Add .gitignore

The code has even turned out simpler.

.clang-format

@@ -0,0 +1,8 @@
+BasedOnStyle: Chromium
+ColumnLimit: 100
+IndentCaseLabels: false
+AccessModifierOffset: -2
+ContinuationIndentWidth: 2
+SpaceAfterTemplateKeyword: false
+SpaceAfterCStyleCast: true
+SpacesBeforeTrailingComments: 2

.gitignore

@@ -0,0 +1,8 @@
+/builddir
+/pdf-simple-sign.cflags
+/pdf-simple-sign.config
+/pdf-simple-sign.creator
+/pdf-simple-sign.creator.user
+/pdf-simple-sign.cxxflags
+/pdf-simple-sign.files
+/pdf-simple-sign.includes

LICENSE

@@ -1,4 +1,4 @@
-Copyright (c) 2017, Přemysl Eric Janouch <p@janouch.name>
+Copyright (c) 2017 - 2020, Přemysl Eric Janouch <p@janouch.name>
 
 Permission to use, copy, modify, and/or distribute this software for any
 purpose with or without fee is hereby granted.

NEWS

@@ -1,3 +1,28 @@
+1.1.1 (2020-09-06)
+
+ * Fix a dysfunctional example in the manual
+
+ * Go: write the xref table in a deterministic order
+
+ * Add a trivial test suite, based on pdfsig from poppler-utils
+
+
+1.1 (2020-09-05)
+
+ * Make it possible to change the signature reservation with an option
+
+ * Return errors rather than mangle documents in some cases,
+   notably with pre-existing PDF forms
+
+ * Avoid downgrading the document's PDF version to 1.6
+
+ * A few fixes for PDF parsing and serialisation
+
+ * Add an instructive man page
+
+ * Add a native Go port of the utility, also usable as a library
+
+
 1.0 (2018-08-03)
 
  * Initial release

README.adoc

@@ -1,27 +1,22 @@
 pdf-simple-sign
 ===============
-:compact-option:
 
 '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
- * the document may not employ cross-reference streams, or must constitute
-   a hybrid-reference file at least
- * the document may not be newer than PDF 1.6 already, or it will get downgraded
-   to that version
- * the signature may take at most 4 kilobytes as a compile-time limit,
-   which should be enough space even for one intermediate certificate
-
-The signature is attached to the first page and has no appearance.
+the Cairo library, GNU troff, ImageMagick, or similar.
 
 I don't aim to extend the functionality any further.  The project is fairly
 self-contained and it should be easy to grasp and change to suit to your needs.
 
+Documentation
+-------------
+See the link:pdf-simple-sign.adoc[man page] for information about usage.
+The rest of this README will concern itself with externalities.
+
+image:https://pkg.go.dev/badge/janouch.name/pdf-simple-sign@master/pdf["PkgGoDev", link="https://pkg.go.dev/janouch.name/pdf-simple-sign@master/pdf"]
+
 Building
 --------
-Build dependencies: Meson, a C++11 compiler, pkg-config +
+Build dependencies: Meson, Asciidoctor, a C++11 compiler, pkg-config +
 Runtime dependencies: libcrypto (OpenSSL 1.1 API)
 
  $ git clone https://git.janouch.name/p/pdf-simple-sign.git
@@ -34,11 +29,6 @@ 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
------
-
- $ ./pdf-simple-sign document.pdf document.signed.pdf KeyAndCerts.p12 password
-
 Contributing and Support
 ------------------------
 Use https://git.janouch.name/p/pdf-simple-sign to report bugs, request features,

cmd/pdf-simple-sign/main.go

@@ -1,5 +1,5 @@
 //
-// Copyright (c) 2018, Přemysl Eric Janouch <p@janouch.name>
+// Copyright (c) 2018 - 2020, Přemysl Eric Janouch <p@janouch.name>
 //
 // Permission to use, copy, modify, and/or distribute this software for any
 // purpose with or without fee is hereby granted.
@@ -20,8 +20,9 @@ import (
 	"flag"
 	"fmt"
 	"io/ioutil"
-	"janouch.name/pdf-simple-sign/pdf"
 	"os"
+
+	"janouch.name/pdf-simple-sign/pdf"
 )
 
 // #include <unistd.h>
@@ -39,10 +40,13 @@ func die(status int, format string, args ...interface{}) {
 }
 
 func usage() {
-	die(1, "Usage: %s [-h] INPUT-FILENAME OUTPUT-FILENAME "+
+	die(1, "Usage: %s [-h] [-r RESERVATION] INPUT-FILENAME OUTPUT-FILENAME "+
 		"PKCS12-PATH PKCS12-PASS", os.Args[0])
 }
 
+var reservation = flag.Int(
+	"r", 4096, "signature reservation as a number of bytes")
+
 func main() {
 	flag.Usage = usage
 	flag.Parse()
@@ -51,7 +55,7 @@ func main() {
 	}
 
 	inputPath, outputPath := flag.Arg(0), flag.Arg(1)
-	pdfDocument, err := ioutil.ReadFile(inputPath)
+	doc, err := ioutil.ReadFile(inputPath)
 	if err != nil {
 		die(1, "%s", err)
 	}
@@ -63,10 +67,10 @@ func main() {
 	if err != nil {
 		die(3, "%s", err)
 	}
-	if pdfDocument, err = pdf.Sign(pdfDocument, key, certs); err != nil {
+	if doc, err = pdf.Sign(doc, key, certs, *reservation); err != nil {
 		die(4, "error: %s", err)
 	}
-	if err = ioutil.WriteFile(outputPath, pdfDocument, 0666); err != nil {
+	if err = ioutil.WriteFile(outputPath, doc, 0666); err != nil {
 		die(5, "%s", err)
 	}
 }

go.mod

@@ -1,6 +1,6 @@
 module janouch.name/pdf-simple-sign
 
-go 1.14
+go 1.17
 
 require (
 	go.mozilla.org/pkcs7 v0.0.0-20200128120323-432b2356ecb1

go.sum

@@ -1,9 +1,5 @@
-go.mozilla.org/pkcs7 v0.0.0-20181029144607-24857c352dd8 h1:W3oGFPlHBLgXdsbPVixWFMYsuPhm81/Qww3XAgBbn/0=
-go.mozilla.org/pkcs7 v0.0.0-20181029144607-24857c352dd8/go.mod h1:5fWP3IVYEMc04wC+lMJAfkmNmKAl2P1swVv8VS+URZ8=
 go.mozilla.org/pkcs7 v0.0.0-20200128120323-432b2356ecb1 h1:A/5uWzF44DlIgdm/PQFwfMkW0JX+cIcQi/SwLAmZP5M=
 go.mozilla.org/pkcs7 v0.0.0-20200128120323-432b2356ecb1/go.mod h1:SNgMg+EgDFwmvSmLRTNKC5fegJjB7v23qTQ0XLGUNHk=
-golang.org/x/crypto v0.0.0-20181127143415-eb0de9b17e85 h1:et7+NAX3lLIk5qUCTA9QelBjGE/NkhzYw/mhnr0s7nI=
-golang.org/x/crypto v0.0.0-20181127143415-eb0de9b17e85/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20200728195943-123391ffb6de h1:ikNHVSjEfnvz6sxdSPCaPt572qowuyMDMJLLm3Db3ig=
 golang.org/x/crypto v0.0.0-20200728195943-123391ffb6de/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=

meson.build

@@ -1,5 +1,23 @@
-project('pdf-simple-sign', 'cpp', default_options : ['cpp_std=c++11'])
+project('pdf-simple-sign', 'cpp', default_options : ['cpp_std=c++11'],
+	version : '1.1.1')
+
+conf = configuration_data()
+conf.set_quoted('PROJECT_NAME', meson.project_name())
+conf.set_quoted('PROJECT_VERSION', meson.project_version())
+configure_file(output : 'config.h', configuration : conf)
+
 cryptodep = dependency('libcrypto')
 executable('pdf-simple-sign', 'pdf-simple-sign.cpp',
 	install : true,
 	dependencies : cryptodep)
+
+asciidoctor = find_program('asciidoctor')
+foreach page : ['pdf-simple-sign']
+	custom_target('manpage for ' + page,
+		input: page + '.adoc', output: page + '.1',
+		command: [asciidoctor, '-b', 'manpage',
+			'-a', 'release-version=' + meson.project_version(),
+			'@INPUT@', '-o', '@OUTPUT@'],
+		install: true,
+		install_dir: join_paths(get_option('mandir'), 'man1'))
+endforeach

pdf-simple-sign.adoc

@@ -0,0 +1,80 @@
+pdf-simple-sign(1)
+==================
+:doctype: manpage
+:manmanual: pdf-simple-sign Manual
+:mansource: pdf-simple-sign {release-version}
+
+Name
+----
+pdf-simple-sign - a simple PDF signer
+
+Synopsis
+--------
+*pdf-simple-sign* [_OPTION_]... _INPUT.pdf_ _OUTPUT.pdf_ _KEY-PAIR.p12_ _PASSWORD_
+
+Description
+-----------
+'pdf-simple-sign' is a simple PDF signer intended for documents produced by
+the Cairo library, GNU troff, ImageMagick, or similar.  As such, it currently
+comes with some restrictions:
+
+ * the document may not have any forms or signatures already, as they would be
+   overwritten,
+ * the document may not employ cross-reference streams, or must constitute
+   a hybrid-reference file at least.
+
+The key and certificate pair is accepted in the PKCS#12 format.  The _PASSWORD_
+must be supplied on the command line, and may be empty if it is not needed.
+
+The signature is attached to the first page and has no appearance.
+
+If signature data don't fit within the default reservation of 4 kibibytes,
+you might need to adjust it using the *-r* option, or throw out any unnecessary
+intermediate certificates.
+
+Options
+-------
+*-r* _RESERVATION_, *--reservation*=_RESERVATION_::
+  Set aside _RESERVATION_ amount of bytes for the resulting signature.
+  Feel free to try a few values in a loop.  The program itself has no
+  conceptions about the data, so it can't make accurate predictions.
+
+*-h*, *--help*::
+  Display a help message and exit.
+
+*-V*, *--version*::
+  Output version information and exit.
+
+Examples
+--------
+Create a self-signed certificate, make a document containing the current date,
+sign it and verify the attached signature:
+
+ $ openssl req -newkey rsa:2048 -subj /CN=Test -nodes \
+   -keyout key.pem -x509 -addext keyUsage=digitalSignature \
+   -out cert.pem 2>/dev/null
+ $ openssl pkcs12 -inkey key.pem -in cert.pem \
+   -export -passout pass: -out key-pair.p12
+ $ date | groff -T pdf > test.pdf
+ $ pdf-simple-sign test.pdf test.signed.pdf key-pair.p12 ""
+ $ pdfsig test.signed.pdf
+ Digital Signature Info of: test.signed.pdf
+ Signature #1:
+   - Signer Certificate Common Name: Test
+   - Signer full Distinguished Name: CN=Test
+   - Signing Time: Sep 05 2020 19:41:22
+   - Signing Hash Algorithm: SHA-256
+   - Signature Type: adbe.pkcs7.detached
+   - Signed Ranges: [0 - 6522], [14716 - 15243]
+   - Total document signed
+   - Signature Validation: Signature is Valid.
+   - Certificate Validation: Certificate issuer isn't Trusted.
+
+Reporting bugs
+--------------
+Use https://git.janouch.name/p/pdf-simple-sign to report bugs, request features,
+or submit pull requests.
+
+See also
+--------
+*openssl*(1), *pdfsig*(1)

pdf-simple-sign.cpp

@@ -2,7 +2,7 @@
 //
 // pdf-simple-sign: simple PDF signer
 //
-// Copyright (c) 2017, Přemysl Eric Janouch <p@janouch.name>
+// Copyright (c) 2017 - 2020, Přemysl Eric Janouch <p@janouch.name>
 //
 // Permission to use, copy, modify, and/or distribute this software for any
 // purpose with or without fee is hereby granted.
@@ -16,30 +16,33 @@
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 //
 
-#include <cstdio>
 #include <cmath>
+#include <cstdio>
 #undef NDEBUG
 #include <cassert>
 
-#include <vector>
 #include <map>
-#include <regex>
 #include <memory>
+#include <regex>
 #include <set>
+#include <vector>
 
 #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>
+#include <openssl/x509v3.h>
+#include <unistd.h>
+
+#include "config.h"
 
 // -------------------------------------------------------------------------------------------------
 
 using uint = unsigned int;
+using ushort = unsigned short;
 
 static std::string concatenate(const std::vector<std::string>& v, const std::string& delim) {
   std::string res;
@@ -52,7 +55,7 @@ static std::string concatenate(const std::vector<std::string>& v, const std::str
 
 template<typename... Args>
 std::string ssprintf(const std::string& format, Args... args) {
-  size_t size = std::snprintf(nullptr, 0, format.c_str(), args... ) + 1;
+  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);
@@ -259,14 +262,12 @@ static std::string pdf_serialize(const pdf_object& o) {
   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:
-  {
+  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:
-  {
+  case pdf_object::NAME: {
     std::string escaped = "/";
     for (char c : o.string) {
       if (c == '#' || strchr(pdf_lexer::delimiters, c) || strchr(pdf_lexer::whitespace, c))
@@ -276,8 +277,7 @@ static std::string pdf_serialize(const pdf_object& o) {
     }
     return escaped;
   }
-  case pdf_object::STRING:
-  {
+  case pdf_object::STRING: {
     std::string escaped;
     for (char c : o.string) {
       if (c == '\\' || c == '(' || c == ')')
@@ -290,15 +290,13 @@ static std::string pdf_serialize(const pdf_object& o) {
   case pdf_object::E_ARRAY: return "]";
   case pdf_object::B_DICT:  return "<<";
   case pdf_object::E_DICT:  return ">>";
-  case pdf_object::ARRAY:
-  {
+  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:
-  {
+  case pdf_object::DICT: {
     std::string s;
     for (const auto i : o.dict)
       // FIXME the key is also supposed to be escaped by pdf_serialize()
@@ -342,6 +340,9 @@ public:
 
   /// Build the cross-reference table and prepare a new trailer dictionary
   std::string initialize();
+  /// Try to extract the claimed PDF version as a positive decimal number, e.g. 17 for PDF 1.7.
+  /// Returns zero on failure.
+  int version(const pdf_object& root) const;
   /// 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
@@ -366,8 +367,8 @@ pdf_object pdf_updater::parse_obj(pdf_lexer& lex, std::vector<pdf_object>& stack
 
   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)
+  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};
@@ -391,8 +392,8 @@ pdf_object pdf_updater::parse_R(std::vector<pdf_object>& stack) const {
 
   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)
+  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};
@@ -409,8 +410,7 @@ pdf_object pdf_updater::parse(pdf_lexer& lex, std::vector<pdf_object>& stack) co
   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:
-  {
+  case pdf_object::B_ARRAY: {
     std::vector<pdf_object> array;
     while (1) {
       auto object = parse(lex, array);
@@ -422,8 +422,7 @@ pdf_object pdf_updater::parse(pdf_lexer& lex, std::vector<pdf_object>& stack) co
     }
     return array;
   }
-  case pdf_object::B_DICT:
-  {
+  case pdf_object::B_DICT: {
     std::vector<pdf_object> array;
     while (1) {
       auto object = parse(lex, array);
@@ -471,8 +470,8 @@ std::string pdf_updater::load_xref(pdf_lexer& lex, std::set<uint>& loaded_entrie
       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)
+    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;
@@ -481,9 +480,9 @@ std::string pdf_updater::load_xref(pdf_lexer& lex, std::set<uint>& loaded_entrie
       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)
+      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;
@@ -512,7 +511,7 @@ std::string pdf_updater::load_xref(pdf_lexer& lex, std::set<uint>& loaded_entrie
 
 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");
+  static std::regex haystack_re(R"([\s\S]*\sstartxref\s+(\d+)\s+%%EOF)");
   std::string haystack = document.substr(document.length() < 1024 ? 0 : document.length() - 1024);
 
   std::smatch m;
@@ -560,6 +559,25 @@ std::string pdf_updater::initialize() {
   return "";
 }
 
+int pdf_updater::version(const pdf_object& root) const {
+  auto version = root.dict.find("Version");
+  if (version != root.dict.end() && version->second.type == pdf_object::NAME) {
+    const auto& v = version->second.string;
+    if (isdigit(v[0]) && v[1] == '.' && isdigit(v[2]) && !v[3])
+      return (v[0] - '0') * 10 + (v[2] - '0');
+  }
+
+  // We only need to look for the comment roughly within the first kibibyte of the document
+  static std::regex version_re(R"((?:^|[\r\n])%(?:!PS-Adobe-\d\.\d )?PDF-(\d)\.(\d)[\r\n])");
+  std::string haystack = document.substr(0, 1024);
+
+  std::smatch m;
+  if (std::regex_search(haystack, m, version_re, std::regex_constants::match_default))
+    return std::stoul(m.str(1)) * 10 + std::stoul(m.str(2));
+
+  return 0;
+}
+
 pdf_object pdf_updater::get(uint n, uint generation) const {
   if (n >= xref_size)
     return {pdf_object::NIL};
@@ -632,8 +650,8 @@ void pdf_updater::flush_updates() {
   }
 
   trailer["Size"] = {pdf_object::NUMERIC, double(xref_size)};
-  document += "trailer\n" + pdf_serialize(trailer)
-    + ssprintf("\nstartxref\n%zu\n%%%%EOF\n", startxref);
+  document +=
+    "trailer\n" + pdf_serialize(trailer) + ssprintf("\nstartxref\n%zu\n%%%%EOF\n", startxref);
 }
 
 // -------------------------------------------------------------------------------------------------
@@ -675,9 +693,9 @@ static pdf_object pdf_get_first_page(pdf_updater& pdf, uint node_n, uint node_ge
   // 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)
+  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
@@ -715,8 +733,8 @@ static std::string pdf_fill_in_signature(std::string& document, size_t sign_off,
   // 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)) {
+  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;
   }
@@ -741,8 +759,8 @@ static std::string pdf_fill_in_signature(std::string& document, size_t sign_off,
 #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))
+  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)
@@ -752,10 +770,10 @@ static std::string pdf_fill_in_signature(std::string& document, size_t sign_off,
 
   // 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))
+  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
@@ -806,12 +824,10 @@ error:
 /// 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) {
+static std::string pdf_sign(std::string& document, ushort reservation) {
   pdf_updater pdf(document);
   auto err = pdf.initialize();
   if (!err.empty())
@@ -827,7 +843,7 @@ static std::string pdf_sign(std::string& document) {
   // 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, [&]{
+  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 ");
@@ -835,7 +851,7 @@ static std::string pdf_sign(std::string& document) {
     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((sign_len = reservation * 2), '0');
     pdf.document.append("> >>");
 
     // We actually need to exclude the hexstring quotes from signing
@@ -860,7 +876,7 @@ static std::string pdf_sign(std::string& document) {
   }}});
 
   auto sigfield_n = pdf.allocate();
-  pdf.update(sigfield_n, [&]{ pdf.document += pdf_serialize(sigfield); });
+  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)
@@ -869,15 +885,21 @@ static std::string pdf_sign(std::string& document) {
   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)
+  if (annots.type != pdf_object::ARRAY) {
+    // TODO indirectly referenced arrays might not be that hard to support
+    if (annots.type != pdf_object::END)
+      return "unexpected Annots";
+
     annots = {pdf_object::ARRAY};
+  }
   annots.array.emplace_back(pdf_object::REFERENCE, sigfield_n, 0);
-  pdf.update(page.n, [&]{ pdf.document += pdf_serialize(page); });
+  pdf.update(page.n, [&] { pdf.document += pdf_serialize(page); });
 
   // 8.6.1 Interactive Form Dictionary
-  // XXX assuming there are no forms already, overwriting everything
+  if (root.dict.count("AcroForm"))
+    return "the document already contains forms, they would be overwritten";
+
   root.dict["AcroForm"] = {std::map<std::string, pdf_object>{
     {"Fields", {std::vector<pdf_object>{
       {pdf_object::REFERENCE, sigfield_n, 0}
@@ -886,10 +908,10 @@ static std::string pdf_sign(std::string& document) {
   }};
 
   // 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); });
+  if (pdf.version(root) < 16)
+    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,
@@ -918,26 +940,39 @@ static void die(int status, const char* format, ...) {
 
 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);
+  auto usage = [=] {
+    die(1, "Usage: %s [-h] [-r RESERVATION] INPUT-FILENAME OUTPUT-FILENAME PKCS12-PATH PKCS12-PASS",
+        invocation_name);
   };
 
   static struct option opts[] = {
     {"help", no_argument, 0, 'h'},
+    {"version", no_argument, 0, 'V'},
+    {"reservation", required_argument, 0, 'r'},
     {nullptr, 0, 0, 0},
   };
 
+  // Reserved space in bytes for the certificate, digest, encrypted digest, ...
+  long reservation = 4096;
   while (1) {
     int option_index = 0;
-    auto c = getopt_long(argc, const_cast<char* const*>(argv),
-                         "h", opts, &option_index);
+    auto c = getopt_long(argc, const_cast<char* const*>(argv), "hVr:", opts, &option_index);
     if (c == -1)
       break;
 
+    char* end = nullptr;
     switch (c) {
-    case 'h': usage(); break;
-    default: usage();
+    case 'r':
+      errno = 0, reservation = strtol(optarg, &end, 10);
+      if (errno || *end || reservation <= 0 || reservation > USHRT_MAX)
+        die(1, "%s: must be a positive number", optarg);
+      break;
+    case 'V':
+      die(0, "%s", PROJECT_NAME " " PROJECT_VERSION);
+      break;
+    case 'h':
+    default:
+      usage();
     }
   }
 
@@ -964,7 +999,7 @@ int main(int argc, char* argv[]) {
     die(1, "%s: %s", input_path, strerror(errno));
   }
 
-  auto err = pdf_sign(pdf_document);
+  auto err = pdf_sign(pdf_document, ushort(reservation));
   if (!err.empty()) {
     die(2, "Error: %s", err.c_str());
   }

pdf/pdf.go

@@ -1,5 +1,5 @@
 //
-// Copyright (c) 2018, Přemysl Eric Janouch <p@janouch.name>
+// Copyright (c) 2018 - 2020, Přemysl Eric Janouch <p@janouch.name>
 //
 // Permission to use, copy, modify, and/or distribute this software for any
 // purpose with or without fee is hereby granted.
@@ -32,6 +32,7 @@ import (
 	"crypto/ecdsa"
 	"crypto/rsa"
 	"crypto/x509"
+
 	"go.mozilla.org/pkcs7"
 	"golang.org/x/crypto/pkcs12"
 )
@@ -669,7 +670,7 @@ func (u *Updater) loadXref(lex *Lexer, loadedEntries map[uint]struct{}) error {
 
 // -----------------------------------------------------------------------------
 
-var haystackRE = regexp.MustCompile(`(?s:.*)\sstartxref\s+(\d+)\s+%%EOF`)
+var trailerRE = regexp.MustCompile(`(?s:.*)\sstartxref\s+(\d+)\s+%%EOF`)
 
 // NewUpdater initializes an Updater, building the cross-reference table and
 // preparing a new trailer dictionary.
@@ -684,7 +685,7 @@ func NewUpdater(document []byte) (*Updater, error) {
 		haystack = haystack[len(haystack)-1024:]
 	}
 
-	m := haystackRE.FindSubmatch(haystack)
+	m := trailerRE.FindSubmatch(haystack)
 	if m == nil {
 		return nil, errors.New("cannot find startxref")
 	}
@@ -738,6 +739,31 @@ func NewUpdater(document []byte) (*Updater, error) {
 	return u, nil
 }
 
+var versionRE = regexp.MustCompile(
+	`(?:^|[\r\n])%(?:!PS-Adobe-\d\.\d )?PDF-(\d)\.(\d)[\r\n]`)
+
+// Version extracts the claimed PDF version as a positive decimal number,
+// e.g. 17 for PDF 1.7. Returns zero on failure.
+func (u *Updater) Version(root *Object) int {
+	if version, ok := root.Dict["Version"]; ok && version.Kind == Name {
+		if v := version.String; len(v) == 3 && v[1] == '.' &&
+			v[0] >= '0' && v[0] <= '9' && v[2] >= '0' && v[2] <= '9' {
+			return int(v[0]-'0')*10 + int(v[2]-'0')
+		}
+	}
+
+	// We only need to look for the comment roughly within
+	// the first kibibyte of the document.
+	haystack := u.Document
+	if len(haystack) > 1024 {
+		haystack = haystack[:1024]
+	}
+	if m := versionRE.FindSubmatch(haystack); m != nil {
+		return int(m[1][0]-'0')*10 + int(m[2][0]-'0')
+	}
+	return 0
+}
+
 // Get retrieves an object by its number and generation--may return
 // Nil or End with an error.
 //
@@ -827,30 +853,19 @@ func (u *Updater) FlushUpdates() {
 		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)]
+	for i := 0; i < len(updated); {
+		start, stop := updated[i], updated[i]+1
+		for i++; i < len(updated) && updated[i] == stop; i++ {
+			stop++
+		}
+
+		fmt.Fprintf(buf, "%d %d\n", start, stop-start)
+		for ; start < stop; start++ {
+			ref := u.xref[start]
 			if ref.nonfree {
 				fmt.Fprintf(buf, "%010d %05d n \n", ref.offset, ref.generation)
 			} else {
@@ -859,6 +874,13 @@ func (u *Updater) FlushUpdates() {
 		}
 	}
 
+	// 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(updated) == 0 {
+		fmt.Fprintf(buf, "%d %d\n", 0, 0)
+	}
+
 	u.Trailer["Size"] = NewNumeric(float64(u.xrefSize))
 	trailer := NewDict(u.Trailer)
 
@@ -1089,15 +1111,14 @@ func FillInSignature(document []byte, signOff, signLen int,
 // There must be at least one certificate, matching the private key.
 // The certificates must form a chain.
 //
+// A good default for the reservation is around 4096 (the value is in bytes).
+//
 // 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) {
+func Sign(document []byte, key crypto.PrivateKey, certs []*x509.Certificate,
+	reservation int) ([]byte, error) {
 	pdf, err := NewUpdater(document)
 	if err != nil {
 		return nil, err
@@ -1129,7 +1150,7 @@ func Sign(document []byte,
 		buf.WriteString("\n   /Contents <")
 
 		signOff = buf.Len()
-		signLen = 8192 // cert, digest, encrypted digest, ...
+		signLen = reservation * 2 // cert, digest, encrypted digest, ...
 		buf.Write(bytes.Repeat([]byte{'0'}, signLen))
 		buf.WriteString("> >>")
 
@@ -1166,9 +1187,13 @@ func Sign(document []byte,
 		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 {
+		// TODO(p): Indirectly referenced arrays might not be
+		// that hard to support.
+		if annots.Kind != End {
+			return nil, errors.New("unexpected Annots")
+		}
 		annots = NewArray(nil)
 	}
 	annots.Array = append(annots.Array, NewReference(sigfieldN, 0))
@@ -1179,17 +1204,21 @@ func Sign(document []byte,
 	})
 
 	// 8.6.1 Interactive Form Dictionary
-	// XXX: Assuming there are no forms already, overwriting everything.
+	if _, ok := root.Dict["AcroForm"]; ok {
+		return nil, errors.New("the document already contains forms, " +
+			"they would be overwritten")
+	}
+
 	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")
+	if pdf.Version(&root) < 16 {
+		root.Dict["Version"] = NewName("1.6")
+	}
+
 	pdf.Update(rootRef.N, func(buf BytesWriter) {
 		buf.WriteString(root.Serialize())
 	})

test.sh

@@ -0,0 +1,77 @@
+#!/bin/sh -e
+# Test basic functionality of both versions
+# Usage: ./test.sh builddir/pdf-simple-sign cmd/pdf-simple-sign/pdf-simple-sign
+
+log() { echo "`tput sitm`-- $1`tput sgr0`"; }
+die() { echo "`tput bold`-- $1`tput sgr0`"; exit 1; }
+
+# Get rid of old test files
+rm -rf tmp
+mkdir tmp
+
+# Create documents in various tools
+log "Creating source documents"
+inkscape --pipe --export-filename=tmp/cairo.pdf <<'EOF' 2>/dev/null || :
+<svg xmlns="http://www.w3.org/2000/svg"><text x="5" y="10">Hello</text></svg>
+EOF
+
+date | tee tmp/lowriter.txt | groff -T pdf > tmp/groff.pdf || :
+lowriter --convert-to pdf tmp/lowriter.txt --outdir tmp >/dev/null || :
+convert rose: tmp/imagemagick.pdf || :
+
+# Create a root CA certificate pair
+log "Creating certificates"
+openssl req -newkey rsa:2048 -subj "/CN=Test CA" -nodes \
+	-keyout tmp/ca.key.pem -x509 -out tmp/ca.cert.pem 2>/dev/null
+
+# Create a private NSS database and insert our test CA there
+rm -rf tmp/nssdir
+mkdir tmp/nssdir
+certutil -N --empty-password -d sql:tmp/nssdir
+certutil -d sql:tmp/nssdir -A -n root -t ,C, -a -i tmp/ca.cert.pem
+
+# Create a leaf certificate pair
+cat > tmp/cert.cfg <<'EOF'
+[smime]
+basicConstraints = CA:FALSE
+keyUsage = digitalSignature
+extendedKeyUsage = emailProtection
+nsCertType = email
+EOF
+
+openssl req -newkey rsa:2048 -subj "/CN=Test Leaf" -nodes \
+	-keyout tmp/key.pem -out tmp/cert.csr 2>/dev/null
+openssl x509 -req -in tmp/cert.csr -out tmp/cert.pem \
+	-CA tmp/ca.cert.pem -CAkey tmp/ca.key.pem -set_serial 1 \
+	-extensions smime -extfile tmp/cert.cfg 2>/dev/null
+openssl verify -CAfile tmp/ca.cert.pem tmp/cert.pem >/dev/null
+openssl pkcs12 -inkey tmp/key.pem -in tmp/cert.pem \
+	-export -passout pass: -out tmp/key-pair.p12
+
+for tool in "$@"; do
+	rm -f tmp/*.signed.pdf
+	for source in tmp/*.pdf; do
+		log "Testing $tool with $source"
+		result=${source%.pdf}.signed.pdf
+		$tool "$source" "$result" tmp/key-pair.p12 ""
+		pdfsig -nssdir sql:tmp/nssdir "$result" | grep Validation
+	done
+
+	log "Testing $tool for expected failures"
+	$tool "$result" "$source.fail.pdf" tmp/key-pair.p12 "" \
+		&& die "Double signing shouldn't succeed"
+	$tool -r 1 "$source" "$source.fail.pdf" tmp/key-pair.p12 "" \
+		&& die "Too low reservations shouldn't succeed"
+
+	# Our generators do not use PDF versions higher than 1.5
+	log "Testing $tool for version detection"
+	grep -q "/Version /1.6" "$result" \
+		|| die "Version detection seems to misbehave (no upgrade)"
+
+	sed '1s/%PDF-1../%PDF-1.7/' "$source" > "$source.alt"
+	$tool "$source.alt" "$result.alt" tmp/key-pair.p12 ""
+	grep -q "/Version /1.6" "$result.alt" \
+		&& die "Version detection seems to misbehave (downgraded)"
+done
+
+log "OK"