Lookup in pairs (username/password)

First of all I've read most records concerning "lookup in pairs". But I just can not understand it, call me stupid, but i can't.

some of the records i have read:

http://appinventor.mit.edu/explore/content/list-blocks.html

https://groups.google.com/forum/#!msg/programming-with-app-inventor/3dj2a2LeIYU/qZOSEwsY96UJ

https://eixerits.wordpress.com/2013/02/03/a-login-template/

and some Pura Vida apps.

I've done some research to find a easy way to encrypt my app. Now I have decided to go for list feature. I got a list of valid usernames and a list of vaild passwords. Then i want to cross check the username and password. I understand that the best way to do this is to use list feature "lookup in pairs". 

But i dont understand how the pairing works. If i insert a word in key, do it look for every index or only the same index as the word mentioned?

at http://appinventor.mit.edu/explore/content/list-blocks.html they say we only can use a list of 2, is it right?

If you guys got any better way to encrypt the app with username and passowrd pls share :). the main thing is that i need to approve or control how got access

Thanks in advance for any replies

How does the lookup in pairs block work?

alternatively just define 2 lists, list1 with the logins and list2 with the passwords

then just check, if the given login and password correspond using the same selection index of the lists