encryption - How do I calculate a password space if the password only has eight alphabetic characters (not case sensitive) - Cryptography Stack Exchange
Password strength - Wikipedia
Shannon Entropy Calculator | Information Theory
All About Password Entropy - Specops Software
Defending Against Password Cracking: Understanding The Math
GitHub - alchimya/iOS-Password-Entropy: A basic implementation (Swift and Objective-C) of the Password Entropy, that allows to evaluate how much secure and strong is a password evaluating also an hypothetical hacking attack.
Password Entropy Calculator by Tim Cutting - Web Developer, Colchester.
Updated table on time to brute force passwords : r/Infographics