---
title: Password Strength, Hackers, and You
date: 2023-10-21
draft: true
---

import Sidenote from '@components/Sidenote.astro';

Every once in a while, as my friends and family can attest, I go off on a random screed about passwords, password strength, password cracking, logins, etc. To which they listen with polite-if-increasingly-glassy-eyed expressions, followed by an equally polite change of conversational topic. To avoid falling into this conversational tarpit _quite_ so often, I've decided to write it all up here, so that instead of spewing it into an unsuspecting interlocutor's face I can simply link them here.<Sidenote>Maybe I can get business cards printed, or something.</Sidenote> Whereupon they can say "Thanks, that sounds interesting," and proceed to forget that it ever existed. So it's a win-win: I get to feel like I've Made A Difference, and they don't have to listen to a half-hour of only-marginally-interesting infosec jargon.

So.

## Password Strength

Everyone knows that the "best" password is at least 27 characters long and contains a mix of upper and lowercase letters, symbols, atomic symbols, and ancient Egyptian hieroglyphs. What may be slightly less known is exactly _why_ this is the recommended approach to picking passwords, and how the same goal might be accomplished by other, less eye-gougingly awful means.

So how do we measure the "strength" of a password? Ultimately, for the purposes of our discussion here, password strength comes down to one thing: How many tries<Sidenote>On average, that is. Obviously (especially with randomly-chosen passwords) the _exact_ number of tries is going to be somewhat random.</Sidenote> would it take for someone to guess this password? There are two ~~facets~~ to this question: 1) How many possible passwords are there (this is sometimes referred to as the "key space"), and 2) How likely is each of them to be the correct password?

The first of those questions is pretty easy to answer in the most basic sense: The number of possible passwords is the maximum password length, raised to the power of the number of possible characters. For instance, if the maximum password length is 16 characters, and the number of possible characters is 95<Sidenote>I.e. uppercase + lowercase + symbols.</Sidenote>, then the 

So what makes a "strong" password? Most people have a pretty good intuition for this, I think: A strong password is one that can't be easily guessed. The absolute _worst_ password is something that might be guessed by someone who knows nothing at all about you, such as `password` or `123456`<Sidenote>This is, in fact, the most common password (or was last I checked), according to [Pwned Passwords](https://haveibeenpwned.com/passwords).</Sidenote> Only slightly stronger is a password that's obvious to anyone who knows the slightest bit about its circumstances, such as your first name or the name of the site/service/etc. to which it logs you in.

Ok, so it's pretty clear what makes a _really_ bad password. But what about an only-sort-of-bad password? This is where intuition starts to veer off the rails a little bit, I think. The "guessability" of a password might be quantified as "how long, on average, would it take to guess"? Unfortuantely, the intuitive situation of "guessing" a password is pretty divergent from the reality of what a password cracker is actually doing when they try to crack passwords. Most people, based on the conversations I've had, envision "password guessing" as someone sitting at a computer, typing in potential passwords one by one. Or, maybe slightly more sophisticatedly, they imagine a computer firing off attempted logins from a list of potential passwords, but critically, _against the live system that is under attack._ This is a problem, because most password cracking (at least, the kind you have to worry about) _doesn't_ take place against live login pages. Instead, it happens in what's known as an "offline" attack, when the password cracker has managed to obtain a copy of the password database and starts testing various candidates against it. To explain this, though, we have to take a little detour into...

## Password storage

Unless the system in question is hopelessly insecure (and there are such systems; we'll talk about that in a bit) it doesn't store a copy of your password in plain text. Instead it stores what's called a _hash_, which is what you get when you run the password through a particular type of data-munging process called a _hashing algorithm_. A good password hashing algorithm has two key properties that make it perfect for this use case: It's _non-reversible_, and it's _computationally expensive_.

### One-way hashing

Suppose your password is `password`, and its hash is something like `X03MO1qnZdYdgyfeuILPmQ`. The non-reversibility of the hashing algorithm means that given the second value, there isn't any direct way to derive the first again. The only way to figure it out is to, essentially, guess-and-check against a list of potential candidate inputs. If that sounds a little bit like black magic, don't worry - I felt the same way when I first encountered the concept. How can a hash be irreversible _even if you know the algorithm_? 
