summaryrefslogtreecommitdiff
path: root/pdf-simple-sign.cpp
blob: 1755dc946caee4bf68b5a3ca22951848ee499ac3 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
// vim: set sw=2 ts=2 sts=2 et tw=100:
//
// pdf-simple-sign: simple PDF signer
//
// Copyright (c) 2017, 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.
//

#include <cstdio>
#include <cmath>
#undef NDEBUG
#include <cassert>

#include <vector>
#include <map>
#include <regex>
#include <memory>
#include <set>

#if defined __GLIBCXX__ && __GLIBCXX__ < 20140422
#error Need libstdc++ >= 4.9 for <regex>
#endif

#include <unistd.h>
#include <getopt.h>
#include <openssl/err.h>
#include <openssl/x509v3.h>
#include <openssl/pkcs12.h>

// -------------------------------------------------------------------------------------------------

using uint = unsigned int;

static std::string concatenate(const std::vector<std::string>& v, const std::string& delim) {
  std::string res;
  if (v.empty())
    return res;
  for (const auto& s : v)
    res += s + delim;
  return res.substr(0, res.length() - delim.length());
}

template<typename... Args>
std::string ssprintf(const std::string& format, Args... args) {
  size_t size = std::snprintf(nullptr, 0, format.c_str(), args... ) + 1;
  std::unique_ptr<char[]> buf(new char[size]);
  std::snprintf(buf.get(), size, format.c_str(), args...);
  return std::string(buf.get(), buf.get() + size - 1);
}

// -------------------------------------------------------------------------------------------------

/// 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.
struct pdf_object {
  enum type {
    END, NL, COMMENT, NIL, BOOL, NUMERIC, KEYWORD, NAME, STRING,
    // Simple tokens
    B_ARRAY, E_ARRAY, B_DICT, E_DICT,
    // Higher-level objects
    ARRAY, DICT, OBJECT, REFERENCE,
  } type = END;

  std::string string;                      ///< END (error message), COMMENT/KEYWORD/NAME/STRING
  double number = 0.;                      ///< BOOL, NUMERIC
  std::vector<pdf_object> array;           ///< ARRAY, OBJECT
  std::map<std::string, pdf_object> dict;  ///< DICT, in the future also STREAM
  uint n = 0, generation = 0;              ///< OBJECT, REFERENCE

  pdf_object(enum type type = END)                          : type(type) {}
  pdf_object(enum type type, double v)                      : type(type), number(v) {}
  pdf_object(enum type type, const std::string& v)          : type(type), string(v) {}
  pdf_object(enum type type, uint n, uint g)                : type(type), n(n), generation(g) {}
  pdf_object(const std::vector<pdf_object>& array)          : type(ARRAY), array(array) {}
  pdf_object(const std::map<std::string, pdf_object>& dict) : type(DICT), dict(dict) {}

  pdf_object(const pdf_object&)            = default;
  pdf_object(pdf_object&&)                 = default;
  pdf_object& operator=(const pdf_object&) = default;
  pdf_object& operator=(pdf_object&&)      = default;

  /// Return whether this is a number without a fractional part
  bool is_integer() const {
    double tmp;
    return type == NUMERIC && std::modf(number, &tmp) == 0.;
  }
};

/// Basic lexical analyser for the Portable Document Format, giving limited error information
struct pdf_lexer {
  const unsigned char* p;
  pdf_lexer(const char* s) : p(reinterpret_cast<const unsigned char*>(s)) {}

  static constexpr const char* oct_alphabet = "01234567";
  static constexpr const char* dec_alphabet = "0123456789";
  static constexpr const char* hex_alphabet = "0123456789abcdefABCDEF";
  static constexpr const char* whitespace = "\t\n\f\r ";
  static constexpr const char* delimiters = "()<>[]{}/%";

  bool eat_newline(int ch) {
    if (ch == '\r') {
      if (*p == '\n') p++;
      return true;
    }
    return ch == '\n';
  }

  pdf_object string() {
    std::string value;
    int parens = 1;
    while (1) {
      if (!*p) return {pdf_object::END, "unexpected end of string"};
      auto ch = *p++;
      if (eat_newline(ch)) ch = '\n';
      else if (ch == '(') { parens++; }
      else if (ch == ')') { if (!--parens) break; }
      else if (ch == '\\') {
        if (!*p) return {pdf_object::END, "unexpected end of string"};
        switch ((ch = *p++)) {
        case 'n': ch = '\n'; break;
        case 'r': ch = '\r'; break;
        case 't': ch = '\t'; break;
        case 'b': ch = '\b'; break;
        case 'f': ch = '\f'; break;
        default:
          if (eat_newline(ch))
            continue;
          std::string octal;
          if (ch && strchr(oct_alphabet, ch)) {
            octal += ch;
            if (*p && strchr(oct_alphabet, *p)) octal += *p++;
            if (*p && strchr(oct_alphabet, *p)) octal += *p++;
            ch = std::stoi(octal, nullptr, 8);
          }
        }
      }
      value += ch;
    }
    return {pdf_object::STRING, value};
  }

  pdf_object string_hex() {
    std::string value, buf;
    while (*p != '>') {
      if (!*p) return {pdf_object::END, "unexpected end of hex string"};
      if (!strchr(hex_alphabet, *p))
        return {pdf_object::END, "invalid hex string"};
      buf += *p++;
      if (buf.size() == 2) {
        value += char(std::stoi(buf, nullptr, 16));
        buf.clear();
      }
    }
    if (!buf.empty()) value += char(std::stoi(buf + '0', nullptr, 16));
    return {pdf_object::STRING, value};
  }

  pdf_object name() {
    std::string value;
    while (!strchr(whitespace, *p) && !strchr(delimiters, *p)) {
      auto ch = *p++;
      if (ch == '#') {
        std::string hexa;
        if (*p && strchr(hex_alphabet, *p)) hexa += *p++;
        if (*p && strchr(hex_alphabet, *p)) hexa += *p++;
        if (hexa.size() != 2)
          return {pdf_object::END, "invalid name hexa escape"};
        ch = char(std::stoi(hexa, nullptr, 16));
      }
      value += ch;
    }
    if (value.empty()) return {pdf_object::END, "unexpected end of name"};
    return {pdf_object::NAME, value};
  }

  pdf_object comment() {
    std::string value;
    while (*p && *p != '\r' && *p != '\n')
      value += *p++;
    return {pdf_object::COMMENT, value};
  }

  // XXX maybe invalid numbers should rather be interpreted as keywords
  pdf_object number() {
    std::string value;
    if (*p == '-')
      value += *p++;
    bool real = false, digits = false;
    while (*p) {
      if (strchr(dec_alphabet, *p))
        digits = true;
      else if (*p == '.' && !real)
        real = true;
      else
        break;
      value += *p++;
    }
    if (!digits) return {pdf_object::END, "invalid number"};
    return {pdf_object::NUMERIC, std::stod(value, nullptr)};
  }

  pdf_object next() {
    if (!*p)
      return {pdf_object::END};
    if (strchr("-0123456789.", *p))
      return number();

    // {} end up being keywords, we might want to error out on those
    std::string value;
    while (!strchr(whitespace, *p) && !strchr(delimiters, *p))
      value += *p++;
    if (!value.empty()) {
      if (value == "null")  return {pdf_object::NIL};
      if (value == "true")  return {pdf_object::BOOL, 1};
      if (value == "false") return {pdf_object::BOOL, 0};
      return {pdf_object::KEYWORD, value};
    }

    switch (char ch = *p++) {
    case '/': return name();
    case '%': return comment();
    case '(': return string();
    case '[': return {pdf_object::B_ARRAY};
    case ']': return {pdf_object::E_ARRAY};
    case '<':
      if (*p++ == '<')
        return {pdf_object::B_DICT};
      p--;
      return string_hex();
    case '>':
      if (*p++ == '>')
        return {pdf_object::E_DICT};
      p--;
      return {pdf_object::END, "unexpected '>'"};
    default:
      if (eat_newline(ch))
        return {pdf_object::NL};
      if (strchr(whitespace, ch))
        return next();
      return {pdf_object::END, "unexpected input"};
    }
  }
};

// FIXME lines /should not/ be longer than 255 characters, some wrapping is in order
static std::string pdf_serialize(const pdf_object& o) {
  switch (o.type) {
  case pdf_object::NL:      return "\n";
  case pdf_object::NIL:     return "null";
  case pdf_object::BOOL:    return o.number ? "true" : "false";
  case pdf_object::NUMERIC:
  {
    if (o.is_integer()) return std::to_string((long long) o.number);
    return std::to_string(o.number);
  }
  case pdf_object::KEYWORD: return o.string;
  case pdf_object::NAME:
  {
    std::string escaped = "/";
    for (char c : o.string) {
      if (c == '#' || strchr(pdf_lexer::delimiters, c) || strchr(pdf_lexer::whitespace, c))
        escaped += ssprintf("#%02x", c);
      else
        escaped += c;
    }
    return escaped;
  }
  case pdf_object::STRING:
  {
    std::string escaped;
    for (char c : o.string) {
      if (c == '\\' || c == '(' || c == ')')
        escaped += '\\';
      escaped += c;
    }
    return "(" + escaped + ")";
  }
  case pdf_object::B_ARRAY: return "[";
  case pdf_object::E_ARRAY: return "]";
  case pdf_object::B_DICT:  return "<<";
  case pdf_object::E_DICT:  return ">>";
  case pdf_object::ARRAY:
  {
    std::vector<std::string> v;
    for (const auto& i : o.array)
      v.push_back(pdf_serialize(i));
    return "[ " + concatenate(v, " ") + " ]";
  }
  case pdf_object::DICT:
  {
    std::string s;
    for (const auto i : o.dict)
      s += " /" + i.first + " " + pdf_serialize(i.second);
    return "<<" + s + " >>";
  }
  case pdf_object::OBJECT:
    return ssprintf("%u %u obj\n", o.n, o.generation) + pdf_serialize(o.array.at(0)) + "\nendobj";
  case pdf_object::REFERENCE:
    return ssprintf("%u %u R", o.n, o.generation);
  default:
    assert(!"unsupported token for serialization");
  }
}

// -------------------------------------------------------------------------------------------------

/// Utility class to help read and possibly incrementally update PDF files
class pdf_updater {
  struct ref {
    size_t offset = 0;     ///< File offset or N of the next free entry
    uint generation = 0;   ///< Object generation
    bool free = true;      ///< Whether this N has been deleted
  };

  std::vector<ref> xref;   ///< Cross-reference table
  size_t xref_size = 0;    ///< Current cross-reference table size, correlated to xref.size()
  std::set<uint> updated;  ///< List of updated objects

  pdf_object parse_obj(pdf_lexer& lex, std::vector<pdf_object>& stack) const;
  pdf_object parse_R(std::vector<pdf_object>& stack) const;
  pdf_object parse(pdf_lexer& lex, std::vector<pdf_object>& stack) const;
  std::string load_xref(pdf_lexer& lex, std::set<uint>& loaded_entries);

public:
  /// The new trailer dictionary to be written, initialized with the old one
  std::map<std::string, pdf_object> trailer;

  std::string& document;
  pdf_updater(std::string& document) : document(document) {}

  /// Build the cross-reference table and prepare a new trailer dictionary
  std::string initialize();
  /// Retrieve an object by its number and generation -- may return NIL or END with an error
  pdf_object get(uint n, uint generation) const;
  /// Allocate a new object number
  uint allocate();
  /// Append an updated object to the end of the document
  void update(uint n, std::function<void()> fill);
  /// Write an updated cross-reference table and trailer
  void flush_updates();
};

// -------------------------------------------------------------------------------------------------

pdf_object pdf_updater::parse_obj(pdf_lexer& lex, std::vector<pdf_object>& stack) const {
  if (stack.size() < 2)
    return {pdf_object::END, "missing object ID pair"};

  auto g = stack.back(); stack.pop_back();
  auto n = stack.back(); stack.pop_back();
  if (!g.is_integer() || g.number < 0 || g.number > UINT_MAX
   || !n.is_integer() || n.number < 0 || n.number > UINT_MAX)
    return {pdf_object::END, "invalid object ID pair"};

  pdf_object obj{pdf_object::OBJECT};
  obj.n = n.number;
  obj.generation = g.number;

  while (1) {
    auto object = parse(lex, obj.array);
    if (object.type == pdf_object::END)
      return {pdf_object::END, "object doesn't end"};
    if (object.type == pdf_object::KEYWORD && object.string == "endobj")
      break;
    obj.array.push_back(std::move(object));
  }
  return obj;
}

pdf_object pdf_updater::parse_R(std::vector<pdf_object>& stack) const {
  if (stack.size() < 2)
    return {pdf_object::END, "missing reference ID pair"};

  auto g = stack.back(); stack.pop_back();
  auto n = stack.back(); stack.pop_back();
  if (!g.is_integer() || g.number < 0 || g.number > UINT_MAX
   || !n.is_integer() || n.number < 0 || n.number > UINT_MAX)
    return {pdf_object::END, "invalid reference ID pair"};

  pdf_object ref{pdf_object::REFERENCE};
  ref.n = n.number;
  ref.generation = g.number;
  return ref;
}

/// Read an object at the lexer's position.  Not a strict parser.
pdf_object pdf_updater::parse(pdf_lexer& lex, std::vector<pdf_object>& stack) const {
  auto token = lex.next();
  switch (token.type) {
  case pdf_object::NL:
  case pdf_object::COMMENT:
    // These are not important to parsing, not even for this procedure's needs
    return parse(lex, stack);
  case pdf_object::B_ARRAY:
  {
    std::vector<pdf_object> array;
    while (1) {
      auto object = parse(lex, array);
      if (object.type == pdf_object::END)
        return {pdf_object::END, "array doesn't end"};
      if (object.type == pdf_object::E_ARRAY)
        break;
      array.push_back(std::move(object));
    }
    return array;
  }
  case pdf_object::B_DICT:
  {
    std::vector<pdf_object> array;
    while (1) {
      auto object = parse(lex, array);
      if (object.type == pdf_object::END)
        return {pdf_object::END, "dictionary doesn't end"};
      if (object.type == pdf_object::E_DICT)
        break;
      array.push_back(std::move(object));
    }
    if (array.size() % 2)
      return {pdf_object::END, "unbalanced dictionary"};
    std::map<std::string, pdf_object> dict;
    for (size_t i = 0; i < array.size(); i += 2) {
      if (array[i].type != pdf_object::NAME)
        return {pdf_object::END, "invalid dictionary key type"};
      dict.insert({array[i].string, std::move(array[i + 1])});
    }
    return dict;
  }
  case pdf_object::KEYWORD:
    // Appears in the document body, typically needs to access the cross-reference table
    // TODO use the xref to read /Length etc. once we actually need to read such objects;
    //   presumably streams can use the pdf_object::string member
    if (token.string == "stream") return {pdf_object::END, "streams are not supported yet"};
    if (token.string == "obj")    return parse_obj(lex, stack);
    if (token.string == "R")      return parse_R(stack);
    return token;
  default:
    return token;
  }
}

std::string pdf_updater::load_xref(pdf_lexer& lex, std::set<uint>& loaded_entries) {
  std::vector<pdf_object> throwaway_stack;
  {
    auto keyword = parse(lex, throwaway_stack);
    if (keyword.type != pdf_object::KEYWORD || keyword.string != "xref")
      return "invalid xref table";
  }
  while (1) {
    auto object = parse(lex, throwaway_stack);
    if (object.type == pdf_object::END)
      return "unexpected EOF while looking for the trailer";
    if (object.type == pdf_object::KEYWORD && object.string == "trailer")
      break;

    auto second = parse(lex, throwaway_stack);
    if (!object.is_integer() || object.number < 0 || object.number > UINT_MAX
     || !second.is_integer() || second.number < 0 || second.number > UINT_MAX)
      return "invalid xref section header";

    const size_t start = object.number;
    const size_t count = second.number;
    for (size_t i = 0; i < count; i++) {
      auto off = parse(lex, throwaway_stack);
      auto gen = parse(lex, throwaway_stack);
      auto key = parse(lex, throwaway_stack);
      if (!off.is_integer() || off.number < 0 || off.number > document.length()
       || !gen.is_integer() || gen.number < 0 || gen.number > 65535
       || key.type != pdf_object::KEYWORD)
        return "invalid xref entry";

      bool free = true;
      if (key.string == "n")
        free = false;
      else if (key.string != "f")
        return "invalid xref entry";

      auto n = start + i;
      if (loaded_entries.count(n))
        continue;
      if (n >= xref.size())
        xref.resize(n + 1);
      loaded_entries.insert(n);

      auto& ref = xref[n];
      ref.generation = gen.number;
      ref.offset = off.number;
      ref.free = free;
    }
  }
  return "";
}

// -------------------------------------------------------------------------------------------------

std::string pdf_updater::initialize() {
  // We only need to look for startxref roughly within the last kibibyte of the document
  static std::regex haystack_re("[\\s\\S]*\\sstartxref\\s+(\\d+)\\s+%%EOF");
  std::string haystack = document.substr(document.length() < 1024 ? 0 : document.length() - 1024);

  std::smatch m;
  if (!std::regex_search(haystack, m, haystack_re, std::regex_constants::match_continuous))
    return "cannot find startxref";

  size_t xref_offset = std::stoul(m.str(1)), last_xref_offset = xref_offset;
  std::set<size_t> loaded_xrefs;
  std::set<uint> loaded_entries;

  std::vector<pdf_object> throwaway_stack;
  while (1) {
    if (loaded_xrefs.count(xref_offset))
      return "circular xref offsets";
    if (xref_offset >= document.length())
      return "invalid xref offset";

    pdf_lexer lex(document.c_str() + xref_offset);
    auto err = load_xref(lex, loaded_entries);
    if (!err.empty()) return err;

    auto trailer = parse(lex, throwaway_stack);
    if (trailer.type != pdf_object::DICT)
      return "invalid trailer dictionary";
    if (loaded_xrefs.empty())
      this->trailer = trailer.dict;
    loaded_xrefs.insert(xref_offset);

    const auto prev_offset = trailer.dict.find("Prev");
    if (prev_offset == trailer.dict.end())
      break;
    if (!prev_offset->second.is_integer())
      return "invalid Prev offset";
    xref_offset = prev_offset->second.number;
  }

  trailer["Prev"] = {pdf_object::NUMERIC, double(last_xref_offset)};
  const auto last_size = trailer.find("Size");
  if (last_size == trailer.end() || !last_size->second.is_integer() ||
      last_size->second.number <= 0)
    return "invalid or missing cross-reference table Size";

  xref_size = last_size->second.number;
  return "";
}

pdf_object pdf_updater::get(uint n, uint generation) const {
  if (n >= xref_size)
    return {pdf_object::NIL};

  auto& ref = xref[n];
  if (ref.free || ref.generation != generation || ref.offset >= document.length())
    return {pdf_object::NIL};

  pdf_lexer lex(document.c_str() + ref.offset);
  std::vector<pdf_object> stack;
  while (1) {
    auto object = parse(lex, stack);
    if (object.type == pdf_object::END)
      return object;
    if (object.type != pdf_object::OBJECT)
      stack.push_back(std::move(object));
    else if (object.n != n || object.generation != generation)
      return {pdf_object::END, "object mismatch"};
    else
      return std::move(object.array.at(0));
  }
}

uint pdf_updater::allocate() {
  assert(xref_size < UINT_MAX);

  auto n = xref_size++;
  if (xref.size() < xref_size)
    xref.resize(xref_size);

  // 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;
}

void pdf_updater::update(uint n, std::function<void()> fill) {
  auto& ref = xref.at(n);
  ref.offset = document.length() + 1;
  ref.free = false;
  updated.insert(n);

  document += ssprintf("\n%u %u obj\n", n, ref.generation);
  // Separately so that the callback can use document.length() to get the current offset
  fill();
  document += "\nendobj";
}

void pdf_updater::flush_updates() {
  std::map<uint, size_t> groups;
  for (auto i = updated.cbegin(); i != updated.cend(); ) {
    size_t start = *i, count = 1;
    while (++i != updated.cend() && *i == start + count)
      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 (groups.empty())
    groups[0] = 0;

  auto startxref = document.length() + 1;
  document += "\nxref\n";
  for (const auto& g : groups) {
    document += ssprintf("%u %zu\n", g.first, g.second);
    for (size_t i = 0; i < g.second; i++) {
      auto& ref = xref[g.first + i];
      document += ssprintf("%010zu %05u %c \n", ref.offset, ref.generation, "nf"[!!ref.free]);
    }
  }

  trailer["Size"] = {pdf_object::NUMERIC, double(xref_size)};
  document += "trailer\n" + pdf_serialize(trailer)
    + ssprintf("\nstartxref\n%zu\n%%%%EOF\n", startxref);
}

// -------------------------------------------------------------------------------------------------

/// Make a PDF object representing the given point in time
static pdf_object pdf_date(time_t timestamp) {
  struct tm parts;
  assert(localtime_r(&timestamp, &parts));

  char buf[64];
  assert(strftime(buf, sizeof buf, "D:%Y%m%d%H%M%S", &parts));

  std::string offset = "Z";
  auto offset_min = parts.tm_gmtoff / 60;
  if (parts.tm_gmtoff < 0)
    offset = ssprintf("-%02ld'%02ld'", -offset_min / 60, -offset_min % 60);
  if (parts.tm_gmtoff > 0)
    offset = ssprintf("+%02ld'%02ld'", +offset_min / 60, +offset_min % 60);
  return {pdf_object::STRING, buf + offset};
}

static pdf_object pdf_get_first_page(pdf_updater& pdf, uint node_n, uint node_generation) {
  auto obj = pdf.get(node_n, node_generation);
  if (obj.type != pdf_object::DICT)
    return {pdf_object::NIL};

  // Out of convenience; these aren't filled normally
  obj.n = node_n;
  obj.generation = node_generation;

  auto type = obj.dict.find("Type");
  if (type == obj.dict.end() || type->second.type != pdf_object::NAME)
    return {pdf_object::NIL};
  if (type->second.string == "Page")
    return obj;
  if (type->second.string != "Pages")
    return {pdf_object::NIL};

  // XXX technically speaking, this may be an indirect reference.  The correct way to solve this
  //   seems to be having "pdf_updater" include a wrapper around "obj.dict.find"
  auto kids = obj.dict.find("Kids");
  if (kids == obj.dict.end() || kids->second.type != pdf_object::ARRAY
   || kids->second.array.empty()
   || kids->second.array.at(0).type != pdf_object::REFERENCE)
    return {pdf_object::NIL};

  // XXX nothing prevents us from recursing in an evil circular graph
  return pdf_get_first_page(pdf, kids->second.array.at(0).n, kids->second.array.at(0).generation);
}

// -------------------------------------------------------------------------------------------------

static std::string pkcs12_path, pkcs12_pass;

// /All/ bytes are checked, except for the signature hexstring itself
static std::string pdf_fill_in_signature(std::string& document, size_t sign_off, size_t sign_len) {
  size_t tail_off = sign_off + sign_len, tail_len = document.size() - tail_off;
  if (pkcs12_path.empty())
    return "undefined path to the signing key";

  auto pkcs12_fp = fopen(pkcs12_path.c_str(), "r");
  if (!pkcs12_fp)
    return pkcs12_path + ": " + strerror(errno);

  // Abandon hope, all ye who enter OpenSSL!  Half of it is undocumented.
  OpenSSL_add_all_algorithms();
  ERR_load_crypto_strings();
  ERR_clear_error();

  PKCS12* p12 = nullptr;
  EVP_PKEY* private_key = nullptr;
  X509* certificate = nullptr;
  STACK_OF(X509)* chain = nullptr;
  PKCS7* p7 = nullptr;
  int len = 0, sign_flags = PKCS7_DETACHED | PKCS7_BINARY | PKCS7_NOSMIMECAP | PKCS7_PARTIAL;
  BIO* p7bio = nullptr;
  unsigned char* buf = nullptr;

  // OpenSSL error reasons will usually be of more value than any distinction I can come up with
  std::string err = "OpenSSL failure";

  if (!(p12 = d2i_PKCS12_fp(pkcs12_fp, nullptr))
   || !PKCS12_parse(p12, pkcs12_pass.c_str(), &private_key, &certificate, &chain)) {
    err = pkcs12_path + ": parse failure";
    goto error;
  }
  if (!private_key || !certificate) {
    err = pkcs12_path + ": must contain a private key and a valid certificate chain";
    goto error;
  }
  // Prevent useless signatures -- makes pdfsig from poppler happy at least (and NSS by extension)
  if (!(X509_get_key_usage(certificate) & (KU_DIGITAL_SIGNATURE | KU_NON_REPUDIATION))) {
    err = "the certificate's key usage must include digital signatures or non-repudiation";
    goto error;
  }
  if (!(X509_get_extended_key_usage(certificate) & (XKU_SMIME | XKU_ANYEKU))) {
    err = "the certificate's extended key usage must include S/MIME";
    goto error;
  }
#if 0  // This happily ignores XKU_ANYEKU and I want my tiny world to make a tiny bit more sense
  if (X509_check_purpose(certificate, X509_PURPOSE_SMIME_SIGN, false /* not a CA certificate */)) {
    err = "the certificate can't be used for S/MIME digital signatures";
    goto error;
  }
#endif

  // The default digest is SHA1, which is mildly insecure now -- hence using PKCS7_sign_add_signer
  if (!(p7 = PKCS7_sign(nullptr, nullptr, nullptr, nullptr, sign_flags))
   || !PKCS7_sign_add_signer(p7, certificate, private_key, EVP_sha256(), sign_flags))
    goto error;
  // For RFC 3161, this is roughly how a timestamp token would be attached (see Appendix A):
  //   PKCS7_add_attribute(signer_info, NID_id_smime_aa_timeStampToken, V_ASN1_SEQUENCE, value)
  for (int i = 0; i < sk_X509_num(chain); i++)
    if (!PKCS7_add_certificate(p7, sk_X509_value(chain, i)))
      goto error;

  // Adaptation of the innards of the undocumented PKCS7_final() -- I didn't feel like making
  // a copy of the whole document.  Hopefully this writes directly into a digest BIO.
  if (!(p7bio = PKCS7_dataInit(p7, nullptr))
   || (ssize_t) sign_off != BIO_write(p7bio, document.data(), sign_off)
   || (ssize_t) tail_len != BIO_write(p7bio, document.data() + tail_off, tail_len)
   || BIO_flush(p7bio) != 1 || !PKCS7_dataFinal(p7, p7bio))
    goto error;

#if 0
  {
    // Debugging: openssl cms -inform PEM -in pdf_signature.pem -noout -cmsout -print
    // Context: https://stackoverflow.com/a/29253469
    auto fp = fopen("pdf_signature.pem", "wb");
    assert(PEM_write_PKCS7(fp, p7) && !fclose(fp));
  }
#endif

  if ((len = i2d_PKCS7(p7, &buf)) < 0)
    goto error;
  if (size_t(len) * 2 > sign_len - 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 API entirely
    err = ssprintf("not enough space reserved for the signature (%zu nibbles vs %zu nibbles)",
                   sign_len - 2, size_t(len) * 2);
    goto error;
  }
  for (int i = 0; i < len; i++) {
    document[sign_off + 2 * i + 1] = "0123456789abcdef"[buf[i] / 16];
    document[sign_off + 2 * i + 2] = "0123456789abcdef"[buf[i] % 16];
  }
  err.clear();

error:
  OPENSSL_free(buf);
  BIO_free_all(p7bio);
  PKCS7_free(p7);
  sk_X509_pop_free(chain, X509_free);
  X509_free(certificate);
  EVP_PKEY_free(private_key);
  PKCS12_free(p12);

  // In any case, clear the error stack (it's a queue, really) to avoid confusion elsewhere
  while (auto code = ERR_get_error())
    if (auto reason = ERR_reason_error_string(code))
      err = err + "; " + reason;

  fclose(pkcs12_fp);
  return err;
}

// -------------------------------------------------------------------------------------------------

/// 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
static std::string pdf_sign(std::string& document) {
  pdf_updater pdf(document);
  auto err = pdf.initialize();
  if (!err.empty())
    return err;

  auto root_ref = pdf.trailer.find("Root");
  if (root_ref == pdf.trailer.end() || root_ref->second.type != pdf_object::REFERENCE)
    return "trailer does not contain a reference to Root";
  auto root = pdf.get(root_ref->second.n, root_ref->second.generation);
  if (root.type != pdf_object::DICT)
    return "invalid Root dictionary reference";

  // 8.7 Digital Signatures - /signature dictionary/
  auto sigdict_n = pdf.allocate();
  size_t byterange_off = 0, byterange_len = 0, sign_off = 0, sign_len = 0;
  pdf.update(sigdict_n, [&]{
    // The timestamp is important for Adobe Acrobat Reader DC.  The ideal would be to use RFC 3161.
    pdf.document.append("<< /Type/Sig /Filter/Adobe.PPKLite /SubFilter/adbe.pkcs7.detached\n"
                        "   /M" + pdf_serialize(pdf_date(time(nullptr))) + " /ByteRange ");
    byterange_off = pdf.document.size();
    pdf.document.append((byterange_len = 32 /* fine for a gigabyte */), ' ');
    pdf.document.append("\n   /Contents <");
    sign_off = pdf.document.size();
    pdf.document.append((sign_len = 8192 /* certificate, digest, encrypted digest, ... */), '0');
    pdf.document.append("> >>");

    // We actually need to exclude the hexstring quotes from signing
    sign_off -= 1;
    sign_len += 2;
  });

  // 8.6.3 Field Types - Signature Fields
  pdf_object sigfield{pdf_object::DICT};
  sigfield.dict.insert({"FT", {pdf_object::NAME, "Sig"}});
  sigfield.dict.insert({"V", {pdf_object::REFERENCE, sigdict_n, 0}});
  // 8.4.5 Annotations Types - Widget Annotations
  // We can merge the Signature Annotation and omit Kids here
  sigfield.dict.insert({"Subtype", {pdf_object::NAME, "Widget"}});
  sigfield.dict.insert({"F", {pdf_object::NUMERIC, 2 /* Hidden */}});
  sigfield.dict.insert({"T", {pdf_object::STRING, "Signature1"}});
  sigfield.dict.insert({"Rect", {std::vector<pdf_object>{
    {pdf_object::NUMERIC, 0},
    {pdf_object::NUMERIC, 0},
    {pdf_object::NUMERIC, 0},
    {pdf_object::NUMERIC, 0},
  }}});

  auto sigfield_n = pdf.allocate();
  pdf.update(sigfield_n, [&]{ pdf.document += pdf_serialize(sigfield); });

  auto pages_ref = root.dict.find("Pages");
  if (pages_ref == root.dict.end() || pages_ref->second.type != pdf_object::REFERENCE)
    return "invalid Pages reference";
  auto page = pdf_get_first_page(pdf, pages_ref->second.n, pages_ref->second.generation);
  if (page.type != pdf_object::DICT)
    return "invalid or unsupported page tree";

  // XXX assuming this won't be an indirectly referenced array
  auto& annots = page.dict["Annots"];
  if (annots.type != pdf_object::ARRAY)
    annots = {pdf_object::ARRAY};
  annots.array.emplace_back(pdf_object::REFERENCE, sigfield_n, 0);
  pdf.update(page.n, [&]{ pdf.document += pdf_serialize(page); });

  // 8.6.1 Interactive Form Dictionary
  // XXX assuming there are no forms already, overwriting everything
  root.dict["AcroForm"] = {std::map<std::string, pdf_object>{
    {"Fields", {std::vector<pdf_object>{
      {pdf_object::REFERENCE, sigfield_n, 0}
    }}},
    {"SigFlags", {pdf_object::NUMERIC, 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"] = {pdf_object::NAME, "1.6"};
  pdf.update(root_ref->second.n, [&]{ pdf.document += pdf_serialize(root); });
  pdf.flush_updates();

  // 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
  size_t tail_off = sign_off + sign_len, tail_len = pdf.document.size() - tail_off;
  auto ranges = ssprintf("[0 %zu %zu %zu]", sign_off, tail_off, tail_len);
  if (ranges.length() > byterange_len)
    return "not enough space reserved for /ByteRange";
  pdf.document.replace(byterange_off, std::min(ranges.length(), byterange_len), ranges);
  return pdf_fill_in_signature(pdf.document, sign_off, sign_len);
}

// -------------------------------------------------------------------------------------------------

__attribute__((format(printf, 2, 3)))
static void die(int status, const char* format, ...) {
  va_list ap;
  va_start(ap, format);
  if (isatty(fileno(stderr)))
    vfprintf(stderr, ssprintf("\x1b[31m%s\x1b[0m\n", format).c_str(), ap);
  else
    vfprintf(stderr, format, ap);
  va_end(ap);
  exit(status);
}

int main(int argc, char* argv[]) {
  auto invocation_name = argv[0];
  auto usage = [=]{
    die(1, "Usage: %s [-h] INPUT-FILENAME OUTPUT-FILENAME PKCS12-PATH PKCS12-PASS",
            invocation_name);
  };

  static struct option opts[] = {
    {"help", no_argument, 0, 'h'},
    {nullptr, 0, 0, 0},
  };

  while (1) {
    int option_index = 0;
    auto c = getopt_long(argc, const_cast<char* const*>(argv),
                         "h", opts, &option_index);
    if (c == -1)
      break;

    switch (c) {
    case 'h': usage(); break;
    default: usage();
    }
  }

  argv += optind;
  argc -= optind;

  if (argc != 4)
    usage();

  const char* input_path  = argv[0];
  const char* output_path = argv[1];
  pkcs12_path = argv[2];
  pkcs12_pass = argv[3];

  std::string pdf_document;
  if (auto fp = fopen(input_path, "rb")) {
    int c;
    while ((c = fgetc(fp)) != EOF)
      pdf_document += c;
    if (ferror(fp))
      die(1, "%s: %s", input_path, strerror(errno));
    fclose(fp);
  } else {
    die(1, "%s: %s", input_path, strerror(errno));
  }

  auto err = pdf_sign(pdf_document);
  if (!err.empty()) {
    die(2, "Error: %s", err.c_str());
  }

  if (auto fp = fopen(output_path, "wb")) {
    auto written = fwrite(pdf_document.c_str(), pdf_document.size(), 1, fp);
    if (fclose(fp) || written != 1) {
      (void) unlink(output_path);
      die(3, "%s: %s", output_path, strerror(errno));
    }
  } else {
    die(3, "%s: %s", output_path, strerror(errno));
  }
  return 0;
}