From 43ca0e5035b6951297715a211b73d8db5f751f15 Mon Sep 17 00:00:00 2001
From: Přemysl Janouch <p@janouch.name>
Date: Tue, 2 Oct 2018 23:19:38 +0200
Subject: Add a Go port
It should be roughly at feature parity.
---
pdf/pdf.go | 1177 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 1177 insertions(+)
create mode 100644 pdf/pdf.go
(limited to 'pdf')
diff --git a/pdf/pdf.go b/pdf/pdf.go
new file mode 100644
index 0000000..d1c4d78
--- /dev/null
+++ b/pdf/pdf.go
@@ -0,0 +1,1177 @@
+//
+// Copyright (c) 2018, Přemysl Janouch <p@janouch.name>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+//
+
+// Package pdf signs PDF documents and provides some processing utilities.
+package pdf
+
+import (
+ "bytes"
+ "encoding/hex"
+ "errors"
+ "fmt"
+ "math"
+ "regexp"
+ "sort"
+ "strconv"
+ "strings"
+ "time"
+
+ "crypto"
+ "crypto/ecdsa"
+ "crypto/rsa"
+ "crypto/x509"
+ "go.mozilla.org/pkcs7"
+ "golang.org/x/crypto/pkcs12"
+)
+
+type ObjectKind int
+
+const (
+ End ObjectKind = iota
+ NL
+ Comment
+ Nil
+ Bool
+ Numeric
+ Keyword
+ Name
+ String
+
+ // simple tokens
+ BArray
+ EArray
+ BDict
+ EDict
+
+ // higher-level objects
+ Array
+ Dict
+ Indirect
+ Reference
+)
+
+// Object is a PDF token/object thingy. Objects may be composed either from
+// one or a sequence of tokens. The PDF Reference doesn't actually speak
+// of tokens.
+//
+// TODO(p): We probably want constructors like NewString, NewBool, NewArray, ...
+type Object struct {
+ Kind ObjectKind
+
+ // End (error message), Comment/Keyword/Name/String
+ String string
+ // Bool, Numeric
+ Number float64
+ // Array, Indirect
+ Array []Object
+ // Dict, in the future also Stream
+ Dict map[string]Object
+ // Indirect, Reference
+ N, Generation uint
+}
+
+// IsInteger checks if the PDF object is an integer number.
+func (o *Object) IsInteger() bool {
+ _, f := math.Modf(o.Number)
+ return o.Kind == Numeric && f == 0
+}
+
+// IsUint checks if the PDF object is an integer number that fits into a uint.
+func (o *Object) IsUint() bool {
+ return o.IsInteger() && o.Number >= 0 && o.Number <= float64(^uint(0))
+}
+
+// -----------------------------------------------------------------------------
+
+const (
+ octAlphabet = "01234567"
+ decAlphabet = "0123456789"
+ hexAlphabet = "0123456789abcdefABCDEF"
+ whitespace = "\t\n\f\r "
+ delimiters = "()<>[]{}/%"
+)
+
+// Lexer is a basic lexical analyser for the Portable Document Format,
+// giving limited error information.
+type Lexer struct {
+ p []byte // input buffer
+}
+
+func (lex *Lexer) read() (byte, bool) {
+ if len(lex.p) > 0 {
+ ch := lex.p[0]
+ lex.p = lex.p[1:]
+ return ch, true
+ }
+ return 0, false
+}
+
+func (lex *Lexer) peek() (byte, bool) {
+ if len(lex.p) > 0 {
+ return lex.p[0], true
+ }
+ return 0, false
+}
+
+func (lex *Lexer) eatNewline(ch byte) bool {
+ if ch == '\r' {
+ if ch, _ := lex.peek(); ch == '\n' {
+ lex.read()
+ }
+ return true
+ }
+ return ch == '\n'
+}
+
+func (lex *Lexer) unescape(ch byte) byte {
+ switch ch {
+ case 'n':
+ return '\n'
+ case 'r':
+ return '\r'
+ case 't':
+ return '\t'
+ case 'b':
+ return '\b'
+ case 'f':
+ return '\f'
+ }
+ if strings.IndexByte(octAlphabet, ch) >= 0 {
+ octal := []byte{ch}
+ lex.read()
+ if ch, _ := lex.peek(); strings.IndexByte(octAlphabet, ch) >= 0 {
+ octal = append(octal, ch)
+ lex.read()
+ }
+ if ch, _ := lex.peek(); strings.IndexByte(octAlphabet, ch) >= 0 {
+ octal = append(octal, ch)
+ lex.read()
+ }
+ u, _ := strconv.ParseUint(string(octal), 8, 8)
+ return byte(u)
+ }
+ return ch
+}
+
+func (lex *Lexer) string() Object {
+ var value []byte
+ parens := 1
+ for {
+ ch, ok := lex.read()
+ if !ok {
+ return Object{Kind: End, String: "unexpected end of string"}
+ }
+ if lex.eatNewline(ch) {
+ ch = '\n'
+ } else if ch == '(' {
+ parens++
+ } else if ch == ')' {
+ if parens--; parens == 0 {
+ break
+ }
+ } else if ch == '\\' {
+ if ch, ok = lex.read(); !ok {
+ return Object{Kind: End, String: "unexpected end of string"}
+ } else if lex.eatNewline(ch) {
+ continue
+ } else {
+ ch = lex.unescape(ch)
+ }
+ }
+ value = append(value, ch)
+ }
+ return Object{Kind: String, String: string(value)}
+}
+
+func (lex *Lexer) stringHex() Object {
+ var value, buf []byte
+ for {
+ ch, ok := lex.read()
+ if !ok {
+ return Object{Kind: End, String: "unexpected end of hex string"}
+ } else if ch == '>' {
+ break
+ } else if strings.IndexByte(hexAlphabet, ch) < 0 {
+ return Object{Kind: End, String: "invalid hex string"}
+ } else if buf = append(buf, ch); len(buf) == 2 {
+ u, _ := strconv.ParseUint(string(buf), 16, 8)
+ value = append(value, byte(u))
+ buf = nil
+ }
+ }
+ if len(buf) > 0 {
+ u, _ := strconv.ParseUint(string(buf)+"0", 16, 8)
+ value = append(value, byte(u))
+ }
+ return Object{Kind: String, String: string(value)}
+}
+
+func (lex *Lexer) name() Object {
+ var value []byte
+ for {
+ ch, ok := lex.peek()
+ if !ok || strings.IndexByte(whitespace+delimiters, ch) >= 0 {
+ break
+ }
+ lex.read()
+ if ch == '#' {
+ var hexa []byte
+ if ch, _ := lex.peek(); strings.IndexByte(hexAlphabet, ch) >= 0 {
+ hexa = append(hexa, ch)
+ lex.read()
+ }
+ if ch, _ := lex.peek(); strings.IndexByte(hexAlphabet, ch) >= 0 {
+ hexa = append(hexa, ch)
+ lex.read()
+ }
+ if len(hexa) != 2 {
+ return Object{Kind: End, String: "invalid name hexa escape"}
+ }
+ u, _ := strconv.ParseUint(string(value), 16, 8)
+ ch = byte(u)
+ }
+ value = append(value, ch)
+ }
+ if len(value) == 0 {
+ return Object{Kind: End, String: "unexpected end of name"}
+ }
+ return Object{Kind: Name, String: string(value)}
+}
+
+func (lex *Lexer) comment() Object {
+ var value []byte
+ for {
+ ch, ok := lex.peek()
+ if !ok || ch == '\r' || ch == '\n' {
+ break
+ }
+ value = append(value, ch)
+ lex.read()
+ }
+ return Object{Kind: Comment, String: string(value)}
+}
+
+// XXX: Maybe invalid numbers should rather be interpreted as keywords.
+func (lex *Lexer) number() Object {
+ var value []byte
+ ch, ok := lex.peek()
+ if ch == '-' {
+ value = append(value, ch)
+ lex.read()
+ }
+ real, digits := false, false
+ for {
+ ch, ok = lex.peek()
+ if !ok {
+ break
+ } else if strings.IndexByte(decAlphabet, ch) >= 0 {
+ digits = true
+ } else if ch == '.' && !real {
+ real = true
+ } else {
+ break
+ }
+ value = append(value, ch)
+ lex.read()
+ }
+ if !digits {
+ return Object{Kind: End, String: "invalid number"}
+ }
+ f, _ := strconv.ParseFloat(string(value), 64)
+ return Object{Kind: Numeric, Number: f}
+}
+
+func (lex *Lexer) Next() Object {
+ ch, ok := lex.peek()
+ if !ok {
+ return Object{Kind: End}
+ }
+ if strings.IndexByte("-0123456789.", ch) >= 0 {
+ return lex.number()
+ }
+
+ // {} end up being keywords, we might want to error out on those.
+ var value []byte
+ for {
+ ch, ok := lex.peek()
+ if !ok || strings.IndexByte(whitespace+delimiters, ch) >= 0 {
+ break
+ }
+ value = append(value, ch)
+ lex.read()
+ }
+ switch v := string(value); v {
+ case "":
+ case "null":
+ return Object{Kind: Nil}
+ case "true":
+ return Object{Kind: Bool, Number: 1}
+ case "false":
+ return Object{Kind: Bool, Number: 0}
+ default:
+ return Object{Kind: Keyword, String: v}
+ }
+
+ switch ch, _ := lex.read(); ch {
+ case '/':
+ return lex.name()
+ case '%':
+ return lex.comment()
+ case '(':
+ return lex.string()
+ case '[':
+ return Object{Kind: BArray}
+ case ']':
+ return Object{Kind: EArray}
+ case '<':
+ if ch, _ := lex.peek(); ch == '<' {
+ lex.read()
+ return Object{Kind: BDict}
+ }
+ return lex.stringHex()
+ case '>':
+ if ch, _ := lex.peek(); ch == '>' {
+ lex.read()
+ return Object{Kind: EDict}
+ }
+ return Object{Kind: End, String: "unexpected '>'"}
+ default:
+ if lex.eatNewline(ch) {
+ return Object{Kind: NL}
+ }
+ if strings.IndexByte(whitespace, ch) >= 0 {
+ return lex.Next()
+ }
+ return Object{Kind: End, String: "unexpected input"}
+ }
+}
+
+// -----------------------------------------------------------------------------
+
+// FIXME: Lines /should not/ be longer than 255 characters,
+// some wrapping is in order.
+func (o *Object) Serialize() string {
+ switch o.Kind {
+ case NL:
+ return "\n"
+ case Nil:
+ return "null"
+ case Bool:
+ if o.Number != 0 {
+ return "true"
+ }
+ return "false"
+ case Numeric:
+ return strconv.FormatFloat(o.Number, 'f', -1, 64)
+ case Keyword:
+ return o.String
+ case Name:
+ escaped := []byte{'/'}
+ for _, ch := range []byte(o.String) {
+ escaped = append(escaped, ch)
+ if ch == '#' || strings.IndexByte(delimiters+whitespace, ch) >= 0 {
+ escaped = append(escaped, fmt.Sprintf("%02x", ch)...)
+ }
+ }
+ return string(escaped)
+ case String:
+ escaped := []byte{'('}
+ for _, ch := range []byte(o.String) {
+ if ch == '\\' || ch == '(' || ch == ')' {
+ escaped = append(escaped, '\\')
+ }
+ escaped = append(escaped, ch)
+ }
+ return string(append(escaped, ')'))
+ case BArray:
+ return "["
+ case EArray:
+ return "]"
+ case BDict:
+ return "<<"
+ case EDict:
+ return ">>"
+ case Array:
+ var v []string
+ for _, i := range o.Array {
+ v = append(v, i.Serialize())
+ }
+ return "[ " + strings.Join(v, " ") + " ]"
+ case Dict:
+ b := bytes.NewBuffer(nil)
+ var keys []string
+ for k := range o.Dict {
+ keys = append(keys, k)
+ }
+ sort.Strings(keys)
+ for _, k := range keys {
+ v := o.Dict[k]
+ // FIXME: The key is also supposed to be escaped by Serialize.
+ fmt.Fprint(b, " /", k, " ", v.Serialize())
+ }
+ return "<<" + b.String() + " >>"
+ case Indirect:
+ return fmt.Sprintf("%d %d obj\n%s\nendobj", o.N, o.Generation,
+ o.Array[0].Serialize())
+ case Reference:
+ return fmt.Sprintf("%d %d R", o.N, o.Generation)
+ default:
+ panic("unsupported token for serialization")
+ }
+}
+
+// -----------------------------------------------------------------------------
+
+type ref struct {
+ offset int64 // file offset or N of the next free entry
+ generation uint // object generation
+ nonfree bool // whether this N is taken (for a good zero value)
+}
+
+// Updater is a utility class to help read and possibly incrementally update
+// PDF files.
+type Updater struct {
+ // cross-reference table
+ xref []ref
+
+ // current cross-reference table size, correlated to len(xref)
+ xrefSize uint
+
+ // list of updated objects
+ // TODO(p): A map to bool makes this simpler to work with.
+ // The same with another map to struct{} somewhere in this code.
+ updated map[uint]struct{}
+
+ // PDF document data
+ Document []byte
+
+ // the new trailer dictionary to be written, initialized with the old one
+ Trailer map[string]Object
+}
+
+func (u *Updater) parseIndirect(lex *Lexer, stack *[]Object) Object {
+ lenStack := len(*stack)
+ if lenStack < 2 {
+ return Object{Kind: End, String: "missing object ID pair"}
+ }
+
+ n := (*stack)[lenStack-2]
+ g := (*stack)[lenStack-1]
+ *stack = (*stack)[:lenStack-2]
+
+ if !g.IsUint() || !n.IsUint() {
+ return Object{Kind: End, String: "invalid object ID pair"}
+ }
+
+ obj := Object{
+ Kind: Indirect, N: uint(n.Number), Generation: uint(g.Number)}
+ for {
+ object := u.parse(lex, &obj.Array)
+ if object.Kind == End {
+ return Object{Kind: End, String: "object doesn't end"}
+ }
+ if object.Kind == Keyword && object.String == "endobj" {
+ break
+ }
+ obj.Array = append(obj.Array, object)
+ }
+ return obj
+}
+
+func (u *Updater) parseR(stack *[]Object) Object {
+ lenStack := len(*stack)
+ if lenStack < 2 {
+ return Object{Kind: End, String: "missing reference ID pair"}
+ }
+
+ n := (*stack)[lenStack-2]
+ g := (*stack)[lenStack-1]
+ *stack = (*stack)[:lenStack-2]
+
+ if !g.IsUint() || !n.IsUint() {
+ return Object{Kind: End, String: "invalid reference ID pair"}
+ }
+ return Object{
+ Kind: Reference, N: uint(n.Number), Generation: uint(g.Number)}
+}
+
+/// parse reads an object at the lexer's position. Not a strict parser.
+func (u *Updater) parse(lex *Lexer, stack *[]Object) Object {
+ switch token := lex.Next(); token.Kind {
+ case NL, Comment:
+ // These are not important to parsing,
+ // not even for this procedure's needs.
+ return u.parse(lex, stack)
+ case BArray:
+ var array []Object
+ for {
+ object := u.parse(lex, &array)
+ if object.Kind == End {
+ return Object{Kind: End, String: "array doesn't end"}
+ }
+ if object.Kind == EArray {
+ break
+ }
+ array = append(array, object)
+ }
+ return Object{Kind: Array, Array: array}
+ case BDict:
+ var array []Object
+ for {
+ object := u.parse(lex, &array)
+ if object.Kind == End {
+ return Object{Kind: End, String: "dictionary doesn't end"}
+ }
+ if object.Kind == EDict {
+ break
+ }
+ array = append(array, object)
+ }
+ if len(array)%2 != 0 {
+ return Object{Kind: End, String: "unbalanced dictionary"}
+ }
+ dict := make(map[string]Object)
+ for i := 0; i < len(array); i += 2 {
+ if array[i].Kind != Name {
+ return Object{
+ Kind: End, String: "invalid dictionary key type"}
+ }
+ dict[array[i].String] = array[i+1]
+ }
+ return Object{Kind: Dict, Dict: dict}
+ case Keyword:
+ // Appears in the document body, typically needs
+ // to access the cross-reference table.
+ //
+ // TODO(p): Use the xref to read /Length etc. once we
+ // actually need to read such objects; presumably
+ // streams can use the Object.String member.
+ switch token.String {
+ case "stream":
+ return Object{Kind: End, String: "streams are not supported yet"}
+ case "obj":
+ return u.parseIndirect(lex, stack)
+ case "R":
+ return u.parseR(stack)
+ }
+ fallthrough
+ default:
+ return token
+ }
+}
+
+func (u *Updater) loadXref(lex *Lexer, loadedEntries map[uint]struct{}) error {
+ var throwawayStack []Object
+ if keyword := u.parse(lex,
+ &throwawayStack); keyword.Kind != Keyword || keyword.String != "xref" {
+ return errors.New("invalid xref table")
+ }
+ for {
+ object := u.parse(lex, &throwawayStack)
+ if object.Kind == End {
+ return errors.New("unexpected EOF while looking for the trailer")
+ }
+ if object.Kind == Keyword && object.String == "trailer" {
+ break
+ }
+
+ second := u.parse(lex, &throwawayStack)
+ if !object.IsUint() || !second.IsUint() {
+ return errors.New("invalid xref section header")
+ }
+
+ start, count := uint(object.Number), uint(second.Number)
+ for i := uint(0); i < count; i++ {
+ off := u.parse(lex, &throwawayStack)
+ gen := u.parse(lex, &throwawayStack)
+ key := u.parse(lex, &throwawayStack)
+ if !off.IsInteger() || off.Number < 0 ||
+ off.Number > float64(len(u.Document)) ||
+ !gen.IsInteger() || gen.Number < 0 || gen.Number > 65535 ||
+ key.Kind != Keyword {
+ return errors.New("invalid xref entry")
+ }
+
+ free := true
+ if key.String == "n" {
+ free = false
+ } else if key.String != "f" {
+ return errors.New("invalid xref entry")
+ }
+
+ n := start + i
+ if _, ok := loadedEntries[n]; ok {
+ continue
+ }
+ if lenXref := uint(len(u.xref)); n >= lenXref {
+ u.xref = append(u.xref, make([]ref, n-lenXref+1)...)
+ }
+ loadedEntries[n] = struct{}{}
+
+ u.xref[n] = ref{
+ offset: int64(off.Number),
+ generation: uint(gen.Number),
+ nonfree: !free,
+ }
+ }
+ }
+ return nil
+}
+
+// -----------------------------------------------------------------------------
+
+var haystackRE = regexp.MustCompile(`(?s:.*)\sstartxref\s+(\d+)\s+%%EOF`)
+
+// Initialize builds the cross-reference table and prepares
+// a new trailer dictionary.
+func (u *Updater) Initialize() error {
+ u.updated = make(map[uint]struct{})
+
+ // We only need to look for startxref roughly within
+ // the last kibibyte of the document.
+ haystack := u.Document
+ if len(haystack) > 1024 {
+ haystack = haystack[len(haystack)-1024:]
+ }
+
+ m := haystackRE.FindSubmatch(haystack)
+ if m == nil {
+ return errors.New("cannot find startxref")
+ }
+
+ xrefOffset, _ := strconv.ParseInt(string(m[1]), 10, 64)
+ lastXrefOffset := xrefOffset
+ loadedXrefs := map[int64]struct{}{}
+ loadedEntries := map[uint]struct{}{}
+
+ var throwawayStack []Object
+ for {
+ if _, ok := loadedXrefs[xrefOffset]; ok {
+ return errors.New("circular xref offsets")
+ }
+ if xrefOffset >= int64(len(u.Document)) {
+ return errors.New("invalid xref offset")
+ }
+
+ lex := Lexer{u.Document[xrefOffset:]}
+ if err := u.loadXref(&lex, loadedEntries); err != nil {
+ return err
+ }
+
+ trailer := u.parse(&lex, &throwawayStack)
+ if trailer.Kind != Dict {
+ return errors.New("invalid trailer dictionary")
+ }
+ if len(loadedXrefs) == 0 {
+ u.Trailer = trailer.Dict
+ }
+ loadedXrefs[xrefOffset] = struct{}{}
+
+ prevOffset, ok := trailer.Dict["Prev"]
+ if !ok {
+ break
+ }
+ // FIXME: We don't check for size_t over or underflow.
+ if !prevOffset.IsInteger() {
+ return errors.New("invalid Prev offset")
+ }
+ xrefOffset = int64(prevOffset.Number)
+ }
+
+ u.Trailer["Prev"] = Object{
+ Kind: Numeric, Number: float64(lastXrefOffset)}
+
+ lastSize, ok := u.Trailer["Size"]
+ if !ok || !lastSize.IsInteger() || lastSize.Number <= 0 {
+ return errors.New("invalid or missing cross-reference table Size")
+ }
+ u.xrefSize = uint(lastSize.Number)
+ return nil
+}
+
+// Get retrieves an object by its number and generation--may return
+// Nil or End with an error.
+func (u *Updater) Get(n, generation uint) Object {
+ if n >= u.xrefSize {
+ return Object{Kind: Nil}
+ }
+
+ ref := u.xref[n]
+ if !ref.nonfree || ref.generation != generation ||
+ ref.offset >= int64(len(u.Document)) {
+ return Object{Kind: Nil}
+ }
+
+ lex := Lexer{u.Document[ref.offset:]}
+ var stack []Object
+ for {
+ object := u.parse(&lex, &stack)
+ if object.Kind == End {
+ return object
+ }
+ if object.Kind != Indirect {
+ stack = append(stack, object)
+ } else if object.N != n || object.Generation != generation {
+ return Object{Kind: End, String: "object mismatch"}
+ } else {
+ return object.Array[0]
+ }
+ }
+}
+
+// Allocate allocates a new object number.
+func (u *Updater) Allocate() uint {
+ n := u.xrefSize
+ u.xrefSize++
+
+ if u.xrefSize == 0 {
+ panic("overflow")
+ } else if lenXref := uint(len(u.xref)); lenXref < u.xrefSize {
+ u.xref = append(u.xref, make([]ref, u.xrefSize-lenXref)...)
+ }
+
+ // We don't make sure it gets a subsection in the update yet because we
+ // make no attempts at fixing the linked list of free items either.
+ return n
+}
+
+// BytesWriter is an interface over a subset of bytes.Buffer methods.
+type BytesWriter interface {
+ Bytes() []byte
+ Len() int
+ Write(p []byte) (n int, err error)
+ WriteByte(c byte) error
+ WriteRune(r rune) (n int, err error)
+ WriteString(s string) (n int, err error)
+}
+
+// Update appends an updated object to the end of the document.
+func (u *Updater) Update(n uint, fill func(buf BytesWriter)) {
+ oldRef := u.xref[n]
+ u.updated[n] = struct{}{}
+ u.xref[n] = ref{
+ offset: int64(len(u.Document) + 1),
+ generation: oldRef.generation,
+ nonfree: true,
+ }
+
+ buf := bytes.NewBuffer(u.Document)
+ fmt.Fprintf(buf, "\n%d %d obj\n", n, oldRef.generation)
+
+ // Separately so that the callback can use w.Len() to get current offset.
+ fill(buf)
+
+ buf.WriteString("\nendobj")
+ u.Document = buf.Bytes()
+}
+
+// FlushUpdates writes an updated cross-reference table and trailer.
+func (u *Updater) FlushUpdates() {
+ updated := make([]uint, 0, len(u.updated))
+ for n := range u.updated {
+ updated = append(updated, n)
+ }
+ sort.Slice(updated, func(i, j int) bool {
+ return updated[i] < updated[j]
+ })
+
+ groups := make(map[uint]uint)
+ for i := 0; i < len(updated); {
+ start, count := updated[i], uint(1)
+ for i++; i != len(updated) && updated[i] == start+count; i++ {
+ count++
+ }
+ groups[start] = count
+ }
+
+ // Taking literally "Each cross-reference section begins with a line
+ // containing the keyword xref. Following this line are one or more
+ // cross-reference subsections." from 3.4.3 in PDF Reference.
+ if len(groups) == 0 {
+ groups[0] = 0
+ }
+
+ buf := bytes.NewBuffer(u.Document)
+ startXref := buf.Len() + 1
+ buf.WriteString("\nxref\n")
+
+ for start, count := range groups {
+ fmt.Fprintf(buf, "%d %d\n", start, count)
+ for i := uint(0); i < count; i++ {
+ ref := u.xref[start+uint(i)]
+ if ref.nonfree {
+ fmt.Fprintf(buf, "%010d %05d n \n", ref.offset, ref.generation)
+ } else {
+ fmt.Fprintf(buf, "%010d %05d f \n", ref.offset, ref.generation)
+ }
+ }
+ }
+
+ u.Trailer["Size"] = Object{Kind: Numeric, Number: float64(u.xrefSize)}
+ trailer := Object{Kind: Dict, Dict: u.Trailer}
+
+ fmt.Fprintf(buf, "trailer\n%s\nstartxref\n%d\n%%%%EOF\n",
+ trailer.Serialize(), startXref)
+ u.Document = buf.Bytes()
+}
+
+// -----------------------------------------------------------------------------
+
+// PdfDate makes a PDF object representing the given point in time.
+func PdfDate(ts time.Time) Object {
+ buf := ts.AppendFormat(nil, "D:20060102150405")
+ // "Z07'00'" doesn't work, we need to do some of it manually.
+ if _, offset := ts.Zone(); offset != 0 {
+ o := ts.AppendFormat(nil, "-0700")
+ buf = append(buf, o[0], o[1], o[2], '\'', o[3], o[4], '\'')
+ } else {
+ buf = append(buf, 'Z')
+ }
+ return Object{Kind: String, String: string(buf)}
+}
+
+// PdfGetFirstPage retrieves the first page of the document or a Nil object.
+func PdfGetFirstPage(pdf *Updater, nodeN, nodeGeneration uint) Object {
+ obj := pdf.Get(nodeN, nodeGeneration)
+ if obj.Kind != Dict {
+ return Object{Kind: Nil}
+ }
+
+ // Out of convenience; these aren't filled normally.
+ obj.N = nodeN
+ obj.Generation = nodeGeneration
+
+ if typ, ok := obj.Dict["Type"]; !ok || typ.Kind != Name {
+ return Object{Kind: Nil}
+ } else if typ.String == "Page" {
+ return obj
+ } else if typ.String != "Pages" {
+ return Object{Kind: Nil}
+ }
+
+ // XXX: Technically speaking, this may be an indirect reference.
+ // The correct way to solve this seems to be having Updater include
+ // a wrapper around "obj.Dict". Though does it still apply in Golang?
+ kids, ok := obj.Dict["Kids"]
+ if !ok || kids.Kind != Array || len(kids.Array) == 0 ||
+ kids.Array[0].Kind != Reference {
+ return Object{Kind: Nil}
+ }
+
+ // XXX: Nothing prevents us from recursing in an evil circular graph.
+ return PdfGetFirstPage(pdf, kids.Array[0].N, kids.Array[0].Generation)
+}
+
+// -----------------------------------------------------------------------------
+
+// PKCS12Parse parses and verifies PKCS#12 data.
+func PKCS12Parse(p12 []byte, password string) (
+ crypto.PrivateKey, []*x509.Certificate, error) {
+ // The pkcs12.Decode function doesn't support included intermediate
+ // certificates, we need to do some processing manually.
+ blocks, err := pkcs12.ToPEM(p12, password)
+ if err != nil {
+ return nil, nil, err
+ }
+
+ // b.Type is literally CERTIFICATE or PRIVATE KEY, the Headers only contain
+ // a localKeyId field. It seems like the pkey and the cert share the same
+ // localKeyId value. Though the leaf certificate should also be the first
+ // one in the PKCS#12 file, so I probably don't need that value.
+ var allX509Blocks [][]byte
+ var allCertBlocks [][]byte
+ for _, b := range blocks {
+ // CERTIFICATE, PRIVATE KEY constants are defined locally in the pkcs12
+ // package. crypto/tls/tls.go seems to only use literals for these and
+ // also accepts words in front such as RSA PRIVATE KEY.
+ switch b.Type {
+ case "PRIVATE KEY":
+ allX509Blocks = append(allX509Blocks, b.Bytes)
+ case "CERTIFICATE":
+ allCertBlocks = append(allCertBlocks, b.Bytes)
+ }
+ }
+ switch {
+ case len(allX509Blocks) == 0:
+ return nil, nil, errors.New("missing private key")
+ case len(allX509Blocks) > 1:
+ return nil, nil, errors.New("more than one private key")
+ case len(allCertBlocks) == 0:
+ return nil, nil, errors.New("missing certificate")
+ }
+
+ // The PKCS#12 file may only contain PKCS#8-wrapped private keys but the
+ // pkcs12 package unwraps them to simple PKCS#1/EC while converting to PEM.
+ var key crypto.PrivateKey
+ if key, err = x509.ParsePKCS1PrivateKey(allX509Blocks[0]); err != nil {
+ if key, err = x509.ParseECPrivateKey(allX509Blocks[0]); err == nil {
+ return nil, nil, errors.New("failed to parse private key")
+ }
+ }
+
+ x509Certs, err := x509.ParseCertificates(allCertBlocks[0])
+ if err != nil {
+ return nil, nil, err
+ }
+ if len(x509Certs) != 1 {
+ return nil, nil,
+ errors.New("expected exactly one certificate in the first bag")
+ }
+
+ for _, cert := range allCertBlocks[1:] {
+ toAdd, err := x509.ParseCertificates(cert)
+ if err != nil {
+ return nil, nil, err
+ }
+ x509Certs = append(x509Certs, toAdd...)
+ }
+
+ // Copied from crypto/tls/tls.go.
+ switch pub := x509Certs[0].PublicKey.(type) {
+ case *rsa.PublicKey:
+ priv, ok := key.(*rsa.PrivateKey)
+ if !ok {
+ return nil, nil,
+ errors.New("private key type does not match public key type")
+ }
+ if pub.N.Cmp(priv.N) != 0 {
+ return nil, nil,
+ errors.New("private key does not match public key")
+ }
+ case *ecdsa.PublicKey:
+ priv, ok := key.(*ecdsa.PrivateKey)
+ if !ok {
+ return nil, nil,
+ errors.New("private key type does not match public key type")
+ }
+ if pub.X.Cmp(priv.X) != 0 || pub.Y.Cmp(priv.Y) != 0 {
+ return nil, nil,
+ errors.New("private key does not match public key")
+ }
+ default:
+ return nil, nil, errors.New("unknown public key algorithm")
+ }
+ return key, x509Certs, nil
+}
+
+// FillInSignature signs PDF contents and writes the signature into the given
+// window that has been reserved for this specific purpose.
+func FillInSignature(document []byte, signOff, signLen int,
+ key crypto.PublicKey, certs []*x509.Certificate) error {
+ if signOff < 0 || signOff > len(document) ||
+ signLen < 2 || signOff+signLen > len(document) {
+ return errors.New("invalid signing window")
+ }
+
+ pkcsError := func(message interface{}) error {
+ return fmt.Errorf("key/cert: %s", message)
+ }
+
+ // Prevent useless signatures--makes pdfsig from poppler happy at least
+ // (and NSS by extension).
+ x509Cert := certs[0]
+ if x509Cert.KeyUsage&(x509.KeyUsageDigitalSignature|
+ x509.KeyUsageContentCommitment /* renamed non-repudiation */) == 0 {
+ return pkcsError("the certificate's key usage must include " +
+ "digital signatures or non-repudiation")
+ }
+
+ extOK := false
+ for _, u := range x509Cert.ExtKeyUsage {
+ if u == x509.ExtKeyUsageAny || u == x509.ExtKeyUsageEmailProtection {
+ extOK = true
+ }
+ }
+ if len(x509Cert.ExtKeyUsage) > 0 && !extOK {
+ return pkcsError("the certificate's extended key usage " +
+ "must include S/MIME")
+ }
+
+ // XXX: We'd like to stream to the hash manually instead of copying data.
+ data := make([]byte, len(document)-signLen)
+ copy(data, document[:signOff])
+ copy(data[signOff:], document[signOff+signLen:])
+
+ signedData, err := pkcs7.NewSignedData(data)
+ if err != nil {
+ return err
+ }
+ // The default digest is SHA1, which is mildly insecure now.
+ signedData.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
+ if err := signedData.AddSignerChain(
+ x509Cert, key, certs[1:], pkcs7.SignerInfoConfig{}); err != nil {
+ return err
+ }
+
+ signedData.Detach()
+ sig, err := signedData.Finish()
+ if err != nil {
+ return err
+ }
+
+ /*
+ Debugging: ioutil.WriteFile("pdf_signature.der", sig, 0666)
+ openssl cms -inform PEM -in pdf_signature.pem -noout -cmsout -print
+ Context: https://stackoverflow.com/a/29253469
+ */
+
+ if len(sig)*2 > signLen-2 /* hexstring quotes */ {
+ // The obvious solution is to increase the allocation... or spend
+ // a week reading specifications while losing all faith in humanity
+ // as a species, and skip the pkcs7 package entirely.
+ return fmt.Errorf("not enough space reserved for the signature "+
+ "(%d nibbles vs %d nibbles)", signLen-2, len(sig)*2)
+ }
+
+ hex.Encode(document[signOff+1:], sig)
+ return nil
+}
+
+// Sign signs the given document, growing and returning the passed-in slice.
+//
+// The presumption here is that the document is valid and that it doesn't
+// employ cross-reference streams from PDF 1.5, or at least constitutes
+// a hybrid-reference file. The results with PDF 2.0 (2017) are currently
+// unknown as the standard costs money.
+//
+// Carelessly assumes that the version of the original document is at most
+// PDF 1.6.
+//
+// https://www.adobe.com/devnet-docs/acrobatetk/tools/DigSig/Acrobat_DigitalSignatures_in_PDF.pdf
+// https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/pdf_reference_1-7.pdf
+// https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PPKAppearances.pdf
+func Sign(document []byte, key crypto.PublicKey, certs []*x509.Certificate) (
+ []byte, error) {
+ pdf := &Updater{Document: document}
+ if err := pdf.Initialize(); err != nil {
+ return nil, err
+ }
+
+ rootRef, ok := pdf.Trailer["Root"]
+ if !ok || rootRef.Kind != Reference {
+ return nil, errors.New("trailer does not contain a reference to Root")
+ }
+ root := pdf.Get(rootRef.N, rootRef.Generation)
+ if root.Kind != Dict {
+ return nil, errors.New("invalid Root dictionary reference")
+ }
+
+ // 8.7 Digital Signatures - /signature dictionary/
+ sigdictN := pdf.Allocate()
+ var byterangeOff, byterangeLen, signOff, signLen int
+ pdf.Update(sigdictN, func(buf BytesWriter) {
+ // The timestamp is important for Adobe Acrobat Reader DC.
+ // The ideal would be to use RFC 3161.
+ now := PdfDate(time.Now())
+ buf.WriteString("<< /Type/Sig /Filter/Adobe.PPKLite" +
+ " /SubFilter/adbe.pkcs7.detached\n" +
+ " /M" + now.Serialize() + " /ByteRange ")
+
+ byterangeOff = buf.Len()
+ byterangeLen = 32 // fine for a gigabyte
+ buf.Write(bytes.Repeat([]byte{' '}, byterangeLen))
+ buf.WriteString("\n /Contents <")
+
+ signOff = buf.Len()
+ signLen = 8192 // cert, digest, encripted digest, ...
+ buf.Write(bytes.Repeat([]byte{'0'}, signLen))
+ buf.WriteString("> >>")
+
+ // We actually need to exclude the hexstring quotes from signing.
+ signOff -= 1
+ signLen += 2
+ })
+
+ sigfield := Object{Kind: Dict, Dict: map[string]Object{
+ // 8.6.3 Field Types - Signature Fields
+ "FT": {Kind: Name, String: "Sig"},
+ "V": {Kind: Reference, N: sigdictN, Generation: 0},
+ // 8.4.5 Annotations Types - Widget Annotations
+ // We can merge the Signature Annotation and omit Kids here.
+ "Subtype": {Kind: Name, String: "Widget"},
+ "F": {Kind: Numeric, Number: 2 /* Hidden */},
+ "T": {Kind: String, String: "Signature1"},
+ "Rect": {Kind: Array, Array: []Object{
+ {Kind: Numeric, Number: 0},
+ {Kind: Numeric, Number: 0},
+ {Kind: Numeric, Number: 0},
+ {Kind: Numeric, Number: 0},
+ }},
+ }}
+
+ sigfieldN := pdf.Allocate()
+ pdf.Update(sigfieldN, func(buf BytesWriter) {
+ buf.WriteString(sigfield.Serialize())
+ })
+
+ pagesRef, ok := root.Dict["Pages"]
+ if !ok || pagesRef.Kind != Reference {
+ return nil, errors.New("invalid Pages reference")
+ }
+ page := PdfGetFirstPage(pdf, pagesRef.N, pagesRef.Generation)
+ if page.Kind != Dict {
+ return nil, errors.New("invalid or unsupported page tree")
+ }
+
+ // XXX: Assuming this won't be an indirectly referenced array.
+ annots := page.Dict["Annots"]
+ if annots.Kind != Array {
+ annots = Object{Kind: Array}
+ }
+ annots.Array = append(annots.Array, Object{
+ Kind: Reference, N: sigfieldN, Generation: 0})
+
+ page.Dict["Annots"] = annots
+ pdf.Update(page.N, func(buf BytesWriter) {
+ buf.WriteString(page.Serialize())
+ })
+
+ // 8.6.1 Interactive Form Dictionary
+ // XXX: Assuming there are no forms already, overwriting everything.
+ root.Dict["AcroForm"] = Object{Kind: Dict, Dict: map[string]Object{
+ "Fields": {Kind: Array, Array: []Object{
+ {Kind: Reference, N: sigfieldN, Generation: 0},
+ }},
+ "SigFlags": {Kind: Numeric,
+ Number: 3 /* SignaturesExist | AppendOnly */},
+ }}
+
+ // Upgrade the document version for SHA-256 etc.
+ // XXX: Assuming that it's not newer than 1.6 already--while Cairo can't
+ // currently use a newer version that 1.5, it's not a bad idea to use
+ // cairo_pdf_surface_restrict_to_version().
+ root.Dict["Version"] = Object{Kind: Name, String: "1.6"}
+ pdf.Update(rootRef.N, func(buf BytesWriter) {
+ buf.WriteString(root.Serialize())
+ })
+ pdf.FlushUpdates()
+
+ // Now that we know the length of everything, store byte ranges of
+ // what we're about to sign, which must be everything but the resulting
+ // signature itself.
+ tailOff := signOff + signLen
+ tailLen := len(pdf.Document) - tailOff
+
+ ranges := fmt.Sprintf("[0 %d %d %d]", signOff, tailOff, tailLen)
+ if len(ranges) > byterangeLen {
+ return nil, errors.New("not enough space reserved for /ByteRange")
+ }
+ copy(pdf.Document[byterangeOff:], []byte(ranges))
+ if err := FillInSignature(pdf.Document, signOff, signLen,
+ key, certs); err != nil {
+ return nil, err
+ }
+ return pdf.Document, nil
+}
--
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