## 1 Introduction  
The main goal of quantum key distribution (QKD) [1, 2, 3] is to generate a secret key between two remote users ([PERSON] and [PERSON]), with the security of the key not based on computational assumptions on a potential eavesdropper ([PERSON]). The first ever QKD protocol, BB84, was proposed by [PERSON] and [PERSON] [4]. In the protocol, the legitimate sender, [PERSON], encodes random bit string into polarization states of single photons and sends them to the legitimate receiver, [PERSON]. For the encoding purposes, [PERSON] randomly chooses one of two orthogonal polarization bases, while all the four states form a non-orthogonal set. [PERSON] uses a random basis guess when conducting his measurement. The bit values corresponding to wrong guesses on [PERSON]'s side are discarded after a round of classical communication, which provides an advantage to the legitimate users.  
The security of quantum key distribution is based on the indistiguishability of the states available to the eavesdropper. For QKD protocols, any eavesdropping attempt leads to a disturbance of the states at the receiver side, and the value of the disturbance in the observed parameters allows the legitimate users to estimate the quantum states of [PERSON] and hence bound her information. When this information is below the information available to the legitimate users, they can use classical post-processing methods to distill a secret key.  
Along with QKD, an adjacent technology of quantum data locking (QDL) is of interest. In the QDL scenario, a potential adversary does not have enough data to perform a correct measurement of the available quantum states, while the total extractable information may be relatively high. Hence, a relatively short amount of classical data can lock a large amount of information. The first QDL protocol was proposed in [5] and resembles BB84, as the same two bases are used. Now, there is a single pre-shared bit that specifies the basis choice for each bit of [PERSON]'s string and for [PERSON]'s measurement at every position. As was shown in [5], the eavesdropper who has intercepted \(N\) signals obtains no more than \(N/2\) bits of information; hence, the single secret bit is sufficient for \"locking\" \(N/2\) bits of classical information. Later, other QDL protocols based on different principles were introduced [6, 7].  
Here, we propose a method that can increase the secret key rate in QKD by simple additional actions of the legitimate users, namely the encryption of information that is disclosed during error correction. Encryption of the postprocessing data was also used in [8] to simplify the security proof, but here, we discuss its usage for a different purpose--for a QDL-like technique that does not allow an eavesdropper to perform the best possible measurement.  
The paper is organized as follows. In Section 2, we recall the main stages of prepare-and-measure QKD protocols and discuss various types of eavesdropping attacks. Section 3 is devoted to the condition sufficient for quantum accessible information additivity and its application to QKD in a QDL scenario. Section 4 addresses the case of the most general eavesdropping attacks and the limitations of the method we propose. Finally, we discuss the results in Section 5.