Bump version, update NEWS

And fix that --reservation was missing from the optstring.

LICENSE

@@ -1,4 +1,4 @@
-Copyright (c) 2017, Přemysl 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 +1,19 @@
+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 +1,20 @@
 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:
-
- * 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.
+'pdf-simple-sign' is a simple PDF signer intended for documents produced by
+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.
+
 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
@@ -30,10 +23,9 @@ Runtime dependencies: libcrypto (OpenSSL 1.1 API)
  $ cd builddir
  $ ninja
 
-Usage
------
+In addition to the C++ version, also included is a native Go port:
 
- $ ./pdf-simple-sign document.pdf document.signed.pdf KeyAndCerts.p12 password
+ $ go get janouch.name/pdf-simple-sign/cmd/pdf-simple-sign
 
 Contributing and Support
 ------------------------

cmd/pdf-simple-sign/main.go

@@ -0,0 +1,76 @@
+//
+// 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.
+//
+// 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"
+	"os"
+
+	"janouch.name/pdf-simple-sign/pdf"
+)
+
+// #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] [-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()
+	if flag.NArg() != 4 {
+		usage()
+	}
+
+	inputPath, outputPath := flag.Arg(0), flag.Arg(1)
+	doc, 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 doc, err = pdf.Sign(doc, key, certs, *reservation); err != nil {
+		die(4, "error: %s", err)
+	}
+	if err = ioutil.WriteFile(outputPath, doc, 0666); err != nil {
+		die(5, "%s", err)
+	}
+}

go.mod

@@ -0,0 +1,8 @@
+module janouch.name/pdf-simple-sign
+
+go 1.14
+
+require (
+	go.mozilla.org/pkcs7 v0.0.0-20200128120323-432b2356ecb1
+	golang.org/x/crypto v0.0.0-20200728195943-123391ffb6de
+)

go.sum

@@ -0,0 +1,13 @@
+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=
+golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=

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')
+
+conf = configuration_data()
+conf.set('PROJECT_NAME', '"' + meson.project_name() + '"')
+conf.set('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 +1,79 @@
+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 -out cert.pem 2>/dev/null
+ $ openssl pkcs12 -inkey key.pem -in cert.pem \
+   -export -passout pass:test -out key-cert.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 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.
@@ -37,9 +37,12 @@
 #include <openssl/x509v3.h>
 #include <openssl/pkcs12.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;
@@ -136,12 +139,11 @@ struct pdf_lexer {
           if (eat_newline(ch))
             continue;
           std::string octal;
-          if (*p && strchr(oct_alphabet, *p)) octal += *p++;
-          if (*p && strchr(oct_alphabet, *p)) octal += *p++;
-          if (*p && strchr(oct_alphabet, *p)) octal += *p++;
-          if (!octal.empty()) {
-            value += char(std::stoi(octal, nullptr, 8));
-            continue;
+          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);
           }
         }
       }
@@ -162,6 +164,7 @@ struct pdf_lexer {
         buf.clear();
       }
     }
+    p++;
     if (!buf.empty()) value += char(std::stoi(buf + '0', nullptr, 16));
     return {pdf_object::STRING, value};
   }
@@ -257,7 +260,7 @@ struct pdf_lexer {
 static std::string pdf_serialize(const pdf_object& o) {
   switch (o.type) {
   case pdf_object::NL:      return "\n";
-  case pdf_object::NIL:     return "nil";
+  case pdf_object::NIL:     return "null";
   case pdf_object::BOOL:    return o.number ? "true" : "false";
   case pdf_object::NUMERIC:
   {
@@ -301,6 +304,7 @@ static std::string pdf_serialize(const pdf_object& o) {
   {
     std::string s;
     for (const auto i : o.dict)
+      // FIXME the key is also supposed to be escaped by pdf_serialize()
       s += " /" + i.first + " " + pdf_serialize(i.second);
     return "<<" + s + " >>";
   }
@@ -341,6 +345,9 @@ 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
@@ -353,6 +360,12 @@ public:
 
 // -------------------------------------------------------------------------------------------------
 
+/// If the object is an error, forward its message, otherwise return err.
+static std::string pdf_error(const pdf_object& o, const char* err) {
+  if (o.type != pdf_object::END || o.string.empty()) return err;
+  return o.string;
+}
+
 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"};
@@ -370,7 +383,7 @@ pdf_object pdf_updater::parse_obj(pdf_lexer& lex, std::vector<pdf_object>& stack
   while (1) {
     auto object = parse(lex, obj.array);
     if (object.type == pdf_object::END)
-      return {pdf_object::END, "object doesn't end"};
+      return {pdf_object::END, pdf_error(object, "object doesn't end")};
     if (object.type == pdf_object::KEYWORD && object.string == "endobj")
       break;
     obj.array.push_back(std::move(object));
@@ -408,7 +421,7 @@ pdf_object pdf_updater::parse(pdf_lexer& lex, std::vector<pdf_object>& stack) co
     while (1) {
       auto object = parse(lex, array);
       if (object.type == pdf_object::END)
-        return {pdf_object::END, "array doesn't end"};
+        return {pdf_object::END, pdf_error(object, "array doesn't end")};
       if (object.type == pdf_object::E_ARRAY)
         break;
       array.push_back(std::move(object));
@@ -421,7 +434,7 @@ pdf_object pdf_updater::parse(pdf_lexer& lex, std::vector<pdf_object>& stack) co
     while (1) {
       auto object = parse(lex, array);
       if (object.type == pdf_object::END)
-        return {pdf_object::END, "dictionary doesn't end"};
+        return {pdf_object::END, pdf_error(object, "dictionary doesn't end")};
       if (object.type == pdf_object::E_DICT)
         break;
       array.push_back(std::move(object));
@@ -459,7 +472,7 @@ std::string pdf_updater::load_xref(pdf_lexer& lex, std::set<uint>& loaded_entrie
   while (1) {
     auto object = parse(lex, throwaway_stack);
     if (object.type == pdf_object::END)
-      return "unexpected EOF while looking for the trailer";
+      return pdf_error(object, "unexpected EOF while looking for the trailer");
     if (object.type == pdf_object::KEYWORD && object.string == "trailer")
       break;
 
@@ -505,7 +518,7 @@ 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;
@@ -529,7 +542,7 @@ std::string pdf_updater::initialize() {
 
     auto trailer = parse(lex, throwaway_stack);
     if (trailer.type != pdf_object::DICT)
-      return "invalid trailer dictionary";
+      return pdf_error(trailer, "invalid trailer dictionary");
     if (loaded_xrefs.empty())
       this->trailer = trailer.dict;
     loaded_xrefs.insert(xref_offset);
@@ -537,6 +550,7 @@ std::string pdf_updater::initialize() {
     const auto prev_offset = trailer.dict.find("Prev");
     if (prev_offset == trailer.dict.end())
       break;
+    // FIXME we don't check for size_t over or underflow
     if (!prev_offset->second.is_integer())
       return "invalid Prev offset";
     xref_offset = prev_offset->second.number;
@@ -552,11 +566,30 @@ 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};
 
-  auto& ref = xref[n];
+  const auto& ref = xref[n];
   if (ref.free || ref.generation != generation || ref.offset >= document.length())
     return {pdf_object::NIL};
 
@@ -798,12 +831,10 @@ 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 +858,7 @@ 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
@@ -861,15 +892,21 @@ 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); });
 
   // 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}
@@ -878,9 +915,9 @@ 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"};
+  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();
 
@@ -911,25 +948,39 @@ 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",
+    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);
+                         "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();
     }
   }
 
@@ -956,7 +1007,7 @@ 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());
   }