// // Copyright (c) 2018, Přemysl Janouch // // 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. type Object struct { Kind ObjectKind String string // Comment/Keyword/Name/String Number float64 // Bool, Numeric Array []Object // Array, Indirect Dict map[string]Object // Dict, in the future also Stream N, Generation uint // Indirect, Reference } // 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)) } // A slew of constructors that will hopefully get all inlined. // New returns a new Object of the given kind, with default values. func New(kind ObjectKind) Object { return Object{Kind: kind} } func NewComment(c string) Object { return Object{Kind: Comment, String: c} } func NewKeyword(k string) Object { return Object{Kind: Keyword, String: k} } func NewBool(b bool) Object { var b64 float64 if b { b64 = 1 } return Object{Kind: Bool, Number: b64} } func NewNumeric(n float64) Object { return Object{Kind: Numeric, Number: n} } func NewName(n string) Object { return Object{Kind: Name, String: n} } func NewString(s string) Object { return Object{Kind: String, String: s} } func NewArray(a []Object) Object { return Object{Kind: Array, Array: a} } func NewDict(d map[string]Object) Object { if d == nil { d = make(map[string]Object) } return Object{Kind: Dict, Dict: d} } func NewIndirect(o Object, n, generation uint) Object { return Object{Kind: Indirect, N: n, Generation: generation, Array: []Object{o}} } func NewReference(n, generation uint) Object { return Object{Kind: Reference, N: n, Generation: generation} } func newError(msg string) (Object, error) { return New(End), errors.New(msg) } // ----------------------------------------------------------------------------- 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, error) { var value []byte parens := 1 for { ch, ok := lex.read() if !ok { return newError("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 newError("unexpected end of string") } else if lex.eatNewline(ch) { continue } else { ch = lex.unescape(ch) } } value = append(value, ch) } return NewString(string(value)), nil } func (lex *Lexer) stringHex() (Object, error) { var value, buf []byte for { ch, ok := lex.read() if !ok { return newError("unexpected end of hex string") } else if ch == '>' { break } else if strings.IndexByte(hexAlphabet, ch) < 0 { return newError("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 NewString(string(value)), nil } func (lex *Lexer) name() (Object, error) { 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 newError("invalid name hexa escape") } u, _ := strconv.ParseUint(string(value), 16, 8) ch = byte(u) } value = append(value, ch) } if len(value) == 0 { return newError("unexpected end of name") } return NewName(string(value)), nil } func (lex *Lexer) comment() (Object, error) { var value []byte for { ch, ok := lex.peek() if !ok || ch == '\r' || ch == '\n' { break } value = append(value, ch) lex.read() } return NewComment(string(value)), nil } // XXX: Maybe invalid numbers should rather be interpreted as keywords. func (lex *Lexer) number() (Object, error) { 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 newError("invalid number") } f, _ := strconv.ParseFloat(string(value), 64) return NewNumeric(f), nil } func (lex *Lexer) Next() (Object, error) { ch, ok := lex.peek() if !ok { return New(End), nil } 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 New(Nil), nil case "true": return NewBool(true), nil case "false": return NewBool(false), nil default: return NewKeyword(v), nil } switch ch, _ := lex.read(); ch { case '/': return lex.name() case '%': return lex.comment() case '(': return lex.string() case '[': return New(BArray), nil case ']': return New(EArray), nil case '<': if ch, _ := lex.peek(); ch == '<' { lex.read() return New(BDict), nil } return lex.stringHex() case '>': if ch, _ := lex.peek(); ch == '>' { lex.read() return New(EDict), nil } return newError("unexpected '>'") default: if lex.eatNewline(ch) { return New(NL), nil } if strings.IndexByte(whitespace, ch) >= 0 { return lex.Next() } return newError("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, error) { lenStack := len(*stack) if lenStack < 2 { return newError("missing object ID pair") } n := (*stack)[lenStack-2] g := (*stack)[lenStack-1] *stack = (*stack)[:lenStack-2] if !g.IsUint() || !n.IsUint() { return newError("invalid object ID pair") } var inner []Object for { object, _ := u.parse(lex, &inner) if object.Kind == End { return newError("object doesn't end") } if object.Kind == Keyword && object.String == "endobj" { break } inner = append(inner, object) } if len(inner) != 1 { return newError("indirect objects must contain exactly one object") } return NewIndirect(inner[0], uint(n.Number), uint(g.Number)), nil } func (u *Updater) parseR(stack *[]Object) (Object, error) { lenStack := len(*stack) if lenStack < 2 { return newError("missing reference ID pair") } n := (*stack)[lenStack-2] g := (*stack)[lenStack-1] *stack = (*stack)[:lenStack-2] if !g.IsUint() || !n.IsUint() { return newError("invalid reference ID pair") } return NewReference(uint(n.Number), uint(g.Number)), nil } // parse reads an object at the lexer's position. Not a strict parser. // // TODO(p): We should fix all uses of this not to eat the error. func (u *Updater) parse(lex *Lexer, stack *[]Object) (Object, error) { switch token, err := 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 newError("array doesn't end") } if object.Kind == EArray { break } array = append(array, object) } return NewArray(array), nil case BDict: var array []Object for { object, _ := u.parse(lex, &array) if object.Kind == End { return newError("dictionary doesn't end") } if object.Kind == EDict { break } array = append(array, object) } if len(array)%2 != 0 { return newError("unbalanced dictionary") } dict := make(map[string]Object) for i := 0; i < len(array); i += 2 { if array[i].Kind != Name { return newError("invalid dictionary key type") } dict[array[i].String] = array[i+1] } return NewDict(dict), nil 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 newError("streams are not supported yet") case "obj": return u.parseIndirect(lex, stack) case "R": return u.parseR(stack) } fallthrough default: return token, err } } 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"] = NewNumeric(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. // // TODO(p): We should fix all uses of this not to eat the error. func (u *Updater) Get(n, generation uint) (Object, error) { if n >= u.xrefSize { return New(Nil), nil } ref := u.xref[n] if !ref.nonfree || ref.generation != generation || ref.offset >= int64(len(u.Document)) { return New(Nil), nil } lex := Lexer{u.Document[ref.offset:]} var stack []Object for { object, err := u.parse(&lex, &stack) if object.Kind == End { return object, err } if object.Kind != Indirect { stack = append(stack, object) } else if object.N != n || object.Generation != generation { return newError("object mismatch") } else { return object.Array[0], nil } } } // 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"] = NewNumeric(float64(u.xrefSize)) trailer := NewDict(u.Trailer) fmt.Fprintf(buf, "trailer\n%s\nstartxref\n%d\n%%%%EOF\n", trailer.Serialize(), startXref) u.Document = buf.Bytes() } // ----------------------------------------------------------------------------- // NewDate makes a PDF object representing the given point in time. func NewDate(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 NewString(string(buf)) } // GetFirstPage retrieves the first page of the document or a Nil object. func (u *Updater) GetFirstPage(nodeN, nodeGeneration uint) Object { obj, _ := u.Get(nodeN, nodeGeneration) if obj.Kind != Dict { return New(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 New(Nil) } else if typ.String == "Page" { return obj } else if typ.String != "Pages" { return New(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 New(Nil) } // XXX: Nothing prevents us from recursing in an evil circular graph. return u.GetFirstPage(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. // This is a very low-level function. func FillInSignature(document []byte, signOff, signLen int, key crypto.PrivateKey, 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 } // 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 // Sign signs the given document, growing and returning the passed-in slice. // There must be at least one certificate, matching the private key. // The certificates must form a chain. // // 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. func Sign(document []byte, key crypto.PrivateKey, 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 := NewDate(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 := NewDict(map[string]Object{ // 8.6.3 Field Types - Signature Fields "FT": NewName("Sig"), "V": NewReference(sigdictN, 0), // 8.4.5 Annotations Types - Widget Annotations // We can merge the Signature Annotation and omit Kids here. "Subtype": NewName("Widget"), "F": NewNumeric(2 /* Hidden */), "T": NewString("Signature1"), "Rect": NewArray([]Object{ NewNumeric(0), NewNumeric(0), NewNumeric(0), NewNumeric(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 := pdf.GetFirstPage(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 = NewArray(nil) } annots.Array = append(annots.Array, NewReference(sigfieldN, 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"] = NewDict(map[string]Object{ "Fields": NewArray([]Object{NewReference(sigfieldN, 0)}), "SigFlags": NewNumeric(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"] = NewName("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 }