Leetspeak
Leetspeak. A classical cipher, computed on your device.
Runs on your device. The file is never uploaded.
Leetspeak swaps six letters of your text for digits: a becomes 4, e 3, i 1, o 0, s 5 and t 7, in either case. Every other character is left as typed. No decoder ships with it, and the swap cannot be undone exactly, because a 1 in the output might have started life as a digit.
Questions
Which letters get replaced?
Six of them: a becomes 4, e becomes 3, i becomes 1, o becomes 0, s becomes 5 and t becomes 7. Both cases are matched. Every other character is left alone.
How do I convert it back?
By hand, or with Find and replace. There is no decoder here, and one cannot be exact: a 1 in the output might have been an i or might have been a digit in your original text.
Does this make a password stronger?
No. Every password cracker applies these substitutions by default, so a leetspeak version of a dictionary word is treated as that dictionary word. Use Passphrase generate instead.
Why did some letters stay normal?
Only those six have a replacement in this tool. Longer leetspeak tables exist and vary by community, so two tools rarely produce identical output.