Hello, I tried to use your library but couldn't get it to work.
I'm migrating some Rust code to C++ (yeah, sad), so I had a reference to compare.

I found a coulple of bugs:

Since key is a string, key[j] returns a signed char, which breaks my keys that are arrays of unsigned chars. So I changed:

data = (data << 8) | (unsigned char)key[j];

imho the key should not be a string

The array should be indexed by k instead of j.

Sboxes in being indexed incorrectly in F:

I changed it to (like the reference):

a = x >> 24;
b = (x >> 16) & 0xFF;
c = (x >> 8) & 0xFF;
d = x & 0xFF;

After those changes, I get the same results as in Rust!
Hope this helps someone who comes across this library