Resource Path Obfuscation

This CL allows aapt2 to obfuscate resource paths within the output apk
and move resources to shorter obfuscated paths. This reduces apk size
when there is a large number of resources since the path metadata exists
in 4 places in the apk.

This CL adds two arguments to aapt2, one to enable resource path
obfuscation and one to point to a path to output the path map to (for
later debugging).

Test: make aapt2_tests
Bug: b/75965637

Change-Id: I9cacafe1d17800d673566b2d61b0b88f3fb8d60c

1 files changed, 112 insertions, 0 deletions

diff --git a/tools/aapt2/optimize/ResourcePathShortener.cpp b/tools/aapt2/optimize/ResourcePathShortener.cpp
new file mode 100644
index 000000000000..c5df3dd00db9
--- /dev/null
+++ b/tools/aapt2/optimize/ResourcePathShortener.cpp
@@ -0,0 +1,112 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "optimize/ResourcePathShortener.h"
+
+#include <math.h>
+#include <unordered_set>
+
+#include "androidfw/StringPiece.h"
+
+#include "ResourceTable.h"
+#include "ValueVisitor.h"
+
+
+static const std::string base64_chars =
+             "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+             "abcdefghijklmnopqrstuvwxyz"
+             "0123456789-_";
+
+namespace aapt {
+
+ResourcePathShortener::ResourcePathShortener(
+    std::map<std::string, std::string>& path_map_out)
+    : path_map_(path_map_out) {
+}
+
+std::string ShortenFileName(const android::StringPiece& file_path, int output_length) {
+  std::size_t hash_num = std::hash<android::StringPiece>{}(file_path);
+  std::string result = "";
+  // Convert to (modified) base64 so that it is a proper file path.
+  for (int i = 0; i < output_length; i++) {
+    uint8_t sextet = hash_num & 0x3f;
+    hash_num >>= 6;
+    result += base64_chars[sextet];
+  }
+  return result;
+}
+
+
+// Calculate the optimal hash length such that an average of 10% of resources
+// collide in their shortened path.
+// Reference: http://matt.might.net/articles/counting-hash-collisions/
+int OptimalShortenedLength(int num_resources) {
+  int num_chars = 2;
+  double N = 64*64; // hash space when hash is 2 chars long
+  double max_collisions = num_resources * 0.1;
+  while (num_resources - N + N * pow((N - 1) / N, num_resources) > max_collisions) {
+    N *= 64;
+    num_chars++;
+  }
+  return num_chars;
+}
+
+std::string GetShortenedPath(const android::StringPiece& shortened_filename,
+    const android::StringPiece& extension, int collision_count) {
+  std::string shortened_path = "res/" + shortened_filename.to_string();
+  if (collision_count > 0) {
+    shortened_path += std::to_string(collision_count);
+  }
+  shortened_path += extension;
+  return shortened_path;
+}
+
+bool ResourcePathShortener::Consume(IAaptContext* context, ResourceTable* table) {
+  // used to detect collisions
+  std::unordered_set<std::string> shortened_paths;
+  std::unordered_set<FileReference*> file_refs;
+  for (auto& package : table->packages) {
+    for (auto& type : package->types) {
+      for (auto& entry : type->entries) {
+        for (auto& config_value : entry->values) {
+          FileReference* file_ref = ValueCast<FileReference>(config_value->value.get());
+          if (file_ref) {
+            file_refs.insert(file_ref);
+          }
+        }
+      }
+    }
+  }
+  int num_chars = OptimalShortenedLength(file_refs.size());
+  for (auto& file_ref : file_refs) {
+    android::StringPiece res_subdir, actual_filename, extension;
+    util::ExtractResFilePathParts(*file_ref->path, &res_subdir, &actual_filename, &extension);
+
+    std::string shortened_filename = ShortenFileName(*file_ref->path, num_chars);
+    int collision_count = 0;
+    std::string shortened_path = GetShortenedPath(shortened_filename, extension, collision_count);
+    while (shortened_paths.find(shortened_path) != shortened_paths.end()) {
+      collision_count++;
+      shortened_path = GetShortenedPath(shortened_filename, extension, collision_count);
+    }
+    shortened_paths.insert(shortened_path);
+    path_map_.insert({*file_ref->path, shortened_path});
+    file_ref->path = table->string_pool.MakeRef(shortened_path, file_ref->path.GetContext());
+  }
+  return true;
+}
+
+}  // namespace aapt