Passphrase FAQ

What is a passphrase?

A passphrase is a sequence of words or other text used to control access to a computer system, program or data. A passphrase is similar to a password in usage, but is generally longer for added security. Passphrases are often used to control both access to, and operation of, cryptographic programs and systems. Passphrases are particularly applicable to systems that use the passphrase as an encryption key. The origin of the term is by analogy with “password”. The modern concept of passphrases is believed to have been invented by Sigmund N. Porter in 1982. (from Wikipedia)

What is entropy?

Entropy, or more precisely “information entropy”, is the measure for randomness. An intuitive understanding of information entropy relates to the amount of uncertainty about picking a password, i.e. an object that could be translated in a string of bits. “If you have a 32-bit word that is completely random, then it has 32 bits of entropy. If the 32-bit word takes only four different values, and each values has a 25% chance of occurring, then then the word has 2 bits of entropy.” (Practical Cryptography, B. Schneier and N. Ferguson, p.155)

Why is Clipperz asking for a passphrase instead of a password?

Clipperz has been designed to obtain a 128-bit security level. That means that an attacker needs to perform at least 2¹²⁸ steps of “work”. Each step could be something as simple as looking something up in a table or as complex as computing some cryptographic function for certain values.

Your passphrase is part of the security chain of Clipperz and, because a security system is only as strong as its weakest link, it is highly recommended you pick a passphrase with a good amount of entropy.
If your passphrase has little entropy, say 24 bits, it can be attacked with an exhaustive search, i.e. with 2²⁴ steps. Hence the overall security will drop at a 24-bit level due to the weak passphrase.
Passwords are usually 6-10 character long and therefore they cannot have more than about 30 bits of entropy. Not enough to protect your data against serious attackers.

How long should the passphrase be?

Long enough to have about 128 bits of entropy. But since entropy is a measure for randomness, it all depends on how you build your passphrase.
- If you choose a random selection of letters from the standard 26 letter alphabet, you need
  
  128 / log₂(26) = 27.23 ~ 28
  
  characters, maybe difficult to memorize, but not impossible.
- If you use all printable ASCII characters, you end up with 95 possible characters to work with. This lead to
  
  128 / log₂(95) = 19.48 ~ 20
  
  random characters. But memorizing 20 random characters is probably more difficult than 28 lowercase letters …
- A good dictionary in one of the most common languages has about 100,000 words in it. If you build your passphrase selecting random words from it, you need
  
  128 / log₂(100,000) = 7.7 ~ 8
  
  words. This is a decent way to generate a passphrase except that it is kind of hard to remember sometimes. This is pretty easy to type though. This approach has been used in the Diceware method of passphrase generation.

Is there a minimum allowed length for a passphrase? What happens if I use Clipperz with a low-entropy passphrase?

There are no constraints on passphrase lengths. You are free to choose even a one letter passphrase, but you should be aware that the overall security of the service is lowered by the weakness of your passphrase.

With the intrinsic strength of modern cryptography, the user password or phrase is becoming more and more the focus of vulnerability and attacks.

What if I use another language?

Using your native language is probably an obvious choice. But the entropy per character is roughly the same for most of the common languages. Dictionary attacks in another language would run in the same manner as a dictionary attack in English.

What if I use common phrases or quotes?

Don’t use them ever. There are plenty of quote repositories and your passphrase could be discovered in a relatively short amount of time without any special hardware. If you can find the phrase in any published work then don’t use it. A simple background search will reveal what kind of music, books, TV shows, movies, games, hobbies, and everything else you might use. All the common phrases will be tried on the first pass of a key search.

Does odd spelling, punctuation and capitalization help?

A popular trick is to substitute digits for letters, or to randomly capitalize certain letters. Using this kind of “0dd sp3LLing5 and CaP!tal!ZaTiOn” will slightly increase the entropy of your passphrase, but it will not provide a good defense against brute force search attacks.