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POSimple encryption algorithm for homework. not getting decryption working properly
 

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  1. POSimple encryption algorithm for homework. not getting decryption working properly
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    <p>This is a homework question that I can't get my head around at all</p> <p>Its a very simple encryption algorithm. You start with a string of characters as your alphabet:</p> <p><code>ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!, .</code></p> <p>Then ask the user to enter there own string that will act as a <code>map</code> such as:</p> <p><code>0987654321! .,POIUYTREWQASDFGHJKLMNBVCXZ</code></p> <p>Then the program uses this to make a <code>map</code> and allows you to enter text that gets encrypted.</p> <p>For example <code>MY NAME IS JOSEPH</code> would be encrypted as <code>.AX,0.6X2YX1PY6O3</code></p> <p>This is all very easy, however he said that its a one to one mapping and thus implied that if I enter <code>.AX,0.6X2YX1PY6O3</code> back into the program I will get out <code>MY NAME IS JOSEPH</code></p> <p>This doesn't happen, because <code>.AX,0.6X2YX1PY6O3</code> becomes <code>Z0QCDZQGAQFOALDH</code></p> <p>The mapping only works to decrypt when you go backwards but the question implies that the program just loops and runs the one algorithm every time. </p> <p>Even if some could say that it is possible I would be happy, I have pages and pages of paper filled up with possible workings, but I came up with nothing, the only solution to run the algorithm backwards back I don't think we are allowed to do that.</p> <p>Any ideas?</p> <p>Edit:</p> <p>Unfortunately I can't get this to work (Using the orbit computation idea) What am I doing wrong?</p> <pre><code>//import scanner class import java.util.Scanner; public class Encryption { static Scanner inputString = new Scanner(System.in); //define alphabet private static String alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!, ."; private static String map; private static int[] encryptionMap = new int[40];//mapping int array private static boolean exit = false; private static boolean valid = true; public static void main(String[] args) { String encrypt, userInput; userInput = new String(); System.out.println("This program takes a large reordered string"); System.out.println("and uses it to encrypt your data"); System.out.println("Please enter a mapping string of 40 length and the same characters as below but in different order:"); System.out.println(alpha); //getMap();//don't get user input for map, for testing! map=".ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!, ";//forced input for testing only! do{ if (valid == true){ System.out.println("Enter Q to quit, otherwise enter a string:"); userInput = getInput(); if (userInput.charAt(0) != 'Q' ){//&amp;&amp; userInput.length()&lt;2){ encrypt = encrypt(userInput); for (int x=0; x&lt;39; x++){//here I am trying to get the orbit computation going encrypt = encrypt(encrypt); } System.out.println("You entered: "+userInput); System.out.println("Encrypted Version: "+encrypt); }else if (userInput.charAt(0) == 'Q'){//&amp;&amp; userInput.length()&lt;2){ exit = true; } } else if (valid == false){ System.out.println("Error, your string for mapping is incorrect"); valid = true;//reset condition to repeat } }while(exit == false); System.out.println("Good bye"); } static String encrypt(String userInput){ //use mapping array to encypt data String encrypt; StringBuffer tmp = new StringBuffer(); char current; int alphaPosition; int temp; //run through the user string for (int x=0; x&lt;userInput.length(); x++){ //get character current = userInput.charAt(x); //get location of current character in alphabet alphaPosition = alpha.indexOf(current); //encryptionMap.charAt(alphaPosition) tmp.append(map.charAt(alphaPosition)); } encrypt = tmp.toString(); return(encrypt); } static void getMap(){ //get a mapping string and validate from the user map = getInput(); //validate code if (map.length() != 40){ valid = false; } else{ for (int x=0; x&lt;40; x++){ if (map.indexOf(alpha.charAt(x)) == -1){ valid = false; } } } if (valid == true){ for (int x=0; x&lt;40; x++){ int a = (int)(alpha.charAt(x)); int y = (int)( map.charAt(x)); //create encryption map encryptionMap[x]=(a-y); } } } static String getInput(){ //get input(this repeats) String input = inputString.nextLine(); input = input.toUpperCase(); if ("QUIT".equals(input) || "END".equals(input) || "NO".equals(input) || "N".equals(input)){ StringBuffer tmp = new StringBuffer(); tmp.append('Q'); input = tmp.toString(); } return(input); } } </code></pre>
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