Add a warning about Decrypt() being used as a padding oracle.

crypto/encryptor.h

Index: crypto/encryptor.h
===================================================================
--- crypto/encryptor.h	(revision 133713)
+++ crypto/encryptor.h	(working copy)
@@ -70,6 +70,14 @@
   bool Encrypt(const base::StringPiece& plaintext, std::string* ciphertext);
 
   // Decrypts |ciphertext| into |plaintext|.  |ciphertext| must not be empty.
+  //
+  // WARNING: In CBC mode, Decrypt() returns false if it detects the padding
+  // in the decrypted plaintext is wrong. Padding errors can result from
+  // tampered ciphertext or a wrong decryption key. But successful decryption
+  // does not imply the authenticity of the data. The caller of Decrypt()
+  // must either authenticate the ciphertext before decrypting it, or take
+  // care to not report decryption failure. Otherwise it could inadvertently
+  // be used as a padding oracle to attack the cryptosystem.
   bool Decrypt(const base::StringPiece& ciphertext, std::string* plaintext);
 
   // Sets the counter value when in CTR mode. Currently only 128-bits