signature.cpp

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/*
 * This file is part of OpenTTD.
 * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
 * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
 */
 
#include "stdafx.h"
 
#include "signature.h"
 
#include "debug.h"
#include "fileio_func.h"
#include "string_func.h"
 
#include "3rdparty/monocypher/monocypher.h"
#include "3rdparty/monocypher/monocypher-ed25519.h"
#include "3rdparty/nlohmann/json.hpp"
 
#include "safeguards.h"
 
static const std::initializer_list<std::array<uint8_t, 32>> _public_keys_v1 = {
  /* 2024-01-20 - Public key for Social Integration Plugins. */
  { 0xed, 0x5d, 0x57, 0x47, 0x21, 0x99, 0x8b, 0x02, 0xdf, 0x6e, 0x3d, 0x69, 0xe1, 0x87, 0xca, 0xd0, 0x0e, 0x88, 0xc3, 0xe2, 0xb2, 0xa6, 0x7b, 0xc0, 0x42, 0xc8, 0xd6, 0x4b, 0x65, 0xe6, 0x48, 0xf7 },
};
 
static std::string CalculateHashV1(const std::string &filename)
{
  auto f = FioFOpenFile(filename, "rb", NO_DIRECTORY);
  if (!f.has_value()) return {};
 
  std::array<uint8_t, 32> digest;
  crypto_blake2b_ctx ctx;
  crypto_blake2b_init(&ctx, digest.size());
 
  while (!feof(*f)) {
    std::array<uint8_t, 1024> buf;
    size_t len = fread(buf.data(), 1, buf.size(), *f);
 
    crypto_blake2b_update(&ctx, buf.data(), len);
  }
 
  crypto_blake2b_final(&ctx, digest.data());
  return FormatArrayAsHex(digest);
}
 
static bool ValidateChecksum(const std::string &filename, const std::string &checksum)
{
  /* Checksums are "<version>$<hash>". Split out the version. */
  auto pos = checksum.find('$');
  assert(pos != std::string::npos); // Already validated by ValidateSchema().
  const std::string version = checksum.substr(0, pos);
  const std::string hash = checksum.substr(pos + 1);
 
  /* Calculate the checksum over the file. */
  std::string calculated_hash;
  if (version == "1") {
    calculated_hash = CalculateHashV1(filename);
  } else {
    Debug(misc, 0, "Failed to validate signature: unknown checksum version: {}", filename);
    return false;
  }
 
  /* Validate the checksum is the same. */
  if (calculated_hash.empty()) {
    Debug(misc, 0, "Failed to validate signature: couldn't calculate checksum for: {}", filename);
    return false;
  }
  if (calculated_hash != hash) {
    Debug(misc, 0, "Failed to validate signature: checksum mismatch for: {}", filename);
    return false;
  }
 
  return true;
}
 
static bool ValidateSignature(const std::string &signature, const nlohmann::json &files, const std::string &filename)
{
  /* Signatures are "<version>$<signature>". Split out the version. */
  auto pos = signature.find('$');
  assert(pos != std::string::npos); // Already validated by ValidateSchema().
  const std::string version = signature.substr(0, pos);
  const std::string sig_value = signature.substr(pos + 1);
 
  /* Create the message we are going to validate. */
  std::string message = files.dump(-1);
 
  /* Validate the signature. */
  if (version == "1") {
    std::array<uint8_t, 64> sig;
    if (sig_value.size() != 128 || !ConvertHexToBytes(sig_value, sig)) {
      Debug(misc, 0, "Failed to validate signature: invalid signature: {}", filename);
      return false;
    }
 
    for (auto &pk_value : _public_keys_v1) {
      /* Check if the message is valid with this public key. */
      auto res = crypto_ed25519_check(sig.data(), pk_value.data(), reinterpret_cast<uint8_t *>(message.data()), message.size());
      if (res == 0) {
        return true;
      }
    }
 
    Debug(misc, 0, "Failed to validate signature: signature validation failed: {}", filename);
    return false;
  } else {
    Debug(misc, 0, "Failed to validate signature: unknown signature version: {}", filename);
    return false;
  }
 
  return true;
}
 
static bool ValidateSchema(const nlohmann::json &signatures, const std::string &filename)
{
  if (signatures["files"].is_null()) {
    Debug(misc, 0, "Failed to validate signature: no files found: {}", filename);
    return false;
  }
 
  if (signatures["signature"].is_null()) {
    Debug(misc, 0, "Failed to validate signature: no signature found: {}", filename);
    return false;
  }
 
  for (auto &signature : signatures["files"]) {
    if (signature["filename"].is_null() || signature["checksum"].is_null()) {
      Debug(misc, 0, "Failed to validate signature: invalid entry in files: {}", filename);
      return false;
    }
 
    const std::string sig_filename = signature["filename"];
    const std::string sig_checksum = signature["checksum"];
 
    if (sig_filename.empty() || sig_checksum.empty()) {
      Debug(misc, 0, "Failed to validate signature: invalid entry in files: {}", filename);
      return false;
    }
 
    auto pos = sig_checksum.find('$');
    if (pos == std::string::npos) {
      Debug(misc, 0, "Failed to validate signature: invalid checksum format: {}", filename);
      return false;
    }
  }
 
  const std::string signature = signatures["signature"];
  auto pos = signature.find('$');
  if (pos == std::string::npos) {
    Debug(misc, 0, "Failed to validate signature: invalid signature format: {}", filename);
    return false;
  }
 
  return true;
}
 
static bool _ValidateSignatureFile(const std::string &filename)
{
  size_t filesize;
  auto f = FioFOpenFile(filename, "rb", NO_DIRECTORY, &filesize);
  if (!f.has_value()) {
    Debug(misc, 0, "Failed to validate signature: file not found: {}", filename);
    return false;
  }
 
  std::string text(filesize, '\0');
  size_t len = fread(text.data(), filesize, 1, *f);
  if (len != 1) {
    Debug(misc, 0, "Failed to validate signature: failed to read file: {}", filename);
    return false;
  }
 
  nlohmann::json signatures;
  try {
    signatures = nlohmann::json::parse(text);
  } catch (nlohmann::json::exception &) {
    Debug(misc, 0, "Failed to validate signature: not a valid JSON file: {}", filename);
    return false;
  }
 
  /*
   * The JSON file should look like:
   *
   *   {
   *     "files": [
   *       {
   *         "checksum": "version$hash"
   *         "filename": "filename",
   *       },
   *       ...
   *     ],
   *     "signature": "version$signature"
   *   }
   *
   * The signature is a signed message of the content of "files", dumped as
   * JSON without spaces / newlines, keys in the order as indicated above.
   */
 
  if (!ValidateSchema(signatures, filename)) {
    return false;
  }
 
  if (!ValidateSignature(signatures["signature"], signatures["files"], filename)) {
    return false;
  }
 
  std::string dirname = FS2OTTD(std::filesystem::path(OTTD2FS(filename)).parent_path());
 
  for (auto &signature : signatures["files"]) {
    const std::string sig_filename = dirname + PATHSEPCHAR + signature["filename"].get<std::string>();
    const std::string sig_checksum = signature["checksum"];
 
    if (!ValidateChecksum(sig_filename, sig_checksum)) {
      return false;
    }
  }
 
  return true;
}
 
bool ValidateSignatureFile(const std::string &filename)
{
  auto res = _ValidateSignatureFile(filename);;
#if defined(ALLOW_INVALID_SIGNATURE)
  (void)res; // Ignore the result.
  return true;
#else
  return res;
#endif
}