20 character password
128.9 bits
20 characters from the full keyboard · about 10 diceware words · Symbolic.
Symbolic.
Twenty characters is 129 bits, which exceeds the key length of AES-128. If an attacker is getting in, brute force was not how.
How long it survives
There is no single answer, and the spread is the point: the same password falls in longer than the universe has existed, by a wide margin or holds for longer than the universe has existed, by a wide margin depending entirely on how the site stored it — which is not something you control.
| Attack | Guesses per second | Time |
|---|---|---|
| Online, rate limited Guessing against a live login that allows a hundred attempts a second — generous for a real service. | 1×10^2 | longer than the universe has existed, by a wide margin |
| Offline, slow hash A stolen database hashed with bcrypt or Argon2 at sensible parameters. | 1×10^4 | longer than the universe has existed, by a wide margin |
| Offline, fast hash A stolen database hashed with unsalted SHA-256 or MD5, attacked on GPUs. This is the case worth designing for. | 1×10^12 | longer than the universe has existed, by a wide margin |
Assuming half the keyspace on average, and that every character was chosen at random. A password you thought of yourself is worth a fraction of this.
Entropy by character set
Length is only half of it. What matters is length × the bits each character carries, and that depends on how many characters were possible.
| Character set | Bits each | 20 characters | Fast offline attack |
|---|---|---|---|
| lowercase only (26) | 4.70 | 94.0 bits | 316 million years |
| letters and digits (62) | 5.95 | 119.1 bits | longer than the universe has existed, by a wide margin |
| full keyboard (87) | 6.44 | 128.9 bits | longer than the universe has existed, by a wide margin |
What four more characters buy
Entropy is linear in length and the search space is exponential in entropy, so the step is not gentle. Going from 20 to 24 adds 25.8 bits — which multiplies the work by 57,289,761. Four keystrokes you will never type, because a password manager types them.
Or use words instead
This password is worth about 10 words from a diceware list. The equivalence is close enough to be worth memorising: two random characters ≈ one random word (12.89 bits against 12.92). A passphrase of the same strength is longer to write and far easier to type on a phone, which is the trade.
Nearby lengths
| Length | Entropy | Fast offline attack | Verdict |
|---|---|---|---|
| 16 | 103.1 bits | 171 billion years | The sensible default. |
| 18 | 116.0 bits | longer than the universe has existed, by a wide margin | Beyond the useful range. |
| 20 | 128.9 bits | longer than the universe has existed, by a wide margin | Symbolic. |
| 24 | 154.6 bits | longer than the universe has existed, by a wide margin | Well past any threat. |
Frequently asked questions
Is a 20 character password strong enough?
Symbolic. Twenty characters is 129 bits, which exceeds the key length of AES-128. If an attacker is getting in, brute force was not how.
How long does it take to crack a 20 character password?
It depends entirely on how it is stored. Against a fast unsalted hash on GPUs, longer than the universe has existed, by a wide margin. Against bcrypt, longer than the universe has existed, by a wide margin. Against a rate-limited login, longer than the universe has existed, by a wide margin. A single "time to crack" figure without that context is meaningless.
How many bits of entropy is a 20 character password?
128.9 bits if every character is chosen at random from the full keyboard — 26 lowercase, 26 uppercase, 10 digits and 25 symbols, which is 6.443 bits each. A password you invented yourself carries far less, because people do not choose randomly.
Is a 20 character password the same as a passphrase?
A 20-character random password is worth about 10 diceware words. Two random characters carry the same entropy as one word from a 7,776-word list — 12.89 bits against 12.92 — so the two approaches are interchangeable at equal strength, and the passphrase is easier to type.