Safely checks a password to see if it exists on the haveibeenpwned.com database of previously breached passwords - this includes nearly 5 billion cracked passwords. If your password appears in this list, then you should change it.
By playing around with Password123 app for a while, you will very soon realise how many people choose the same password, humans are very bad at being unpredictable sometimes. How many people thought they were being original when they chose the password "letmein"? Over 184 thousand(!) people.
Try searching for naughty words; embarrassingly personal phrases; famous people; ilove[insert your name] is a fun one.
The app searches an offline database of the most common 10,000 passwords, before going online so it still works partially if you have no internet connection.
This app uses the k-Anonymity API from https://haveibeenpwned.com/API/v2#PwnedPasswords so it never shares the actual passwords you type in, the first few characters of the password hash are the only thing to leave your device (and nothing at all if there is a match in the offline 10,000 list)
The full Kotlin source code of this app is available under the Apache 2 licence here: https://github.com/erdo/password123