Certificate Transparency: Code for unpacking EV cert hashes whitelist

net/cert/ct_ev_whitelist.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "net/cert/ct_ev_whitelist.h"
+
+#include <set>
+
+#include "base/big_endian.h"
+#include "base/file_util.h"
+#include "base/lazy_instance.h"
+#include "base/logging.h"
+
+namespace net {
+
+namespace ct {
+
+namespace {
+base::LazyInstance<std::set<std::string> >::Leaky g_current_ev_whitelist =
+    LAZY_INSTANCE_INITIALIZER;
+
+const uint8 kCertHashLengthBits = 64;     // 8 bytes
+const uint64 kGolombMParameterBits = 47;  // 2^47
+}  // namespace
+
+namespace internal {
+
+BitStreamReader::BitStreamReader(const char* source, size_t length)
+    : source_(source), length_(length), current_byte_(0), current_bit_(7) {
+}
+
+bool BitStreamReader::ReadUnaryEncoding(uint64* out) {
+  uint64 res = 0;
+  if (BitsLeft() == 0)
+    return false;
+
+  uint8 b = 1;
+  while ((BitsLeft() > 0) && b) {
+    b = ReadBit();
+    res += b;
+  }

[wtc, 2014/09/02 23:03:02]

Nit: we should be able to simplify this (lines 37-41) as follows:

  while (BitsLeft() > 0 && ReadBit())
    res++;

But this code is very subtle, so I may have missed something.

[Eran Messeri, 2014/09/03 09:38:50]

Done.

+
+  *out = res;
+  return true;
+}
+
+bool BitStreamReader::ReadBits(uint8 num_bits, uint64* out) {
+  if (num_bits > 64)
+    return false;
+
+  if (BitsLeft() < num_bits)
+    return false;
+
+  uint64 res = 0;
+  for (uint8 i = 0; i < num_bits; ++i)
+    res |= (static_cast<uint64>(ReadBit()) << (num_bits - (i + 1)));
+
+  *out = res;
+  return true;
+}
+
+uint64 BitStreamReader::BitsLeft() const {
+  if (current_byte_ == length_)
+    return 0;
+  return (length_ - (current_byte_ + 1)) * 8 + current_bit_ + 1;
+}
+
+uint8 BitStreamReader::ReadBit() {
+  DCHECK(BitsLeft() > 0);

[wtc, 2014/09/02 23:03:02]

Nit: you can use DCHECK_GT.

[Eran Messeri, 2014/09/03 09:38:50]

Done.

+  DCHECK(current_bit_ < 8 && current_bit_ >= 0);
+  uint8 res = (source_[current_byte_] & (1 << current_bit_)) >> current_bit_;
+  current_bit_--;
+  if (current_bit_ < 0) {
+    current_byte_ += 1;

[wtc, 2014/09/02 23:03:02]

Nit: use ++.

[Eran Messeri, 2014/09/03 09:38:50]

Done.

+    current_bit_ = 7;
+  }
+
+  return res;
+}
+
+bool UncompressEVWhitelist(const std::string& compressed_whitelist,
+                           std::set<std::string>* uncompressed_list) {
+  BitStreamReader reader(compressed_whitelist.data(),
+                         compressed_whitelist.size());
+  std::set<std::string> result;
+
+  VLOG(1) << "Uncompressing EV whitelist of size "
+          << compressed_whitelist.size();
+  uint64 curr_hash(0);
+  if (!reader.ReadBits(kCertHashLengthBits, &curr_hash)) {
+    VLOG(1) << "Failed reading first hash.";
+    return false;
+  }
+  char hash_bytes[8];
+  base::WriteBigEndian(hash_bytes, curr_hash);
+  result.insert(std::string(hash_bytes, 8));
+  uint64 M = static_cast<uint64>(1) << kGolombMParameterBits;

[wtc, 2014/09/02 23:03:02]

What I meant is to declare this as either "const" or "static const":

  static const uint64 M = static_cast<uint64>(1) << kGolombMParameterBits;

We can even name it "kM" to suggest that it is a constant. But it should be OK
to make an exception to the constant naming rule in this case.

[Eran Messeri, 2014/09/03 09:38:50]

Done.

+
+  while (reader.BitsLeft() > kGolombMParameterBits) {
+    uint64 read_prefix = 0;
+    if (!reader.ReadUnaryEncoding(&read_prefix)) {
+      VLOG(1) << "Failed reading unary-encoded prefix.";
+      return false;
+    }
+
+    uint64 r = 0;
+    if (!reader.ReadBits(kGolombMParameterBits, &r)) {
+      VLOG(1) << "Failed reading " << kGolombMParameterBits << " bits.";
+      return false;
+    }
+
+    uint64 curr_diff = read_prefix * M + r;
+
+    base::WriteBigEndian(hash_bytes, curr_hash + curr_diff);
+    result.insert(std::string(hash_bytes, 8));
+    curr_hash += curr_diff;

[wtc, 2014/09/02 23:03:01]

What I meant is to replace lines 113-116 with the following:

  curr_hash += curr_diff;
  base::WriteBigEndian(hash_bytes, curr_hash);
  result.insert(std::string(hash_bytes, 8));

[Eran Messeri, 2014/09/03 09:38:50]

Done, good point.

+  }
+
+  uncompressed_list->swap(result);
+  return true;
+}
+
+void SetEVWhitelistData(std::set<std::string>& ev_whitelist) {
+  g_current_ev_whitelist.Get().swap(ev_whitelist);
+}
+
+}  // namespace internal
+
+void SetEVWhitelistFromFile(const base::FilePath& compressed_whitelist_file) {
+  VLOG(1) << "Setting EV whitelist from file: "
+          << compressed_whitelist_file.value();
+  std::string compressed_list;
+  if (!base::ReadFileToString(compressed_whitelist_file, &compressed_list)) {
+    VLOG(1) << "Failed reading from " << compressed_whitelist_file.value();
+    return;
+  }
+
+  std::set<std::string> uncompressed_list;
+  if (!internal::UncompressEVWhitelist(compressed_list, &uncompressed_list)) {
+    VLOG(1) << "Failed uncompressing.";
+    return;
+  }
+  VLOG(1) << "Uncompressing succeeded, hashes: " << uncompressed_list.size();
+
+  internal::SetEVWhitelistData(uncompressed_list);
+}
+
+bool IsCertificateHashInWhitelist(const std::string& certificate_hash) {
+  const std::set<std::string>& current_ev_whitelist =
+      g_current_ev_whitelist.Get();
+  return current_ev_whitelist.find(certificate_hash) !=
+         current_ev_whitelist.end();
+}
+
+}  // namespace ct
+
+}  // namespace net