You should read about the attacks tptacek mentioned. If IVs are predictable, it's the same impact as if you reuse the same IV, you just have to compute (m' xor

You should read about the attacks tptacek mentioned. If IVs are predictable, it's the same impact as if you reuse the same IV, you just have to compute (m' xor IV' xor IV) instead of just m' if IV' = IV Then in both cases you check if c' = c