El Gamal Signature: Attack to recover private key if $k$ is generated as $k_{i+1}=k_{i}+a$ where $a$ is a large unknown constant
Actually, you can recover $g^{-a}$.

You know that $r_i = g^{k_i} \mod p$ and $r_{i+1} = g^{k_i + a} \mod p = r_i \cdot g^a \bmod p$, hence $g^a = r_{i+1} \cdot r_i^{-1} \bmod p$; from there, computing $g^{-a}$ and $g^{-2a}$ is easy.

With that, the linear equations become easy to solve...