Lightweight multimedia encryption: Algorithms and performance analysis.  Page 43 
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2.6.2 Parsing Attack The parsing attack is a particular attack for the RHT encryption scheme. In standard Huffman decoding, there is only one possible way to decode the Huffmancoded bit stream due to the unique decodability property of the Huffman code. However, to decode an RHT encrypted bit stream, the correct KHS is needed. Thus, a proper Huffman table is used to parse the bit stream at each step. If a wrong KHS is used, we may end up with several remaining bits that cannot be parsed exactly at the end. A cryptanalyst can exploit this characteristic to determine whether a KHS is correct or not. For a general Nbit RHTencrypted bit stream C, a particular parsing is called a viable parsing scheme if C can be exactly parsed without leaving any bits at the end. The corre sponding KHS associated with a viable parsing scheme is called the viable KHS. Decoding C using the correct KHS is a viable parsing scheme. The question is whether there exist other viable parsing schemes that can parse C exactly. If the answer is positive, the next question is how many of them? To address this problem, we first define the following concept. Definition 3 Let C be an Nbit RHTencrypted bit stream. The viable kparsing scheme of C is defined as a parsing scheme that can exactly parse the first k bits of C. We use V (k) to denote the total number of viable kparsing schemes of C. If V (k) ≥ 1, we call k a parsable length, i.e., there exists at least one vialbe kparsing scheme. By parsing bit stream C using all possible ways and recording the parsed code length, a cryptanalyst can gradually compute values of all V (k) for 1 ≤ k ≤ N. The detailed parsing attack is described below: Parsing Attack 1. To initialize, we set all V (k) = 0, 1 ≤ k ≤ N and V P to the empty set ∅. 33
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Title  Lightweight multimedia encryption: Algorithms and performance analysis.  Page 43 
Repository email  cisadmin@lib.usc.edu 
Full text  2.6.2 Parsing Attack The parsing attack is a particular attack for the RHT encryption scheme. In standard Huffman decoding, there is only one possible way to decode the Huffmancoded bit stream due to the unique decodability property of the Huffman code. However, to decode an RHT encrypted bit stream, the correct KHS is needed. Thus, a proper Huffman table is used to parse the bit stream at each step. If a wrong KHS is used, we may end up with several remaining bits that cannot be parsed exactly at the end. A cryptanalyst can exploit this characteristic to determine whether a KHS is correct or not. For a general Nbit RHTencrypted bit stream C, a particular parsing is called a viable parsing scheme if C can be exactly parsed without leaving any bits at the end. The corre sponding KHS associated with a viable parsing scheme is called the viable KHS. Decoding C using the correct KHS is a viable parsing scheme. The question is whether there exist other viable parsing schemes that can parse C exactly. If the answer is positive, the next question is how many of them? To address this problem, we first define the following concept. Definition 3 Let C be an Nbit RHTencrypted bit stream. The viable kparsing scheme of C is defined as a parsing scheme that can exactly parse the first k bits of C. We use V (k) to denote the total number of viable kparsing schemes of C. If V (k) ≥ 1, we call k a parsable length, i.e., there exists at least one vialbe kparsing scheme. By parsing bit stream C using all possible ways and recording the parsed code length, a cryptanalyst can gradually compute values of all V (k) for 1 ≤ k ≤ N. The detailed parsing attack is described below: Parsing Attack 1. To initialize, we set all V (k) = 0, 1 ≤ k ≤ N and V P to the empty set ∅. 33 