I've Just Launched "Pwned Passwords" V2 With Half a Billion Passwords for Download (via) Troy Hunt has collected 501,636,842 passwords from a wide collection of major breaches. He suggests using the to build a password strength checker that can say “your password has been used by 53,274 other people”. The full collection is available as a list of SHA1 codes (brute-force reversible but at least slightly obfuscated) in an 8GB file or as an API. Where things get really clever is the API design: you send just the first 5 characters of the SHA1 hash of the user’s password and the API responds with the full list of several hundred hashes that match that prefix. This lets you build a checking feature without sharing full passwords with a remote service, if you don’t want to host the full 8GB of data yourself.