The application of this for Bitcoin could be to code your wallet seed phrase in this way by choosing a book and finding those seed words in the book and converting them to 5 digit numbers.
Without knowing from which book and the exact print edition (your key), your seed phrase is reasonably safe.
The cryptographers here will be able to punch holes in this but for good enough protection for regular folk it could work quite well.
Yes, they will. So
don’t do it.No, it is not “good enough protection for regular folk”. Not when in the real world—
not in theory, but in practice—there are blackhats doing a batch offline attacks using any text corpus you can think of.