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<h1 class="page-title">Source: operations/Entropy.js</h1>
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<pre
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class="sunlight-highlight-javascript linenums">/**
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* Entropy operations.
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*
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* @author n1474335 [n1474335@gmail.com]
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* @copyright Crown Copyright 2016
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* @license Apache-2.0
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*
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* @namespace
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*/
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var Entropy = {
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/**
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* @constant
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* @default
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*/
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CHUNK_SIZE: 1000,
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/**
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* Entropy operation.
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*
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* @param {byte_array} input
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* @param {Object[]} args
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* @returns {html}
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*/
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run_entropy: function(input, args) {
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var chunk_size = args[0],
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output = "",
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entropy = Entropy._calc_entropy(input);
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output += "Shannon entropy: " + entropy + "\n" +
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"<br><canvas id='chart-area'></canvas><br>\n" +
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"- 0 represents no randomness (i.e. all the bytes in the data have the same value) whereas 8, the maximum, represents a completely random string.\n" +
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"- Standard English text usually falls somewhere between 3.5 and 5.\n" +
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"- Properly encrypted or compressed data of a reasonable length should have an entropy of over 7.5.\n\n" +
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"The following results show the entropy of chunks of the input data. Chunks with particularly high entropy could suggest encrypted or compressed sections.\n\n" +
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"<br><script>\
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var canvas = document.getElementById('chart-area'),\
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parent_rect = canvas.parentNode.getBoundingClientRect(),\
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entropy = " + entropy + ",\
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height = parent_rect.height * 0.25;\
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\
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canvas.width = parent_rect.width * 0.95;\
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canvas.height = height > 150 ? 150 : height;\
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\
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CanvasComponents.draw_scale_bar(canvas, entropy, 8, [\
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{\
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label: 'English text',\
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min: 3.5,\
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max: 5\
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},{\
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label: 'Encrypted/compressed',\
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min: 7.5,\
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max: 8\
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}\
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]);\
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</script>";
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var chunk_entropy = 0;
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if (chunk_size !== 0) {
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for (var i = 0; i < input.length; i += chunk_size) {
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chunk_entropy = Entropy._calc_entropy(input.slice(i, i+chunk_size));
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output += "Bytes " + i + " to " + (i+chunk_size) + ": " + chunk_entropy + "\n";
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}
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} else {
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output += "Chunk size cannot be 0.";
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}
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return output;
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},
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/**
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* @constant
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* @default
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*/
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FREQ_ZEROS: false,
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/**
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* Frequency distribution operation.
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*
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* @param {byte_array} input
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* @param {Object[]} args
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* @returns {html}
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*/
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run_freq_distrib: function (input, args) {
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if (!input.length) return "No data";
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var distrib = new Array(256),
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percentages = new Array(256),
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len = input.length,
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show_zeroes = args[0];
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// Initialise distrib to 0
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for (var i = 0; i < 256; i++) {
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distrib[i] = 0;
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}
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// Count bytes
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for (i = 0; i < len; i++) {
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distrib[input[i]]++;
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}
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// Calculate percentages
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var repr = 0;
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for (i = 0; i < 256; i++) {
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if (distrib[i] > 0) repr++;
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percentages[i] = distrib[i] / len * 100;
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}
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// Print
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var output = "<canvas id='chart-area'></canvas><br>" +
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"Total data length: " + len +
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"\nNumber of bytes represented: " + repr +
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"\nNumber of bytes not represented: " + (256-repr) +
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"\n\nByte Percentage\n" +
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"<script>\
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var canvas = document.getElementById('chart-area'),\
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parent_rect = canvas.parentNode.getBoundingClientRect(),\
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scores = " + JSON.stringify(percentages) + ";\
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\
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canvas.width = parent_rect.width * 0.95;\
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canvas.height = parent_rect.height * 0.9;\
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\
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CanvasComponents.draw_bar_chart(canvas, scores, 'Byte', 'Frequency %', 16, 6);\
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</script>";
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for (i = 0; i < 256; i++) {
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if (distrib[i] || show_zeroes) {
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output += " " + Utils.hex(i, 2) + " (" +
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Utils.pad_right(percentages[i].toFixed(2).replace(".00", "") + "%)", 8) +
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Array(Math.ceil(percentages[i])+1).join("|") + "\n";
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}
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}
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return output;
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},
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/**
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* Calculates the Shannon entropy for a given chunk of data.
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*
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* @private
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* @param {byte_array} data
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* @returns {number}
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*/
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_calc_entropy: function(data) {
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var prob = [],
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uniques = data.unique(),
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str = Utils.byte_array_to_chars(data);
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for (var i = 0; i < uniques.length; i++) {
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prob.push(str.count(Utils.chr(uniques[i])) / data.length);
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}
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var entropy = 0,
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p;
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for (i = 0; i < prob.length; i++) {
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p = prob[i];
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entropy += p * Math.log(p) / Math.log(2);
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}
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return -entropy;
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},
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};
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</pre>
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</article>
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</section>
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