Securing encryption keys with DPAPI: An obvious hole?

Question

I have a Windows.Forms-based .NET desktop application that stores privileged information in a file on disk (without using .NET configuration files) encrypted using a symmetric cryptography algorithm such as TripleDES using MS CryptoAPI. This file must be read / written for several program cycles / machine power cycles, aka, use the same / IV switch each time. The obvious question here is how to protect the key (and possibly IV), and a few questions here on SO just say “use DPAPI” and give a trivial round-trip encryption / decryption example.

I know how to use DPAPI already, but there seems to be an obvious problem with using it to protect the / IV switch, which will be transferred to another encryption scheme. Consider the following code:

TripleDESCryptoServiceProvider^ cryptoprov = gcnew TripleDESCryptoServiceProvider; cryptoprov->Key = ProtectedData::Unprotect(encryptedKey, salt, DataProtectionScope::CurrentUser); cryptoprov->IV = ProtectedData::Unprotect(encryptedIV, salt, DataProtectionScope::CurrentUser); 

Due to the fact that you must assign the SymmetricAlgorithm derived class “Key and IV”, could the attacker simply set a breakpoint at this point and easily determine what Key / IV is?

My questions are as follows:

• Am I missing a point for using DPAPI to protect keys? How do you do this?
• Should I use DPAPI to encrypt my file? Therefore, storage of keys / IV is not required.
• I noticed the existence of CspParameters for asymmetric encryption. Is this inherently a better option than symmetrical? (in the context of my scenario, not symmetric or asymmetric)

Thanks!

Answer 1

DPAPI's goal is to protect persistent data from tracking and tampering, it does not offer anything to protect sensitive data in the application’s memory.

Answer 2

If an attacker can set a breakpoint, you have already lost.
An attacker can simply set a breakpoint after decrypting the data and reading plaintext.

What intruder are you afraid of?

If you want, you can write if (Debugger.IsAttached) Environment.FailFast() , but an attacker can remove the check using Reflexil.