The Intercept’s Micah Lee says: A passphrase is like a password, but longer and more secure. In essence, it’s an encryption key that you memorize. Once you start caring more deeply about your privacy and improving your computer security habits, one of the first roadblocks you’ll run into is having to create a passphrase. You can’t secure much without one.
For example, when you encrypt your hard drive, a USB stick, or a document on your computer, the disk encryption is often only as strong as your passphrase. If you use a password database, or the password-saving feature in your web browser, you’ll want to set a strong master passphrase to protect them. If you want to encrypt your email with PGP, you protect your private key with a passphrase. In his first email to Laura Poitras, Edward Snowden wrote, “Please confirm that no one has ever had a copy of your private key and that it uses a strong passphrase. Assume your adversary is capable of one trillion guesses per second.”
In this post, I outline a simple way to come up with easy-to-memorize but very secure passphrases. It’s the latest entry in an ongoing series of stories offering solutions — partial and imperfect but useful solutions — to the many surveillance-related problems we aggressively report about here at The Intercept.
It turns out, coming up with a good passphrase by just thinking of one is incredibly hard, and if your adversary really is capable of one trillion guesses per second, you’ll probably do a bad job of it. If you use an entirely random sequence of characters it might be very secure, but it’s also agonizing to memorize (and honestly, a waste of brain power).
But luckily this usability/security trade-off doesn’t have to exist. There is a method for generating passphrases that are both impossible for even the most powerful attackers to guess, yet very possible for humans to memorize. The method is called Diceware, and it’s based on some simple math.
Your secret password trick probably isn’t very clever
People often pick some phrase from pop culture — favorite lyrics from a song or a favorite line from a movie or book — and slightly mangle it by changing some capitalization or adding some punctuation, or use the first letter of each word from this phrase. Some of these passphrases might seem good and entirely unguessable, but it’s easy to underestimate the capabilities of those invested in guessing passphrases.
Imagine your adversary has taken the lyrics from every song ever written, taken the scripts from every movie and TV show, taken the text from every book ever digitized and every page on Wikipedia, in every language, and used that as a basis for their guess list. Will your passphrase still survive?
If you created your passphrase by just trying to think of a good one, there’s a pretty high chance that it’s not good enough to stand up against the might of a spy agency. For example, you might come up with “To be or not to be/ THAT is the Question?” If so, I can guarantee that you are not the first person to use this slightly-mangled classic Shakespeare quote as your passphrase, and attackers know this.
The reason the Shakespeare quote sucks as a passphrase is that it lacks something called entropy. You can think of entropy as randomness, and it’s one of the most important concepts in cryptography. It turns out humans are a species of patterns, and they are incapable of doing anything in a truly random fashion.
Even if you don’t use a quote, but instead make up a phrase off the top of your head, your phrase will still be far from random because language is predictable. As one research paper on the topic states, “users aren’t able to choose phrases made of completely random words, but are influenced by the probability of a phrase occurring in natural language,” meaning that user-chosen passphrases don’t contain as much entropy as you think they might. Your brain tends to continue using common idioms and rules of grammar that reduce randomness. For example, it disproportionately decides to follow an adverb with a verb and vice versa, or, to cite one actual case from the aforementioned research paper, to put the word “fest” after the word “sausage.”
Passphrases that come from pop culture, facts about your life, or anything that comes directly from your mind are much weaker than passphrases that are imbued with actual entropy, collected from nature.