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@m0rjc
Last active December 19, 2015 22:08
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Code used trying to work out a cipher for Cyber Security Challenge
package m0rjc.test;
import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;
/**
* Decode the hex string from a Cyber Security Challenge question.
* This code is enough to get the job done, rather than something neat.
*
* @author Richard Corfield <m0rjc@m0rjc.me.uk>
*/
public class Decoding
{
/**
* Base64 decode result from http://www.base64decode.org/
*/
private static final String HEX = "53 23 e2 13 d2 77 a6 14 03 23 52 64 84 94 83 53 b4 84 73 03 23 52 a6 36 a6 e2 17 17 66 26 a6 77 e6 b6 d2 37 e6 26 84 53 52 66 97 66 94 03 23 52 37 47 e6 97 66 86 e6 17 57 57 64 84 53 52 15 37 47 97 17 e6 27 66 d4 84 53 52 64 33 52 84 03 23 52 a4 a5 15 74 d2 25 75 a4 95 d2 95 35 e4";
public static void main(final String[] args) throws UnsupportedEncodingException
{
// Initially I'd swapped nibbles, decoded then noted it had to be reversed.
// This way is shorter.
String input = swapNibblesAndDecodeHex();
System.out.println("Reverse, -> ASCII: " + input);
// First pass tried all values for n to see if any looked interesting,
// though given I needed to make %5H valid it needed to be a small back step.
String rot21 = rotn(input.toString(), 21);
System.out.println("ROT-21: " + rot21);
String urlDecoded = URLDecoder.decode(rot21);
System.out.println("URL Decoded: " + urlDecoded);
}
/**
* Given the constant HEX, swap the nibbles and convert to characters.
* @return ASCII text from the HEX data.
*/
private static String swapNibblesAndDecodeHex()
{
StringBuilder sb1 = new StringBuilder();
for(String hex : reverse(HEX).split(" "))
{
sb1.append((char)Integer.parseInt(hex, 16));
}
return sb1.toString();
}
/**
* Reverse a string.
* @param forward
* @return drawrof
*/
private static String reverse(final String forward)
{
StringBuilder reversed = new StringBuilder(forward.length());
for (char ch : forward.toCharArray())
{
reversed.insert(0, ch);
}
return reversed.toString();
}
/**
* Apply the "ROT-n" Ceaser Cipher to the given string. Only applies to alphabetical characters.
* @param input input text
* @param n amount of places to rotate
* @return decoded text.
*/
private static String rotn(final String input, final int n)
{
StringBuilder sb = new StringBuilder(input.length());
for (char ch : input.toCharArray())
{
if (Character.isLetter(ch))
{
if (Character.isUpperCase(ch))
{
ch = (char) (((ch - 'A' + n) % 26) + 'A');
}
else
{
ch = (char) (((ch - 'a' + n) % 26) + 'a');
}
}
sb.append(ch);
}
return sb.toString();
}
}
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