[CacheStorage] Pad and bin opaque resource sizes.

content/browser/cache_storage/README.md

Index: content/browser/cache_storage/README.md
diff --git a/content/browser/cache_storage/README.md b/content/browser/cache_storage/README.md
index 7b3758dab2193227091b2605166083839794ac19..4a06b27224795c7fe163fdae51d259b90fb38e6b 100644
--- a/content/browser/cache_storage/README.md
+++ b/content/browser/cache_storage/README.md
@@ -1,5 +1,5 @@
 # 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).
 
@@ -152,3 +152,25 @@ 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 may only be cached, and in order to prevent "leaking" the size

[jkarlin, 2017/07/31 14:03:24]

what do you mean "may only be cached". What is it that they can't do?

[cmumford, 2017/08/10 16:37:10]

haha - yeah, good point.

+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.

[jkarlin, 2017/07/31 14:03:24]

actual resource size via quota apis.

[cmumford, 2017/08/10 16:37:10]

Done.

+
+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. This key is also persisted to the cache index.

[jkarlin, 2017/07/31 14:03:24]

How about replacing the last line with:

Each cache's key is persisted to disk in the cache index file.

[cmumford, 2017/08/10 16:37:10]

Done.

+
+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 algorithm version is also written to each cache. This allows for the

[jkarlin, 2017/07/31 14:03:24]

s/algorithm/padding algorithm/

[cmumford, 2017/08/10 16:37:10]

Done.

+algorithm to be changed at a future date. CacheStorage will use the persisted
+key and padding from the cache's index unless the 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.