Change behavior of MockEncrypter/MockDecrypter to work with header protection

Bug: 964618
Change-Id: I0ec62e4d8ddce9b44eb3f796ac8317ab822e79c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625100
Commit-Queue: Nick Harper <[email protected]>
Auto-Submit: Nick Harper <[email protected]>
Reviewed-by: Ryan Hamilton <[email protected]>
Cr-Commit-Position: refs/heads/master@{#662433}
diff --git a/net/quic/mock_decrypter.h b/net/quic/mock_decrypter.h
index 7b9256f..5b7f0e22 100644
--- a/net/quic/mock_decrypter.h
+++ b/net/quic/mock_decrypter.h
@@ -17,10 +17,10 @@
 
 namespace net {
 
-// A MockDecrypter is a QuicDecrypter that does no validation of
-// the given ciphertext and returns it untouched, ignoring the
-// associated data. This is used to allow fuzzing to mutate
-// plaintext packets.
+// A MockDecrypter is a QuicDecrypter that strips the last 12 bytes of
+// ciphertext (which should be zeroes, but are ignored), and returns the
+// remaining ciphertext untouched and ignores the associated data. This is used
+// to allow fuzzing to mutate plaintext packets.
 class MockDecrypter : public quic::QuicDecrypter {
  public:
   explicit MockDecrypter(quic::Perspective perspective);