Two of the first things you learn in cryptography are that simple substitution ciphers are very easy to break, and that security by obscurity is a bad idea. This post will revisit both of these ideas.
Security depends on your threat model. If the threat you want to protect against is a human reading your encrypted messages, then simple substitution ciphers are indeed weak. Anyone capable of working a cryptogram in a puzzle book is capable of breaking your encryption.
But if your threat model is impersonal surveillance, things are different. It might not take much to thwart a corporation scanning your email for ideas of what to advertise to you. Even something as simple as rot13 might be enough.
The point of this article is not to recommend rot13, or any other simple substitution cipher, but to elaborate on the idea that evading commercial surveillance is different from evading intelligence agencies. It’s easier to evade bots than spooks.
If you’re the target of a federal investigation, the most sophisticated encryption at your disposal may be inadequate. But if you’re the target of an advertising company, things are much easier.
Rot13 works by moving letters ahead 13 positions in the alphabet. The nth letter goes to the (n + 13)th letter mod 26. So A’s become N’s, and N’s become A’s, etc. Rot13 offers no security at all, but it is used to prevent text from being immediately recognizable. A common application is to publish the punchline of jokes.
Rot13 converts the word “pyrex” to “clerk.” If you use the word “pyrex” in an email, but rot13 encrypt your message, you’re unlikely to see advertisements for glassware unless you also talk about a clerk.
It is conceivable, but doubtful, that impersonal surveillance software would try to detect rot13 encoding even though it would be easy to do. But detecting and decrypting simple substitution ciphers in general, while certainly possible, would not be worth the effort. Security-by-obscurity could protect you from surveillance because it’s not profitable for mass surveillance to pursue anything obscure. For example, the Playfair cipher, broken over a century ago, would presumably throw off bots.
Modern but deprecated encryption
Simple substitution ciphers are ancient. Modern encryption methods, even deprecated methods like DES, are far more secure. For example, DES (Data Encryption Standard) is considered obsolete because it can be broken by a multiprocessor machine in under 24 hours. However, commercial surveillance is unwilling to spend processor-days to decrypt one person’s communication.
This is not to recommend DES. If it’s just as easy to use much better algorithms like AES (Advanced Encryption Standard) then why not do that? My purpose in bringing up DES is to say that squabbles over the merits of various modern encryption methods are beside the point if your goal is to evade impersonal surveillance.
Everyone agrees DES should be put out to pasture, but it doesn’t matter if you’re just trying to avoid surveillance. Things that not everyone agrees on matter even less.
What matters more than the algorithms is who holds the keys. A system in which you alone hold a DES key may give you more privacy than a system that holds an AES key for you. Companies may want to protect your private data from competitors but have no qualms about using it themselves.
Why surveillance matters
Why go to any effort to evade corporate surveillance?
Companies share data, and companies get hacked. One innocuous piece of data may be the link that connects two databases together. Things that you don’t mind revealing may unlock things you don’t want to reveal.