This is a port of booto's save_adjust stripped down to just signing a 3ds TAD's footer.bin. This required altering the code to work with sha256 instead of sha1, among other small changes. fyi - this was the code yellows8's ctr-dsiwaretool was based on originally. This code, however, doesn't use openssl which facilitates it running on a limited platform like the 3ds.
The app works by loading input files ctcert.bin (+ privkey) and footer.bin and it produces footer_signed.bin as output. All files are accessed on the 3ds sdmc root.
This was made to aid in the development of TADpole-3DS.