Is Apple’s Cloud Key Vault a crypto backdoor?

TL;DR: No, it isn’t. If that’s all you wanted to know, you can stop reading.

Still, as you can see there’s been some talk on Twitter about the subject, and I’m afraid it could lead to a misunderstanding. That would be too bad, since Apple’s new technology is kind of a neat experiment.

So while I promise that this blog is not going to become all-Apple-all-the-time, I figured I’d take a minute to explain what I’m talking about. This post is loosely based on an explanation of Apple’s new escrow technology that Ivan Krstic gave at BlackHat. You should read the original for the fascinating details.

What is Cloud Key Vault (and what is iCloud Keychain)?

A few years ago Apple quietly introduced a new service called iCloud Keychain. This service is designed to allow you to back up your passwords and secret keys to the cloud. Now, if backing up your sensitive passwords gives you the willies, you aren’t crazy. Since these probably include things like bank and email passwords, you really want these to be kept extremely secure.

And — at least going by past experience — security is not where iCloud shines:

The problem here is that passwords need to be secured at a much higher assurance level than most types of data backup. But how can Apple ensure this? We can’t simply upload our secret passwords the way we upload photos of our kids. That would create a number of risks, including:

  1. The risk that someone will guess, reset or brute-force your iCloud password. Password resets are a particular problem. Unfortunately these seem necessary for normal iCloud usage, since people do forget their passwords. But that’s a huge risk when you’re talking about someone’s entire password collection.
  2. The risk that someone will break into Apple’s infrastructure. Even if Apple gets their front-end brute-forcing protections right (and removes password resets), the password vaults themselves are a huge target. You want to make sure that even someone who hacks Apple can’t get them out of the system.
  3. The risk that a government will compel Apple to produce data. Maybe you’re thinking of the U.S. government here. But that’s myopic: Apple stores iCloud data all over the world.

So clearly Apple needs a better way to protect these passwords. How do to it?

Why not just encrypt the passwords?

It is certainly possible for an Apple device to encrypt your password vault before sending it to iCloud. The problem here is that Apple doesn’t necessarily have a strong encryption key to do this with. Remember that the point of a backup is to survive the loss of your device, and thus we can’t assume the existence of a strong recovery key stored on your phone.

This leaves us with basically one option: a user password. This could be either the user’s iCloud password or their device passcode. Unfortunately for the typical user, these tend to be lousy. They may be strong enough to use as a login password — in a system that allows only a very limited number of login attempts. But the kinds of passwords typical users choose to enter on mobile devices are rarely strong enough to stand up to an offline dictionary attack, which is the real threat when using passwords as encryption keys.

(Even using a strong memory-hard password hash like scrypt — with crazy huge parameters — probably won’t save a user who chooses a crappy password. Blame phone manufacturers for making it painful to type in complicated passwords by forcing you to type them so often.)

So what’s Apple to do?

So Apple finds itself in a situation where they can’t trust the user to pick a strong password. They can’t trust their own infrastructure. And they can’t trust themselves. That’s a problem. Fundamentally, computer security requires some degree of trust — someone has to be reliable somewhere.

Apple’s solution is clever: they decided to make something more trustworthy than themselves. To create a new trust anchor, Apple purchased a bunch of fancy devices called Hardware Security Modules, or HSMs. These are sophisticated, tamper-resistant specialized computers that store and operate with cryptographic keys, while preventing even malicious users from extracting them. The high-end HSMs Apple uses also allow the owner to include custom programming.

Rather than trusting Apple, your phone encrypts its secrets under a hardcoded 2048-bit RSA public key that belongs to Apple’s HSM. It also encrypts a function of your device passcode, and sends the resulting encrypted blob to iCloud. Critically, only the HSM has a copy of the corresponding RSA decryption key, thus only the HSM can actually view any of this information. Apple’s network sees only an encrypted blob of data, which is essentially useless.

When a user wishes to recover their secrets, they authenticate themselves directly to the HSM. This is done using a user’s “iCloud Security Code” (iCSC), which is almost always your device passcode — something most people remember after typing it every day. This authentication is done using the Secure Remote Password protocol, ensuring that Apple (outside of the HSM) never sees any function of your password.

Now, I said that device passcodes are lousy secrets. That’s true when we’re talking about using them as encryption keys — since offline decryption attacks allow the attacker to make an unlimited number of attempts. However, with the assistance of an HSM, Apple can implement a common-sense countermeasure to such attacks: they limit you to a fixed number of login attempts. This is roughly the same protection that Apple implements on the devices themselves.

The encrypted contents of the data sent to the HSM (source).

The upshot of all these ideas is that — provided that the HSM works as designed, and that it can’t be reprogrammed — even Apple can’t access your stored data except by logging in with a correct passcode. And they only get a limited number of attempts to guess correctly, after which the account locks.

This rules out both malicious insiders and government access, with one big caveat.

What stops Apple from just reprogramming its HSM?

This is probably the biggest weakness of the system, and the part that’s driving the “backdoor’ concerns above. You see, the HSMs Apple uses are programmable. This means that — as long as Apple still has the code signing keys — the company can potentially update the custom code it includes onto the HSM to do all sort sorts of things.

These things might include: programming the HSM to output decrypted escrow keys. Or disabling the maximum login attempt counting mechanism. Or even inserting a program that runs a brute-force dictionary attack on the HSM itself. This would allow Apple to brute-force your passcode and/or recover your passwords.

Fortunately Apple has thought about this problem and taken steps to deal with it. Note that on HSMs like the one Apple is using, the code signing keys live on a special set of admin smartcards. To remove these keys as a concern, once Apple is done programming the HSM, they run these cards through a process that they call a “physical one-way hash function”.

If that sounds complicated, here’s Ivan’s slightly simpler explanation.

So, with the code signing keys destroyed, updating the HSM to allow nefarious actions should not be possible. Pretty much the only action Apple can take is to  wipe the HSM, which would destroy the HSM’s RSA secret keys and thus all of the encrypted records it’s responsible for. To make sure all admin cards are destroyed, the company has developed a complex ceremony for controlling the cards prior to their destruction. This mostly involves people making assertions that they haven’t made copies of the code signing key — which isn’t quite foolproof. But overall it’s pretty impressive.

The downside for Apple, of course, is that there had better not be a bug in any of their programming. Because right now there’s nothing they can do to fix it — except to wipe all of their HSMs and start over.

Couldn’t we use this idea to implement real crypto backdoors?

A key assertion I’ve heard is that if Apple can do this, then surely they can do something similar to escrow your keys for law enforcement. But looking at the system shows isn’t true at all.

To be sure, Apple’s reliance on a Hardware Security Module indicates a great deal of faith in a single hardware/software solution for storing many keys. Only time will tell if that faith is really justified. To be honest, I think it’s an overly-strong assumption. But iCloud Keychain is opt-in, so individuals can decide for themselves whether or not to take the risk. That wouldn’t be true of a mandatory law enforcement backdoor.

But the argument that Apple has enabled a law enforcement backdoor seems to miss what Apple has actually done. Instead of building a system that allows the company to recover your secret information, Apple has devoted enormous resources to locking themselves out. Only customers can access their own information. In other words, Apple has decided that the only way they can hold this information is if they don’t even trust themselves with it.

That’s radically different from what would be required to build a mandatory key escrow system for law enforcement. In fact, one of the big objections to such a backdoor — which my co-authors and I recently outlined in a report — is the danger that any of the numerous actors in such a system could misuse it. By eliminating themselves from the equation, Apple has effectively neutralized that concern.

If Apple can secure your passwords this way, then why don’t they do the same for your backed up photos, videos, and documents?

That’s a good question. Maybe you should ask them?

Neat research ideas that went nowhere: Preventing offline dictionary attacks with CAPTCHAs

Once in a while I come across a research paper that’s so off the beaten path and so obviously applicable to a real problem, that I know it’s going to sink like a stone and never be heard from again.

Today I’d like to talk about an idea that was introduced back in CRYPTO 2006, but doesn’t seem to have gotten any real traction out in the real world since. (If I’m wrong and someone is using it, feel free to email me. I’d be happy to eat my words and hear more.)

This particular idea comes from a 2006 paper by Ran Canetti, Shai Halevi and Michael Steiner,* and it addresses a very real problem that we have with deployed encryption schemes: namely, they tend to rely on passwords. This means that even if you use strong cryptography, all of your data is vulnerable to someone with a password dictionary and a lot of computing power.

Password-Based Encryption (and why we hate it)

To explain how Canetti, Halevi and Steiner propose to deal with this, I need to give a little bit of background on password-based encryption.

First of all of all, no matter what Dan Brown says, we’ve gotten pretty good at building encryption schemes that stand up against very motivated adversaries. If you’re using a modern scheme and are properly generating/storing your secret keys, then it’s awfully unlikely that a third party is going to decrypt your data — even if that party is a government agency.** (I’m leaving aside the wrench factor).

Unfortunately, the previous paragraph embeds one major assumption that makes it inapplicable to the way that many people secure their data. You see, in a cryptographer’s imagination, everyone encrypts with a strong, random 128+ bit secret key, which they memorize and never write down.

But in real life, people mostly prefer to use their cat’s name.

The use of passwords to derive encryption keys is a lot more common than it should be. It’s depressingly common in many applications where people really need security. Passwords are the default authorization method for many fancy encrypting hard-drives and WDE packages. Windows just loves to encrypt things under your login credentials. (Many of these tools also support hardware tokens and TPM, but these are such a drag that most people don’t bother.)

To your attacker, this is a feature not a bug. If a key is derived from dictionary words, or some predictable mutation of dictionary words, then it can be guessed. All it takes is time and  computing power. Moreover, since password guessing is embarrassingly parallel, it’s just the kind of enterprise that you can throw a data center or five at. (Ever wondered why the state of Maryland uses so much electricity?)

So far I haven’t said anything you didn’t already know. But just for fun, let’s quickly talk about what we currently do to prevent dictionary attacks. There are basically three approaches:

  1. Use some trusted, on-line server (or tamper-resistant hardware) to assist with the key derivation process. If done correctly, the server/hardware can shut off an attacker after a few invalid guesses, assuming that it isn’t also compromised. Unfortunately, this solution assumes that the server or hardware will be available when you need your data. That isn’t always possible.
  2. Use key stretching, which increases the time required to convert a password into a cryptographic key. This can slow down a dictionary attack, but it just slows it down. Moreover, it’s pretty ineffective against an attacker with specialized hardware.
  3. Use salt, which is hardly worth mentioning since if you aren’t salting your passwords then you’re crazy. Unfortunately all this does is prevent pre-computation. It does nothing against normal dictionary attacks.

The human touch

In a perfect world, the online solution (1) above would probably be the best way to go. Unfortunately, we don’t live in a perfect world, and this may not be compatible with people’s requirements.

While it’s not possible to prevent dictionary attacks in an offline system, we may be able to slow them down. To do this, we have to make the decryption process much more resource-intensive. Unfortunately, it’s hard to win this game through computation alone, at least not in a world full of FPGAs and elastic compute clouds.

Not this sort of brains. (XKCD).

What Canetti, Halevi and Steiner realized is that there’s one resource that the adversary might not have in abundance: human brains.

What if we could mandate some sort of human interaction each time a user tries to decrypt a file? This shouldn’t be too burdensome for the legitimate decryptor, since presumably he’s human, and moreover he only needs to try once.

But a password cracker needs to try millions of passwords. Requiring a human interaction for each attempt should throw a serious wrench into the process.

Using Human Puzzles to stop dictionary attacks

Fortunately we already have a whole class of technologies that differentiate human beings from computers. Generally, these are known as Reverse Turing Tests (RTTs). Canetti, Halevi and Steiner propose to use RTTs to construct “human puzzles”. At a high level, the idea is pretty simple.

In every PBE scheme, a password must first be converted into a key. Normally you do this by hashing the password and some salt via a standard key derivation function (e.g., PBKDF2). Usually this is where you’d stop. The output of this function, which we’ll call K, would be the key used to encrypt and decrypt.

However, now we’re going to take things one step further. The first key will be further processed by converting it into a puzzle that only a human can solve. When a human solves this puzzle, they create a new key that, ideally, a computer all by itself should not be able to recover. The process should be reliable — it needs to produce the same result every time it’s run, possibly across many different users.

Once you get past the theoretical stuff, the concrete example that Canetti, Halevi and Steiner propose is to use CAPTCHAs to implement human puzzles.

CAPTCHAs: don’t you love them?

In their example, the process of deriving an encryption key from a password now looks like this:

  1. First hash the password with some random salt r to derive a first key K. Now break this into a bunch of small chunks, each (for example) about 20 bits long.
  2. Use each of these 20-bit chunks to index into a huge table containing 2^20 (about 1 million) distinct CAPTCHA images. Each CAPTCHA contains a hard-to-read string with at least 20 bits of entropy in it. This string could be a bunch of numbers, or some randomly-selected dictionary words.
  3. Give each CAPTCHA to the human user to solve. Collect the results.
  4. Hash all of the user’s responses together with another salt s to derive a final decryption key.

If we instantiate the hash function with, say, PBKDF2 and a 160-bit output, then this requires the legitimate user to solve 8 puzzles the first time they encrypt, and every subsequent time they decrypt.

The dictionary attacker, however, has to solve eight puzzles for every password they try. If we assume that they have to guess a large number of passwords, this means that over time they’ll have to solve a substantial fraction (if not all) of the 1 million CAPTCHAs. Even if they can outsource this work at a rate of 5 cents per CAPTCHA, solving the whole collection will still cost $50,000 and a whole lot of hassle.

(This is a hugely simplified analysis. Obviously the attacker doesn’t have to solve all of the CAPTCHAs. Rather, their probability of guessing the key is a function of the number of CAPTCHAs, the fraction of the CAPTCHAs that they’ve been able to solve, the complexity of the password, etc. See the full paper for a much better analysis.)

The sticky wicket

Now, this really might work, in the sense that it will slow down attacks. But if you’re a practical-minded sort, you probably have one small question about this proposal. Namely: where do the CAPTCHAs come from?

Well, a simple answer is that you generate them all randomly the first time you encrypt, and you ship them along with the encrypted file. If we assume that each CAPTCHA is a 4KB image file, then storing the entire collection will require about 4GB.

That’s seems like a lot of space, but really it isn’t. After all, it’s not like we’re working with floppy drives. We live in a world where a 16GB USB Flash Drive can be had for about $10. So it’s perfectly reasonable to assume that we’ll have lots storage for things we really care about.

In summary

Now maybe CAPTCHAs aren’t your thing. Maybe you don’t like the idea of storing 4GB worth of files just to ship someone an encrypted ZIP file. That’s fine. One of the neatest open problems in the paper is to find new sorts of human puzzles. In general, a human puzzle is any function that converts some input (say a 20-bit string) into an output, such that only a human being can do the transformation.

These don’t need to rely on bulky, stored CAPTCHAs. Hypothetically you could come up with a human puzzle that dynamically generates instances without a big stored database. For example, your puzzle might be based on recognizing human expressions, or inkblots. It just has to be reliable.

And that’s why I think this is such a neat paper. Whether or not this particular idea changes the world, it’s a new idea, it tries to solve a hard practical problem that most people have given up on, and it inspires some new ideas.

The crypto publishing game rarely produces papers with that particular combination. Just for that, I’d love to see it go somewhere.


* Ran Canetti, Shai Halevi and Michael Steiner. 2006. Mitigating Dictionary Attacks on Password-Protected Local Storage. In CRYPTO ’06.

** Leaving aside what we all know the NSA’s got in its basement.