Simple PDF signer
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// vim: set sw=2 ts=2 sts=2 et tw=100:
//
// pdf-simple-sign: simple PDF signer
//
// 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.
//
// 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>
#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;
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();
}
}
p++;
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)
// FIXME the key is also supposed to be escaped by pdf_serialize()
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();
/// 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
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();
};
// -------------------------------------------------------------------------------------------------
/// 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"};
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, pdf_error(object, "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, pdf_error(object, "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, pdf_error(object, "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 pdf_error(object, "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(R"([\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 pdf_error(trailer, "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;
// 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;
}
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 "";
}
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};
const 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.
///
/// 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, ushort reservation) {
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 = reservation * 2), '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";
auto& annots = page.dict["Annots"];
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
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}
}}},
{"SigFlags", {pdf_object::NUMERIC, 3 /* SignaturesExist | AppendOnly */}}
}};
// Upgrade the document version for SHA-256 etc.
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,
// 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] [-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),
"hVr:", opts, &option_index);
if (c == -1)
break;
char* end = nullptr;
switch (c) {
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();
}
}
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, ushort(reservation));
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;
}