How long would your password take to crack?

When a service is breached, attackers rarely log in one guess at a time. They copy the stored password hashes and guess against them on their own hardware, as fast as it will go. Graphics cards can test billions of guesses a second against a fast hash. The demo on the Aranor page assumes 10 billion a second, and so does this note.

The arithmetic

A password chosen at random from a set of characters has a fixed number of possibilities. On average an attacker finds it after trying half of them.

Password	Possibilities	Average time at 10 billion guesses a second
8 random lowercase letters	2 × 10¹¹	about 10 seconds
8 random characters (letters, digits, symbols)	7 × 10¹⁵	about 4 days
12 random lowercase letters	1 × 10¹⁷	about 2 months
12 random characters	5 × 10²³	about 860,000 years
16 random characters	4 × 10³¹	about 70 trillion years
4 random words from a 7,776-word list	4 × 10¹⁵	about 2 days
6 random words from a 7,776-word list	2 × 10²³	about 350,000 years

Two things stand out. Each extra character multiplies the work, so length beats complexity: four more lowercase letters do more than swapping in symbols. And a passphrase of random words is strong only when it’s long enough and the words are picked at random, not by you.

The catch: people aren’t random

The table only holds for passwords nobody chose. Real passwords follow patterns: a word, a capital at the start, a year or 123 at the end, a ! to satisfy the rules. Attackers start with the billions of passwords already leaked, then try the patterns people use. A password like Summer2024! looks complex and falls almost immediately.

That’s why the demo on the Aranor page checks whether a password has appeared in a breach, and treats one that has as cracked instantly. It does that without revealing the password: it sends only the first five characters of the password’s SHA-1 hash to Have I Been Pwned, and compares the list that comes back on your device.

What the service can do

How fast attackers can guess also depends on how the password was stored. A fast hash lets them run billions of guesses a second. A deliberately slow, memory-hard function such as Argon2id makes each guess cost real time and memory, cutting that rate by many orders of magnitude. You can’t choose how a website stores your password, which is why the next point matters most.

What actually protects you

1. A different password for every account. A leak at one site then can’t open the others.
2. Long and random. Let a password manager generate them, 16 characters or more.
3. One strong master password you can remember, such as six or more random words, protecting the rest.
4. Check for leaks, and change any password that turns up in one.