c'mon man. what is there to not understand?
you add two numbers and mod the result by a third one - which will give you a 256-bit/32-byte number. it wont be too long - if too short, just pad it with zeros on the first (msb) bytes.
i think it just cannot be more simple.
The "mod order" is not specified in
https://wiki.unsystem.net/index.php/DarkWallet/Stealth#Theory, I added it, and it worked (mod N, of the curve).
I then encourtered 1 case on 100 were it broke.
This was when d + c were less than 32 bytes, which made me unsure of my solution (mod N), that I found, by pure luck instead than comprehension of underlying math.
I then padded bytes, but did not work, the bug was because I was using little endian. (bouncy castle is using msb)
With msb, as you said, it works perfectly, so it responds to my question.
But the root on my problem was that I was unsure of my solution (using mod N in the private key calculation) since it seemed to broke randomly.
Then I asked the question.
Anyway, the Mod N + the padding msb did the trick and it works perfectly.
