# How complex must round constants be to resist slide attacks?

A key schedule that generates round keys by XORing a round constant with the key is linear and can be vulnerable to related key attacks, but let's ignore that for now. Constants are necessary to avoid the slide attack. How complex do these constants need to be and how much must they differ from each other?

For example, would a trivial key schedule composed of $$r_i = k \oplus c_i$$ to generate the $$i^\text{th}$$ round key $$r_i$$ using round constant $$c_i$$ and secret key $$k$$ be weak to any form of cryptanalysis if $$c_i = i$$?