Storing secrets in software

2011-09-06 by . 0 comments

This question comes up on Stack Overflow and IT Security relatively regularly, and goes along one of these lines:

  1. I have a symmetric encryption key I would like to store in my application so attackers can’t find it.
  2. I have an asymmetric encryption key I would like to store in my application so attackers can’t find it.
  3. I would like to store authentication details of some kind in my application so attackers can’t find them.
  4. I have developed an algorithm. How do I make it so attackers can never find it.
  5. I am selling commercial music. I need to make it so The Nasty Pirates can’t decode it.

Firstly, a review of what cryptography is at heart: encryption and decryption are all about sending data over untrusted networks or storing data in untrusted places such that only the intended recipient can read that information. This gives you confidentiality; cryptography as a whole also aims to provide integrity checks through signatures and message digests. Confusingly, cryptography is often confused with access control, in which it so often plays a part. Cryptography ceases to be able to protect you when you decide to put the key and the encrypted data together – at this point, the data has reached its destination, the trusted place. The expectation that cryptography can protect data once it is decrypted is similar to the expectation that a locked door will protect your house if you leave the key under the mat. The act of accessing the key and decrypting the data (and even encrypting it) is a weak link in the chain: it assumes the system you’re performing these actions on guarantees your confidentiality and integrity – it assumes that system is trusted. Reading the argument presented here, our resident cryptographer provided the following explanation:  “Encryption does not create confidentiality, it just concentrates confidentiality into the key. Presumably, it is easier to keep confidential a small key of fixed size, and the key uniform structure allows for the key confidentiality to be measured. Yet you have to start confidentiality at something. Once the key is known, confidentiality has left.”

All of the above questions are really forms of the same thing: how do I on an untrusted system safely decode some encrypted data without interception? In other words, you’re now asking for cryptography to guarantee the security of that information even after you’ve decrypted it. This isn’t possible. The problem then becomes one of how do you ensure that the system in question will maintain confidentiality and integrity for you.

The answer, then, is to create a system which acts as the recipient such that data can be decoded in it and never needs to be transferred outside of it. I’ll call it the black box. The rest of this blog post will be about looking for the black box setup.

  1. We will begin with the idea we want to write a program that stores something securely whilst preventing the user from accessing that information. So the first place people want to do this is in their source code. Which is fine, except source code can be disassembled. Yes, there are ways to make this more difficult but it is not possible to prevent. The same goes for hiding or stashing files around the system, since a cursory analysis with the right tools will tell you exactly where to look. I should add that disassembly prevention probably makes your software less stable and/or portable.
  2. The next option is to coerce the system into helping you hide your information. In any form, this will essentially look like a rootkit. This is dangerous: you may well have your application categorised as malware, for starters, but more importantly at this stage it is very easy to introduce extra vulnerability into the system you have just hooked. You could also crash your customer’s system, which is “not cool” whichever way you look at it. Finally, whilst rootkits are difficult to remove, taking a live listing of files and then an offline one is not. Rootkits can be found and their installation prevented. In fact, a cunning reverse engineer might replace your rootkit with their own equivalent, stealing the information you send it. Handy.
  3. Stage 3 is one for the slashdot home page: the operating system vendor is complicit in helping you. Unless you are a music giant I suspect this probably is not an option for you. A complicit operating system is harder to circumvent, but entirely possible. All you need is access to ring 0 (for people not familiar with the term, ring 0 is the mode where the processor will not stop you lifting any restrictions placed upon memory or code. You can ignore read only page checks, rewrite chunks of kernel memory, whatever you want to do). Depending on the system, this might be difficult to achieve, but it certainly is not impossible. Another route to this stage is to compromise the boot process. You can pass the OS the correct validation codes if it checks anything. Etc. Clearly, this stage is difficult to pull off and requires time/effort, but it can be done.
  4. So, OS security not enough? Hardware then. At this stage you actually have a real black box (or chip) somewhere on the computer. Of course, if the OS has responses sent to it, see stages 1-3. So now your black box needs to talk to all your other hardware, like your monitor or speaker system. Oh and they need to be intercept-proof too – maybe they can’t be trusted either and are really decoding the data straight back to the hard disk. This might sound far fetched, but High-bandwidth Digital Content Protection (wikipedia) aims to provide exactly this kind of protection.

The move/counter-move sequence carries on – so what is to stop a hardware engineer taking your box apart? Self-destructing hardware? Ok, how do you prevent the fact that you’ll display it on a monitor, sending light-waves and electronic signals out?

What does this mean for software on the PC?

  1. You cannot store passwords, encryption keys etc in your code, or anywhere on the system. The only way around that is to allow for user input and aim to prevent interception, which could in itself be difficult.
  2. License keys do not work either, for the same reason.
  3. Nor does obfuscated code. It has to be decrypted to execute, at some point.

However, there is a case where hardware complicit defences are perfectly possible and may well be encouraged, where the hardware and software combination come together. There are many scenarios in which this happens, the most obvious being the mobile phone. A mobile phone is, relatively speaking, hard to take apart and put back together again whole (unless you’re a mobile phone engineer) so it is possible to have hardware-based security work reasonably effectively. Smart cards with on-board cryptographic function again ensure the keys are exceptionally hard to steal. In the case of smart cards, the boundary problem still exists in terms of transferring the decrypted data back to the untrusted system, but on a mobile phone-like device it would be entirely feasible not to route that information through the OS itself and instead play it directly on the screen, isolating those buffers from the “untrusted” sections of the OS. However, let’s leave that idea here. Trusted platforms is a blog post for another day.

Is there a case for devices capable of such secure display? Absolutely. Want to securely read data on a financial transaction, or have a trusted communication channel with your bank? Upping the bar like this definitely helps protect against malware and other interception threats. However, the most common use case appears to be “how do I defend my asset in an untrusted environment in order to enforce my desired price model?” Which brings me full circle – that is not the problem set cryptography solves.

When people say secure, they often mean “impossible for bad guy, possible for me”. That is almost never the case. Clearly, looking at the above, the number of people skilled enough to counter 3 is actually pretty small. You will, therefore, achieve part of this aim: “hard for bad guy, easyish for me”. When people say secure, they also frequently mean technical security measures. Security is more than just technical security – that’s why we have policies, community, law, education, awareness etc. Going as far as points 2/3 reduces the number of people capable of subverting your protection system to the point where legal action is feasible.

However, I think we still have a narrow definition of security in the first place. If you are talking about delivering content or software, the level of issues a customer is likely to come up against increases exponentially as you move from 1 through to point 4. Does your customer really want to be told that you do not support Windows 8 yet? Or that they cannot use their favourite media player for your content? Or that they need to upgrade their BigMediaCorpSatelliteTVBox because you altered the algorithms? Or that they’ve been locked out of that Abba Specials subscription channel they paid for because of a hardware fault, or… Security here is not just about protecting your content, it is about protecting your business. Will the cost of securing the content using any of 1-4 will make up for the potential revenue you could have made if everyone brought the copy legally? Or will the number of legitimate sales simply go down as consumers react to all those technical barriers shattering their plug-and-play expectation? I do not have any numbers on that one, but I am willing to bet the net result of adding this extra “security” is a loss.

Security needs to be appropriate to the risk of the situation presented, having been fully evaluated from all angles. The general consensus is that DRM is an excessive protection measure given the risks involved – indeed, a very simple solution is to make software/music/whatever at the right price point and value such that the vast majority of users buy it, and allow for the fact that some people will pirate/steal/make unauthorised use of it. In certain situations, providing a value add around the product can make the difference (some companies selling open source software generate their entire revenue using this model), and even persuade users of illegal copies to buy a licence in order to gain access to these services (e.g. support, upgrades).

 
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