<?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('fpdfdraw.php');

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 PDF_Draw
{
	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);
	}
}

?>