and it also indicates which signals produced a click in his measurement apparatus. The raw key and the protocol information is sent to Alice and Bob’s classical post-processing units, CP$_{\rm A}$ and CP$_{\rm B}$, which are connected via an authenticated classical channel. These units generate a secret key, $k_{{\rm A}i}$ and $k_{{\rm B}i}$, by applying various post-processing techniques. In a memory attack, Eve hides a memory in say Alice’s module QKD$_{\rm A}$ to first store up the key material generated in each QKD run and then leak this information to her by hiding it in say the decision of abort or not abort of a subsequent QKD session. For example, if a particular bit value, say the $j$-th bit, of the key generated in the first QKD run is $0$, then this memory makes that a permuted $\sigma(j)$-th QKD run aborts. (Here, $\sigma$ is a permutation and $\sigma(j)$ is the permuted value of $j$.) This could be achieved, for instance, by outputting a raw key with a high quantum bit error rate (QBER). And, if the $j$-th bit value of the key is $1$, then the $\sigma(j) $-th QKD run does not abort. That is, by simply learning whether or 