Enforce variations signature verification.

chrome/browser/metrics/variations/variations_seed_store.h

Index: chrome/browser/metrics/variations/variations_seed_store.h
===================================================================
--- chrome/browser/metrics/variations/variations_seed_store.h	(revision 254843)
+++ chrome/browser/metrics/variations/variations_seed_store.h	(working copy)
@@ -45,6 +45,27 @@
   // Registers Local State prefs used by this class.
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
+ protected:
+  // Note: UMA histogram enum - don't re-order or remove entries.
+  enum VerifySignatureResult {
+    VARIATIONS_SEED_SIGNATURE_MISSING,
+    VARIATIONS_SEED_SIGNATURE_DECODE_FAILED,
+    VARIATIONS_SEED_SIGNATURE_INVALID_SIGNATURE,
+    VARIATIONS_SEED_SIGNATURE_INVALID_SEED,
+    VARIATIONS_SEED_SIGNATURE_VALID,
+    VARIATIONS_SEED_SIGNATURE_ENUM_SIZE,
+  };
+
+  // Verifies a variations seed (the serialized proto bytes) with the specified
+  // base-64 encoded signature that was received from the server and returns the
+  // result. The signature is assumed to be an "ECDSA with SHA-256" signature
+  // (see kECDSAWithSHA256AlgorithmID in the .cc file). Returns the result of
+  // signature verification or VARIATIONS_SEED_SIGNATURE_ENUM_SIZE if signature

[jwd, 2014/03/05 17:56:27]

opt nit: Overloading VARIATIONS_SEED_SIGNATURE_ENUM_SIZE like this bugs me a
bit. 

[Alexei Svitkine (slow), 2014/03/05 18:21:43]

I considered making this function return a bool and having the result be an out
param, but felt that would make code using this more complicated.

The other alternative is to add an extra enum value, but since it's an UMA enum
now we either have to log the histogram with that value on mobile platforms
(will use more memory) or have an entry in the enum that's never logged, which
is kind of confusing, though probably OK as long as we document it in
histograms.xml. Do you prefer I change it to that?

[jwd, 2014/03/05 18:36:00]

No, not really. It's probably fine how it is. 

+  // verification is not enabled.
+  virtual VariationsSeedStore::VerifySignatureResult VerifySeedSignature(
+      const std::string& seed_bytes,
+      const std::string& base64_seed_signature);
+
  private:
   // Clears all prefs related to variations seed storage.
   void ClearPrefs();