //P entry in pdf document
    protected $enc_obj_id;         //encryption object id

    /**
    * Function to set permissions as well as user and owner passwords
    *
    * - permissions is an array with values taken from the following list:
    *   copy, print, modify, annot-forms
    *   If a value is present it means that the permission is granted
    * - If a user password is set, user will be prompted before document is opened
    * - If an owner password is set, document can be opened in privilege mode with no
    *   restriction if that password is entered
    */
    function SetProtection($permissions=array(), $user_pass='', $owner_pass=null)
    {
        $options = array('print' => 4, 'modify' => 8, 'copy' => 16, 'annot-forms' => 32 );
        $protection = 192;
        foreach($permissions as $permission)
        {
            if (!isset($options[$permission]))
                $this->Error('Incorrect permission: '.$permission);
            $protection += $options[$permission];
        }
        if ($owner_pass === null)
            $owner_pass = uniqid(rand());
        $this->encrypted = true;
        $this->padding = "\x28\xBF\x4E\x5E\x4E\x75\x8A\x41\x64\x00\x4E\x56\xFF\xFA\x01\x08".
                        "\x2E\x2E\x00\xB6\xD0\x68\x3E\x80\x2F\x0C\xA9\xFE\x64\x53\x69\x7A";
        $this->_generateencryptionkey($user_pass, $owner_pass, $protection);
    }

/****************************************************************************
*                                                                           *
*                              Private methods                              *
*                                                                           *
****************************************************************************/

    function _putstream($s)
    {
        if ($this->encrypted)
            $s = RC4($this->_objectkey($this->n), $s);
        parent::_putstream($s);
    }

    function _textstring($s)
    {
        if (!$this->_isascii($s))
            $s = $this->_UTF8toUTF16($s);
        if ($this->encrypted)
            $s = RC4($this->_objectkey($this->n), $s);
        return '('.$this->_escape($s).')';
    }

    /**
    * Compute key depending on object number where the encrypted data is stored
    */
    function _objectkey($n)
    {
        return substr($this->_md5_16($this->encryption_key.pack('VXxx', $n)), 0, 10);
    }

    function _putresources()
    {
        parent::_putresources();
        if ($this->encrypted) {
            $this->_newobj();
            $this->enc_obj_id = $this->n;
            $this->_put('<<');
            $this->_putencryption();
            $this->_put('>>');
            $this->_put('endobj');
        }
    }

    function _putencryption()
    {
        $this->_put('/Filter /Standard');
        $this->_put('/V 1');
        $this->_put('/R 2');
        $this->_put('/O ('.$this->_escape($this->Ovalue).')');
        $this->_put('/U ('.$this->_escape($this->Uvalue).')');
        $this->_put('/P '.$this->Pvalue);
    }

    function _puttrailer()
    {
        parent::_puttrailer();
        if ($this->encrypted) {
            $this->_put('/Encrypt '.$this->enc_obj_id.' 0 R');
            $this->_put('/ID [()()]');
        }
    }

    /**
    * Get MD5 as binary string
    */
    function _md5_16($string)
    {
        return md5($string, true);
    }

    /**
    * Compute O value
    */
    function _Ovalue($user_pass, $owner_pass)
    {
        $tmp = $this->_md5_16($owner_pass);
        $owner_RC4_key = substr($tmp, 0, 5);
        return RC4($owner_RC4_key, $user_pass);
    }

    /**
    * Compute U value
    */
    function _Uvalue()
    {
        return RC4($this->encryption_key, $this->padding);
    }

    /**
    * Compute encryption key
    */
    function _generateencryptionkey($user_pass, $owner_pass, $protection)
    {
        // Pad passwords
        $user_pass = substr($user_pass.$this->padding, 0, 32);
        $owner_pass = substr($owner_pass.$this->padding, 0, 32);
        // Compute O value
        $this->Ovalue = $this->_Ovalue($user_pass, $owner_pass);
        // Compute encyption key
        $tmp = $this->_md5_16($user_pass.$this->Ovalue.chr($protection)."\xFF\xFF\xFF");
        $this->encryption_key = substr($tmp, 0, 5);
        // Compute U value
        $this->Uvalue = $this->_Uvalue();
        // Compute P value
        $this->Pvalue = -(($protection^255)+1);
    }
}

?>

<?php
require('fpdf_protection.php');

$pdf = new FPDF_Protection();
$pdf->SetProtection(array('print'));
$pdf->AddPage();
$pdf->SetFont('Arial');
$pdf->Write(10, 'You can print me but not copy my text.');
$pdf->Output();
?>