[CacheStorage] Pad and bin opaque resource sizes.

content/browser/cache_storage/README.md

 # Architecture (as of July 29th 2016)
-This document descibes the browser-process implementation of the [Cache
+This document describes the browser-process implementation of the [Cache
 Storage specification](
 https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html).
 
 ## Major Classes and Ownership
 ### Ownership
 Where '=>' represents ownership, '->' is a reference, and '~>' is a weak
 reference.
 
 ##### `CacheStorageContextImpl`=>`CacheStorageManager`=>`CacheStorage`=>`CacheStorageCache`
 * A `CacheStorageManager` can own multiple `CacheStorage` objects.
@@ skipping 132 matching lines @@
 `CacheStorage` as well. This has happened in the past (`Cache::Put` called
 `QuotaManager` to determine how much room was available, which in turn called
 `Cache::Size`). Be careful to avoid situations in which one operation triggers
 a dependency on another operation from the same scheduler.
 
 At the end of an operation, the scheduler needs to be kicked to start the next
 operation. The idiom for this in CacheStorage/ is to wrap the operation's
 callback with a function that will run the callback as well as advance the
 scheduler. So long as the operation runs its wrapped callback the scheduler
 will advance.
+
+## Opaque Resource Size Obfuscation
+Applications can cache cross-origin resources as per
+[Cross-Origin Resources and CORS](https://www.w3.org/TR/service-workers-1/#cross-origin-resources).
+Opaque responses are also also cached, but in order to prevent "leaking" the size

[jkarlin, 2017/08/11 15:04:37]

s/also also/also/

[cmumford, 2017/08/11 22:06:33]

Done.

+of opaque responses their sizes are obfuscated. Random padding is added to the
+actual size making it difficult for an attacker to ascertain the actual resource
+size via quota APIs.
+
+When Chromium starts, a new random padding key is generated and used
+for all new caches created. This key is used by each cache to calculate padding
+for opaque resources. Each cache's key is persisted to disk in the cache index file
+
+Each cache maintains the total padding for all opaque resources within the
+cache. This padding is added to the actual resource size when reporting sizes
+to the quota manager.
+
+The padding algorithm version is also written to each cache allowing for it
+to be changed at a future date. CacheStorage will use the persisted key and
+padding from the cache's index unless the padding algorithm has been changed,
+one of values is missing, or deemed to be incorrect. In this situation the cache
+is enumerated and the padding recalculated during open.