A friend asked me what I considered a secure channel a couple of months ago, and it made me think. Many of us have information that we wish to communicate which we’d rather other people can’t look at, for all sorts of reasons. These might range from present ideas for our spouse or partner sent by a friend to my phone to diplomatic communications about espionage targets sent between embassies over the Internet, with lots in between: intellectual property discussions, bank transactions and much else. Sometimes, we want to ensure that people can’t change what’s in the messages we send: it might be OK for other people to know that I pay £300 in rent, but not for them to be able to change the amount (or the bank account into which it goes). These two properties are referred to as confidentiality (keeping information secret) and integrity (keeping information unchangeable), and often you want to combine them – in the case of our espionage plans, I’d prefer that my enemies don’t know what targets are at risk, but also that they don’t change the targets I’ve selected to something less bothersome for them.
Modern encryption systems generally provide both confidentiality and integrity for messages and data, so I’m going to treat these as standard properties for an encrypted channel. Which means that if I use encryption on a channel, it’s secure, right?
Hmm. Let’s step back a bit, because, unfortunately, there’s rather a lot more to unpack than that. Three of the questions we need to tackle should give us pause. They are: “secure from whom?”, “secure for how long?” and “secure where?”. The answers we give to these questions will be important, and though they are all somewhat intertwined, I’m going to deal with them in order, and I’m going to use the examples of the espionage message and the present ideas to discuss them. I’m also going to talk more about confidentiality than integrity – though we’ll assume that both properties are important to what we mean by “secure”.
Secure from whom?
In our examples, we have very different sets of people wanting to read our messages – a nation state and my spouse. Unless my spouse has access to skills and facilities of which I’m unaware (and I wouldn’t put it past her), the resources that she has at her disposal to try to break the security of my communication are both fewer and less powerful than those of the nation state. A nation state may be able to apply cryptologic attacks to messages, attack the software (and even firmware or hardware) implementations of the encryption system, mess with the amount of entropy available for key generation at either or both ends of the channel, perform interception (e.g. Person-In-The-Middle) attacks, coerce the sender or recipient of the message and more. I’m hoping that most of the above are not options for my wife (though coercion might be, I suppose!). The choice of encryption system, including entropy sources, cipher suite(s), hardware and software implementation are all vital in the diplomatic message case, as are vetting of staff and many other issues. In the case of gift ideas for my wife’s birthday, I’m assuming that a standard implementation of a commercial messaging system should be enough.
Secure for how long?
It’s only a few days till my wife’s birthday (yes, I have got her a present, though that does remind me; I need a card…), so I only have to keep the gift ideas secure for a little longer. It turns out that, in this case, the time sensitivity of the integrity of the message is different to that of the confidentiality: even if she managed to change what the gift idea in the message was, it wouldn’t make a difference to what I’ve got her at this point. However, I’d still prefer if she didn’t know what the gift ideas are.
In the case of the diplomatic espionage message, we can assume that confidentiality and the integrity are both important for a much longer time, but we’ll concentrate on the confidentiality. Obviously an attacking country would prefer it if the target were unaware of an attack before it happened, but if the enemy managed to prove an attack was performed by the message sender’s or recipient’s country, even a decade or more in the future, this could also lead to major (and negative) consequences. We want to ensure that whatever steps we take to protect the message are sufficient that access to a copy of the message taken when it was sent (via wire-tapping, for instance) or retrieved at a later date (via access to a message store in the future), is insufficient to allow it to be cracked. This is tricky, and the history of cryptologic attacks on encryption schemes, not to mention human failures (such as leaks) and advances in computation (such as quantum computing) should serve as a strong warning that we need to consider very carefully what mechanisms we should use to protect our messages.
Secure where?
Are the embassies secure? Are all the machines between the embassies secure? Is the message stored before delivery? If so, is it stored on a machine within the embassy or on a server elsewhere? Is it end-to-end encrypted, or is it decrypted before delivery and then re-encrypted (I really, really hope not). While this is unlikely in the case of diplomatic messages, a good number of commercially sensitive messages (including much email) is not end-to-end encrypted, leading to vulnerabilities if someone trying to break the security can get access to the system where they are stored, or intercept them between decryption and re-encryption.
Typically, we have better control over different parts of the infrastructure which carry or host our communications than we do over others. For most of the article above, I’ve generally assumed that the nation state trying to read embassy message is going to have more relevant resources to try to breach the security of the message than my wife does, but there’s a significant weakness in protecting my wife’s gift idea: she has easy access to my phone. I tend to keep it locked, and it has a PIN, but, if I’m honest, I don’t tend to go out of my way to keep her out: the PIN is to deter someone who might steal it. Equally, it’s entirely possible that I may be sharing some material (a video or news article) with her at exactly the time that the gift idea message arrives from our mutual friend, leading her to see the notification. In either case, there’s a good chance that the property of confidentiality is not that strong after all.
Conclusion
I’ve said it before, and I plan to say it again (and again, and again): there is no “secure”. When we talk about secure channels, we must be aware that what we mean should be “channels secured with appropriate measures to protect against the risks associated with the security being compromised”. This is a long way of saying “if I’m protecting diplomatic messages, I need to make greater efforts than if I’m trying to stop my wife finding out ahead of time what she’s getting for her birthday”, but it’s important to understand this. Part of the problem is that we’re bombarded with words like “secure”, which are unqualified, and may lead us to think that they’re absolute, when they’re absolutely not. Another part of the problem is that once we’ve put one type of security in place, particularly when it’s sold or marketed as “best in breed” or “best practice”, that it addresses all of the issues we might have. This is clearly not the case – using the strongest encryption possible for messages between my friend and me isn’t going to stop my wife from knowing I’ve bought her if knows the PIN for my phone. Please, please, consider what you need when you’re protecting your communications (and other data, of course), and always beware of products and services which call themselves “secure”. Because they’re not.
Alice, Eve and Bob - a security blog 