pdf_protection.go

package gopdf

import (
	"crypto/md5"
	"crypto/rc4"
	"encoding/binary"
	"math/rand"
	"time"
)

const (
	//PermissionsPrint setProtection print
	PermissionsPrint = 4
	//PermissionsModify setProtection modify
	PermissionsModify = 8
	//PermissionsCopy setProtection copy
	PermissionsCopy = 16
	//PermissionsAnnotForms setProtection  annot-forms
	PermissionsAnnotForms = 32
)

var protectionPadding = []byte{
	0x28, 0xBF, 0x4E, 0x5E, 0x4E, 0x75, 0x8A, 0x41, 0x64, 0x00, 0x4E, 0x56, 0xFF, 0xFA, 0x01, 0x08,
	0x2E, 0x2E, 0x00, 0xB6, 0xD0, 0x68, 0x3E, 0x80, 0x2F, 0x0C, 0xA9, 0xFE, 0x64, 0x53, 0x69, 0x7A,
}

// PDFProtection protection in pdf
type PDFProtection struct {
	encrypted bool   //whether document is protected
	uValue    []byte //U entry in pdf document
	oValue    []byte //O entry in pdf document
	pValue    int    //P entry in pdf document
	//var $enc_obj_id;         //encryption object id
	encryptionKey []byte
}

// SetProtection set protection infomation
func (p *PDFProtection) SetProtection(permissions int, userPass []byte, ownerPass []byte) error {
	return p.setProtection(permissions, userPass, ownerPass)
}

func (p *PDFProtection) setProtection(permissions int, userPass []byte, ownerPass []byte) error {
	protection := 192 | permissions
	if ownerPass == nil || len(ownerPass) == 0 {
		ownerPass = p.randomPass(24)
	}
	return p.generateEncryptionKey(userPass, ownerPass, protection)
}

func (p *PDFProtection) generateEncryptionKey(userPass []byte, ownerPass []byte, protection int) error {

	userPass = append(userPass, protectionPadding...)
	userPassWithPadding := userPass[0:32]
	ownerPass = append(ownerPass, protectionPadding...)
	ownerPassWithPadding := ownerPass[0:32]

	//oValue
	oValue, err := p.createOValue(userPassWithPadding, ownerPassWithPadding)
	if err != nil {
		return err
	}
	p.oValue = oValue

	uValue, err := p.createUValue(userPassWithPadding, oValue, protection)
	if err != nil {
		return err
	}
	p.uValue = uValue
	p.pValue = -((protection ^ 255) + 1)

	return nil
}

// EncryptionObj get Encryption Object
func (p *PDFProtection) EncryptionObj() *EncryptionObj {
	return p.encryptionObj()
}

func (p *PDFProtection) encryptionObj() *EncryptionObj {
	var en EncryptionObj
	en.oValue = p.oValue
	en.pValue = p.pValue
	en.uValue = p.uValue
	return &en
}

func (p *PDFProtection) createOValue(userPassWithPadding []byte, ownerPassWithPadding []byte) ([]byte, error) {
	tmp := md5.Sum(ownerPassWithPadding)
	ownerRC4key := tmp[0:5]
	cip, err := rc4.NewCipher(ownerRC4key)
	if err != nil {
		return nil, err
	}
	dest := make([]byte, len(userPassWithPadding))
	cip.XORKeyStream(dest, userPassWithPadding)
	return dest, nil
}

func (p *PDFProtection) createUValue(userPassWithPadding []byte, oValue []byte, protection int) ([]byte, error) {
	m := md5.New()
	m.Write(userPassWithPadding)
	m.Write(oValue)
	m.Write([]byte{byte(protection), byte(0xff), byte(0xff), byte(0xff)})

	tmp2 := m.Sum(nil)
	p.encryptionKey = tmp2[0:5]
	cip, err := rc4.NewCipher(p.encryptionKey)
	if err != nil {
		return nil, err
	}
	dest := make([]byte, len(protectionPadding))
	cip.XORKeyStream(dest, protectionPadding)
	return dest, nil
}

func (p *PDFProtection) randomPass(strlen int) []byte {
	rand.Seed(time.Now().UTC().UnixNano())
	const chars = "abcdef0123456789"
	result := make([]byte, strlen)
	for i := 0; i < strlen; i++ {
		result[i] = chars[rand.Intn(len(chars))]
	}
	return result
}

// Objectkey create object key from ObjID
func (p *PDFProtection) Objectkey(objID int) []byte {
	return p.objectkey(objID)
}

func (p *PDFProtection) objectkey(n int) []byte {
	tmp := make([]byte, 8, 8)
	binary.LittleEndian.PutUint32(tmp, uint32(n))
	tmp2 := append(p.encryptionKey, tmp[0], tmp[1], tmp[2], 0, 0)
	tmp3 := md5.Sum(tmp2)
	return tmp3[0:10]
}

func rc4Cip(key []byte, src []byte) ([]byte, error) {
	cip, err := rc4.NewCipher(key)
	if err != nil {
		return nil, err
	}
	dest := make([]byte, len(src))
	cip.XORKeyStream(dest, src)
	return dest, nil
}