Certificate Transparency: Parse Signed Tree Heads and validate them

net/cert/ct_log_response_parser.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_log_response_parser.h"
+
+#include "base/base64.h"
+#include "base/json/json_reader.h"
+#include "base/json/json_value_converter.h"
+#include "base/logging.h"
+#include "base/time/time.h"
+#include "base/values.h"
+#include "net/cert/ct_serialization.h"
+#include "net/cert/signed_tree_head.h"
+
+namespace net {
+
+namespace ct {
+
+namespace {
+
+// Structure for making JSON decoding easier. The string fields
+// are base64-encoded so will require further decoding.
+struct JsonSignedTreeHead {
+  int tree_size;
+  double timestamp;
+  std::string sha256_root_hash;
+  std::string tree_head_signature;
+
+  static void RegisterJSONConverter(
+      base::JSONValueConverter<JsonSignedTreeHead>* converted);
+};
+
+void JsonSignedTreeHead::RegisterJSONConverter(
+    base::JSONValueConverter<JsonSignedTreeHead>* converter) {
+  converter->RegisterIntField("tree_size", &JsonSignedTreeHead::tree_size);
+  converter->RegisterDoubleField("timestamp", &JsonSignedTreeHead::timestamp);
+  converter->RegisterStringField("sha256_root_hash",
+                                 &JsonSignedTreeHead::sha256_root_hash);
+  converter->RegisterStringField("tree_head_signature",
+                                 &JsonSignedTreeHead::tree_head_signature);

[Ryan Sleevi, 2014/04/25 23:33:19]

SECURITY: Should there be any customer converters attached to these, so that you
can validate their well-formedness?

tree_size or timestamp being negative numbers immediately springs to mind as
being potential security issues

[Eran Messeri, 2014/04/29 15:22:24]

Good catch - I've added custom converters for the sha256_root_hash and
tree_head_signature. Since I'm further processing this struct later on I've
opted for checks in the FillSignedTreeHead function below for the timestamp and
tree_size.
I could use custom converters for these fields as well but that would require
manually converting the StringPiece to the Int/Double value before doing any
validation checks on them.

+}
+
+}  // namespace
+
+bool FillSignedTreeHead(const base::StringPiece& json_signed_tree_head,
+                        SignedTreeHead* signed_tree_head) {
+  base::JSONReader json_reader;
+  scoped_ptr<base::Value> json(json_reader.Read(json_signed_tree_head));
+  if (json.get() == NULL) {
+    DVLOG(1) << "Empty Signed Tree Head JSON.";
+    return false;
+  }
+
+  JsonSignedTreeHead parsed_sth;
+  base::JSONValueConverter<JsonSignedTreeHead> converter;
+  if (!converter.Convert(*json.get(), &parsed_sth)) {
+    DVLOG(1) << "Invalid Signed Tree Head JSON.";
+    return false;
+  }
+
+  std::string decoded_root_hash;
+  if (!base::Base64Decode(parsed_sth.sha256_root_hash, &decoded_root_hash)) {
+    DVLOG(1) << "Failed decoding sha256_root_hash";
+    return false;
+  }
+
+  if (decoded_root_hash.length() != kSthRootHashLength) {
+    DVLOG(1) << "sha256_root_hash is expected to be 32 bytes, but is "
+             << decoded_root_hash.length() << " bytes.";
+    return false;
+  }
+
+  std::string decoded_signature;
+  if (!base::Base64Decode(parsed_sth.tree_head_signature, &decoded_signature)) {
+    DVLOG(1) << "Failed decoding tree_head_signature";
+    return false;
+  }
+
+  // It's OK to decode the signature directly into the output as this is the
+  // last check before filling in all other fields.
+  // If decoding of the DigitallySigned part fails, the STH will not contain
+  // partial data. If it passes, all other fields will be filled as well.
+  base::StringPiece sp(decoded_signature);
+  if (!DecodeDigitallySigned(&sp, &(signed_tree_head->signature))) {
+    DVLOG(1) << "Failed decoding signature to DigitallySigned";
+    return false;
+  }
+
+  signed_tree_head->version = SignedTreeHead::V1;
+  signed_tree_head->tree_size = parsed_sth.tree_size;
+  signed_tree_head->timestamp =
+      base::Time::UnixEpoch() +
+      base::TimeDelta::FromMilliseconds(parsed_sth.timestamp);
+  memcpy(signed_tree_head->sha256_root_hash,
+         decoded_root_hash.c_str(),
+         kSthRootHashLength);
+  return true;
+}
+
+}  // namespace ct
+
+}  // namespace net