So basically you are trying to bruteforce all possible permutations with repetitions of 20 chars out of 64 (also called partial permutations, variations, arrangements)? these are 64**20 words (1329227995784915872903807060280344576)
If you can't explain it to a six year old, you don't understand it yourself, Albert Einstein
How to Ask Questions The Smart Way: link and another link
Create MCV example
Debug small programs
How to Ask Questions The Smart Way: link and another link
Create MCV example
Debug small programs