## 1 Introduction  
A coding scheme basically replaces the symbols of an input sequence with their corresponding code-words. Such a scheme can be referred as ambiguous if it is not possible to uniquely decode the code-words back into the original data without using a disambiguation information. In this study, we study non-prefix-free (NPF) codes, where a code-word can be a prefix of other(s), and the ambiguity arises since the code-word boundaries cannot be determined on the code-word stream without explicit knowledge of the individual code-word lengths. Due to the lack of that unique decodability feature, NPF codes has received limited attention [1; 2; 3; 4; 5] in the related literature. The basic approach in dealing with the decodability problem of NPF schemes is to find an efficient way of representing the code-word boundaries. That had been studied in [5] by maintaining an additional compressed bit array supporting rank and select queries [6] to mark the code-word boundaries on the code-word stream. Similar approaches were also independently mentioned in [7; 8]. Yet another way is to create a wavelet tree [9] over the code-word lengths, which was studied in [5]as an alternative to maintaining a bit array. Empirical observations in [5] showed that actually NPF codes can provide compression ratios quite close to and sometimes better than the Huffman codes, while providing random access, which is a missing feature in ordinary Huffman codes, in constant or logarithmic time. Despite efforts to solve unique decodability of ambigious codes, in this study we focus on the privacy and security opportunities provided by the NPF codes. Achieving the security of massive data volumes with less encryption makes sense on platforms where the cost of encryption becomes hefty according to some metrics, e.g., time, memory, energy, or bandwidth. For example, in battery-constrained environments such as mobile devices [10], sensor networks [11], or unmanned aerial vehicles [12], performing less encryption may help to increase the battery life. It had been shown that symmetric security algorithms roughly doubles the energy consumption of normal operation in those environments, and asymmetric security algorithms increase the energy usage per bit in order of magnitudes (around 5 fold) [13]. Previously, selective encryption schemes [14] have been proposed to reduce the encryption load, particularly on transmission of video/image files [15; 16; 17]. In selective encryption, segments of the data, which are assumed to include important information, e.g., the I-frames in a video stream, are encrypted, while rest of the data is kept plain. We introduce an alternative approach to reduce the amount of encryption required to secure a source data. As opposed to the partial security provided by the selective encryption schemes, we observe that the intrinsic ambiguity of non-prefix-free (NPF) codes gives us an interesting opportunity for the privacy of the whole data. The main idea of the proposed technique here is to process the \(n\) bit long input bit sequence, which is assumed to be independently and identically distributed, in blocks of \(d\) bits according to a predetermined \(d\) parameter that is typically between 6 and 20. We create \(2^{d}\) non-prefix code-words by using a permutation of the numbers \([1\ldots 2^{d}]\), and then replace every \(d\)-bits long symbol in the input with its corresponding NPF code-word of varying bit-length in between 1 to \(d\). We call the resulting bit stream the _payload_ since it includes the actual content of the source. This sequence can not be decoded properly without the code-word boundaries due to the inherent ambiguity of the NPF codes. Therefore, we need to maintain an efficient representation of the code-word boundaries on the payload. We refer to that second bit stream as the _disambiguation information_ throughout the study.