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I’ve heard about a rule in Information Security, that once a hacker has access to your physical machine, then it’s all over. However, there seems to be a big exception to this rule: iPhones.
It was all over the news a while back that the CIA (or the FBI or something) could not access information from a terrorist’s phone for their counter-terrorism ops. They had to ask Apple to create them an unlocking program that could unlock the phone for them.
My question is, why are iPhones so hard to hack?
I don't think that you interpret the rule you've heard in the right way. If an attacker has physical access to an encrypted but switched off device he cannot simply break the encryption provided that the encryption was done properly. This is true for an iPhone as much as it is for an fully encrypted notebook or an encrypted Android phone.
The situation is different if the device is not switched off, i.e. the system is on and the operating system has access to the encrypted data because the encryption key was entered at startup. In this case the attacker might try to use an exploit to let the system provide him with the decrypted data. Such exploits are actually more common on Android mainly because you have many vendors and a broad range of cheap and expensive devices on this system vs. only few models and a tightly controlled environment with iPhones. But such exploits exist for iPhone too.
With physical access it would also be possible to manipulate the device in a stealthy way in the hope that the owner does not realize that the device was manipulated and enters the passphrase which protected the device. Such manipulations might be software or hardware based keyloggers or maybe some transparent overlay over the touchscreen which captures the data or similar modifications. This can be done both for switched off and switched on devices but a successful attack requires that the owner is unaware of the changes and will thus enter the secret data into the device. Such attack is also often called evil maid attack since it could for example be done by the maid if one leaves the device in the hotel room.
The rule you are referring to goes back to Scott Culp and is from this essay he wrote in 2000:
https://technet.microsoft.com/library/cc722487.aspx
In 2000, there was no such thing as an iPhone. Moreover, the "10 Immutable Laws of Security" are meant as guidelines, aphoristic memory jogs, and (despite the name) not really as laws. They are also outlined in more detail in the essay, so go and read the part about the hardware law:
https://technet.microsoft.com/library/cc722487.aspx#EIAA
excerpt:
If you travel with a laptop, it's absolutely critical that you protect it. The same features that make laptops great to travel with – small size, light weight, and so forth—also make them easy to steal. There are a variety of locks and alarms available for laptops, and some models let you remove the hard drive and carry it with you. You also can use features like the Encrypting File System in Microsoft Windows® 2000 to mitigate the damage if someone succeeded in stealing the computer. But the only way you can know with 100% certainty that your data is safe and the hardware hasn't been tampered with is to keep the laptop on your person at all times while traveling.
Laptops being the closest things to iPhones in 2000, he is talking about basically the same thing that iPhones actually do (encrypt data, have anti-tampering protocols, etc.) but his conclusion is still correct today: If the president's iPhone with the nuclear launch codes on it went missing, I would very strongly urge changing the launch codes immediately. The attacker may or may not be able to break into the iPhone, but once he has physical access, you cannot be 100% sure.
So, why?
Your main question was why iPhones are hard to break and there are two answers. The technical answer is that they use strong encryption with hardware components (the "secure enclave") to encrypt the device and prevent unauthorized access. The non-technical answer is that they have both the resources and the interest to throw enough money at the problem and do it right. Unlike Google or Facebook, they are not in the business of selling user data, so they do not have to leave backdoors or access ways for their own purposes. A good security reputation is worth considerable revenue.
The simple fact that Apple controls both the hardware and the software also makes it considerably easier to implement proper security.
so they do not have to leave backdoors or access ways for their own purposes - the fact that you can call apple to disable your iphone when it gets stolen proves the contrary.
This is a really broad question. But the information access thing is because the iPhone's data was encrypted. And if encryption is done properly it is very hard to break. Does not necessarily mean the iPhone itself is hard to hack.
One of the most important reason is that the iPhone data is correctly encrypted. Correctly means that the right algorithm was used and it was correctly implemented.
In particular, the key used to encrypt the data is long enough - in practical terms unbreakable.
On the other hand the user is authenticated with a 4 digits PIN - how can this not be brute forced?
The reason is that this PIN is used to recover the encryption key above, which is then used to decrypt the data. This extraction process must therefore be extremely protected. Apple does this by using a special chip (TPM) which gets a PIN request, checks the PIN and releases the key if the PIN is correct. It also makes sure to deny access for queries which are too numerous, increasing the answer time and ultimately (if configured) wiping the device. This is detailed in the IOS Security Guide.
This allows for a ridiculously short PIN to still be an appropriate indirect encryption key.
Other concerns have been addressed in other questions, particularly the accepted one which goes though other attack vectors.
The premise of the question is wrong. It assumes, that FBI actually failed to crack an iPhone, but there is no evidence for that (aside from statements of US officials).
More importantly, if FBI, CIA or your uncle could get access to all data on iPhone with just Apple's consent, terrorists could have done the same.
Take right Apple employees hostage, or hack into their computers, and you get access to any iPhone in the world.
That does not sound "hard to crack" at all.
EDIT:
Some people have expressed opinion, that this answer lacks sufficient technical details (not sure why, the question as asked does not mention "technical details"). To illustrate the point above, let's add some technical details.
Suppose, that you have encrypted an USB flash drive with some well-known encryption algorithm. By doing so you ensure (assuming that algorithm is secure), that nobody can access the information within without your personal consent (we are ignoring the non-technical details, remember?)
On other hand, if you encrypt your iPhone with same encryption algorithm, the people, who may give consent to decrypt it are: you and unspecified number of strangers (Apple employees and anyone who they share the key with). This makes for a weaker encryption scheme, since the "key" is forcibly shared amongst unspecified number of people and any of them can obtain your information without consent of others.
As such, based on technical consideration above, iPhone is (on average) less secure that an encrypted USB thumb stick.
