Correct horse
This generates a passphrase from a specified word list. Passphrases are demonstrably more secure and more memorable than a password with standard naive attempts at randomization (letter substitutions, numbers, exclamation marks).
Entropy
The correct metric for the security of a randomly generated password is its entropy, that is, the logarithm of the total pool of possible passwords it was drawn from.
As formulated by Auguste Kerckhoff,
a cryptosystem should be secure if everything but the key is public knowledge.
Likewise, a password should be secure even if the attacker knows your method of
choosing a password. Naive passwords that depend on personal data (names, dates)
trivially fail this challenge, but even an ostensibly good password (like correct
horse battery staple) can be weak if the words are picked by a human, because
humans are bad at generating randomness.
The minimum entropy of your password can be approximated as a sum of several binary logarithms:
- How fast you expect an attacker to guess (
log2(guesses per second)) - How long the password must remain secure (
log2(years)) - 25 (because there are almost 225 seconds in a year).
- How unlikely it should be for the attacker to guess correctly (
log2(1/p)).
For example, if I want an attacker who can guess a thousand times per second
to have no more than a one in a thousand chance of guessing correctly in 8 years,
I would add 10 + 10 + 25 + 3 to get a minimum entropy of 48 bits.
Note: Do not focus obsessively on the difficulty of brute-forcing. The biggest risk to your password's security is you. Avoid storing it in clear text, avoid entering it on devices you do not control, avoid using similar passwords on different systems (even non-simultaneously). Do not let your device get compromised, and never let a third-party service access your account with your password.
Format
Many systems use incompetently written password authentication, and therefore do not allow passwords to contain spaces (or, in other cases, enforce security theater policies, such as mixed case or numbers, or actively sabotage security by limiting the maximum length).
My suggestion for the first problem is to set the delimiter to some other
character (like . or -, which also takes care of the fourth problem), for
the second to capitalize the first letter of each word (an easily memorable
rule) or append 1! (pronounced "FU"), and for the fifth to completely refuse
to use that system if at all possible.
Security
This is a client-only utility whose source is available at password.js. For particularly sensitive cases, I would recommend auditing the code and adapting it to run in a local context to prevent its output from being leaked by browser extensions.
Credits
Word list provided by Word frequency data.