diff options
| -rw-r--r-- | README.adoc | 8 | ||||
| -rw-r--r-- | cmd/pdf-simple-sign/main.go | 72 | ||||
| -rw-r--r-- | pdf/pdf.go | 1177 |
3 files changed, 1255 insertions, 2 deletions
diff --git a/README.adoc b/README.adoc index d20ff8a..59bfdef 100644 --- a/README.adoc +++ b/README.adoc @@ -2,8 +2,8 @@ pdf-simple-sign =============== :compact-option: -'pdf-simple-sign' is a simple open source PDF signer intended for documents -generated by Cairo. As such, it currently comes with some restrictions: +'pdf-simple-sign' is a simple PDF signer intended for documents produced by +the Cairo library. As such, it currently comes with some restrictions: * the document may not have any forms or signatures already, as they will be overwitten @@ -30,6 +30,10 @@ Runtime dependencies: libcrypto (OpenSSL 1.1 API) $ cd builddir $ ninja +In addition to the C++ version, also included is a native Go port: + + $ go get janouch.name/pdf-simple-sign/cmd/pdf-simple-sign + Usage ----- diff --git a/cmd/pdf-simple-sign/main.go b/cmd/pdf-simple-sign/main.go new file mode 100644 index 0000000..11aadaa --- /dev/null +++ b/cmd/pdf-simple-sign/main.go @@ -0,0 +1,72 @@ +// +// 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. +// + +// pdf-simple-sign is a simple PDF signer. +package main + +import ( + "flag" + "fmt" + "io/ioutil" + "janouch.name/pdf-simple-sign/pdf" + "os" +) + +// #include <unistd.h> +import "C" + +func isatty(fd uintptr) bool { return C.isatty(C.int(fd)) != 0 } + +func die(status int, format string, args ...interface{}) { + msg := fmt.Sprintf(format+"\n", args...) + if isatty(os.Stderr.Fd()) { + msg = "\x1b[0;31m" + msg + "\x1b[m" + } + os.Stderr.WriteString(msg) + os.Exit(status) +} + +func usage() { + die(1, "Usage: %s [-h] INPUT-FILENAME OUTPUT-FILENAME "+ + "PKCS12-PATH PKCS12-PASS", os.Args[0]) +} + +func main() { + flag.Usage = usage + flag.Parse() + if flag.NArg() != 4 { + usage() + } + + inputPath, outputPath := flag.Arg(0), flag.Arg(1) + pdfDocument, err := ioutil.ReadFile(inputPath) + if err != nil { + die(1, "%s", err) + } + p12, err := ioutil.ReadFile(flag.Arg(2)) + if err != nil { + die(2, "%s", err) + } + key, certs, err := pdf.PKCS12Parse(p12, flag.Arg(3)) + if err != nil { + die(3, "%s", err) + } + if pdfDocument, err = pdf.Sign(pdfDocument, key, certs); err != nil { + die(2, "error: %s", err) + } + if err = ioutil.WriteFile(outputPath, pdfDocument, 0666); err != nil { + die(3, "%s", err) + } +} 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 +} |