B-6.2 Decryption

To recover the message $M$, it is sufficient to decrypt the last GLev ciphertext (the one encrypting $M$) using the secret $S$. Decrypting the other rows yields $-S_i\cdot M$, but recovering $M$ from these rows is only possible if $S_i$ is invertible (i.e., $S_i\ne 0$).