I am preparing for the midterm exam in my Data Security class so I am trying to read and understand the textbook's exercise questions. The sample solution at the end the textbook makes this problem more confusing instead of clarifying it. $\oplus$ denotes XOR operation but what are we trying to XOR with?
$c = \oplus c_i$ and $i \in I(c)$, but $c_i$ is getting XOR with what?
Any help would be appreciated.