Password Gen

What makes a password strong — entropy explained

Password strength is measured in bits of entropy, which represents how many guesses an attacker would need to try all possible passwords of the same length and character set. Entropy = log₂(character_set_size^password_length). A 12-character password using uppercase, lowercase, digits, and symbols (a character set of roughly 95) has about 78 bits of entropy — considered very strong.

To understand the scale: a modern GPU can attempt around 10–100 billion password guesses per second against a simple hash. At 100 billion guesses/second, a 60-bit entropy password would take about 33 years to crack on average. A 78-bit password would take longer than the age of the universe. Length matters more than complexity: a 16-character all-lowercase password has more entropy than a 10-character password with all character types.

The best passwords are long, random, and unique per site. A password manager generates and remembers them for you — you only need to remember one strong master password. Never reuse passwords: if one site is breached, credential stuffing attacks try those credentials against thousands of other sites automatically.

How this generator uses the Web Crypto API

This tool generates passwords using window.crypto.getRandomValues(), the browser's cryptographically secure pseudorandom number generator (CSPRNG). This is fundamentally different from Math.random(), which is a pseudorandom number generator (PRNG) with predictable output if the seed is known.

The Web Crypto API draws its randomness from the operating system's entropy pool, which is seeded by unpredictable physical events: mouse movements, keyboard timing, network packet arrival times, and hardware noise. This makes the output computationally indistinguishable from true randomness and safe for security-sensitive applications.

The generation process works by creating a typed array of random bytes, then mapping each byte to the character set you've selected using a rejection-sampling approach. Rejection sampling ensures that every character in your character set has exactly equal probability of being chosen — there is no bias toward characters that appear early in the set.

Password best practices — what security experts actually recommend

NIST (the US National Institute of Standards and Technology) published updated password guidelines in 2017 that overturned decades of conventional wisdom. The key findings:

Length beats complexity. A 15-character passphrase made of random words ("correct horse battery staple") is more secure and far more memorable than a shorter, symbol-heavy password like "P@ssw0rd!". Require length, not mandatory symbols.

Do not enforce frequent rotation. Forcing users to change passwords every 90 days leads to weak patterns (Password1, Password2, Password3). Only require a change if a breach is suspected.

Check against known-compromised lists. If a user chooses a password that appears in a data breach database (like haveibeenpwned.com's API), reject it regardless of how "complex" it looks. "P@ssw0rd!" appears in breach databases in exactly that form.

For practical use: generate a unique random password per site using a password manager. Use a long, memorable passphrase only for accounts you must type manually (like the password manager itself).

Passphrase vs random password — when each approach makes sense

Random character passwords ("k#9mPqLz4@w!") and passphrases ("correct-horse-battery-staple") achieve similar entropy at appropriate lengths but differ dramatically in memorability and typing ergonomics.

A random 16-character password drawn from a 95-character set has approximately 105 bits of entropy. A four-word passphrase drawn from the EFF's 7,776-word list (one roll of five dice per word) achieves about 77 bits of entropy — strong enough for almost every application, but significantly easier to type on a mobile keyboard and to read aloud to someone.

Use a random character password for any credential stored in a password manager — the manager types it for you, so memorability is irrelevant and maximum entropy is the goal. Use a passphrase for credentials you must type manually: the password manager's master password, disk encryption passwords, server login passwords typed into a terminal. The EFF diceware wordlist is available for download and produces passphrases specifically designed to avoid confusable words and short word combinations.

Never use a passphrase built from meaningful phrases (song lyrics, movie quotes, birth dates) — these are the first entries in targeted attacks. The randomness must come from a random source, not from human selection.