Greetings.

I am having problems with john the ripper (first time I try it). I have an excercise where I am supposed to crack this: user:NTUzNTM2MDA=. But when I ran:

john file.txt
john -single file.txt
john -wordlistassword.lst file.txt

All of them say Loaded 0 passwords, exiting...

I don't know what is the problem and I don't find any answers to this situation in the net. Please help.