Can anyone explain to me what the following means in relation to one time pad security? It's from the book 'Serious Cryptography: A Practical Introduction' and my logic knowledge seems to have lost me here so I'm struggling :-
Given a ciphertext $C_1 = E(K, P_1)$, it should be impossible to create another ciphertext, $C_2$, whose corresponding plaintext, $P_2$, is related to $P_1$ in a meaningful way (for example, to create a $P_2$ that is equal to $P_1 \oplus 1$ or to $P_1 \oplus X$ for some known value $X$). Surprisingly, the one-time pad is malleable: given a ciphertext $C_1 = P_1 ⊕ K$, you can define $C_2 = C_1 \oplus 1$, which is a valid ciphertext of $P_2 = P_1 ⊕ 1$ under the same key $K$. Oops, so much for our perfect cipher.