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elrolfe's solution

to Rail Fence Cipher in the Java Track

Published at Nov 29 2018 · 0 comments
Instructions
Test suite
Solution

Implement encoding and decoding for the rail fence cipher.

The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded. It was already used by the ancient Greeks.

In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag). Finally the message is then read off in rows.

For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

Then reads off:

WECRLTEERDSOEEFEAOCAIVDEN

To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.

? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

The first row has seven spots that can be filled with "WECRLTE".

W . . . E . . . C . . . R . . . L . . . T . . . E
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Now the 2nd row takes "ERDSOEEFEAOC".

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Leaving "AIVDEN" for the last row.

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

If you now read along the zig-zag shape you can read the original message.

Running the tests

You can run all the tests for an exercise by entering

$ gradle test

in your terminal.

Source

Wikipedia https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

RailFenceCipherTest.java

import org.junit.Assert;
import org.junit.Ignore;
import org.junit.Test;

public class RailFenceCipherTest {

    private RailFenceCipher railFenceCipher;

    @Test
    public void encodeWithTwoRails() {
        railFenceCipher = new RailFenceCipher(2);
        Assert.assertEquals("XXXXXXXXXOOOOOOOOO",
            railFenceCipher.getEncryptedData("XOXOXOXOXOXOXOXOXO"));
    }

    @Ignore("Remove to run test")
    @Test
    public void encodeWithThreeRails() {
        railFenceCipher = new RailFenceCipher(3);
        Assert.assertEquals("WECRLTEERDSOEEFEAOCAIVDEN",
            railFenceCipher.getEncryptedData("WEAREDISCOVEREDFLEEATONCE"));
    }

    @Ignore("Remove to run test")
    @Test
    public void encodeWithEndingInTheMiddle() {
        railFenceCipher = new RailFenceCipher(4);
        Assert.assertEquals("ESXIEECSR",
            railFenceCipher.getEncryptedData("EXERCISES"));
    }

    @Ignore("Remove to run test")
    @Test
    public void decodeWithThreeRails() {
        railFenceCipher = new RailFenceCipher(3);
        Assert.assertEquals("THEDEVILISINTHEDETAILS",
            railFenceCipher.getDecryptedData("TEITELHDVLSNHDTISEIIEA"));
    }

    @Ignore("Remove to run test")
    @Test
    public void decodeWithFiveRails() {
        railFenceCipher = new RailFenceCipher(5);
        Assert.assertEquals("EXERCISMISAWESOME",
            railFenceCipher.getDecryptedData("EIEXMSMESAORIWSCE"));
    }

    @Ignore("Remove to run test")
    @Test
    public void decodeWithSixRails() {
        railFenceCipher = new RailFenceCipher(6);
        Assert.assertEquals("112358132134558914423337761098715972584418167651094617711286",
            railFenceCipher.getDecryptedData("133714114238148966225439541018335470986172518171757571896261"));
    }
}
public class RailFenceCipher {
    private final int rails;

    public RailFenceCipher(int rails) {
        this.rails = rails;
    }

    public String getEncryptedData(String text) {
        StringBuilder railBuilder[] = new StringBuilder[rails];
        for (int i = 0; i < rails; i++)
            railBuilder[i] = new StringBuilder();

        int railIndex = 0;
        int direction = -1;
        for (char c : text.toCharArray()) {
            railBuilder[railIndex].append(c);
            if (railIndex == 0 || railIndex == rails - 1) {
                direction *= -1;
            }
            railIndex += direction;
        }

        StringBuilder encryptedText = new StringBuilder();
        for (int i = 0; i < rails; i++)
            encryptedText.append(railBuilder[i]);

        return encryptedText.toString();
    }

    public String getDecryptedData(String text) {
        String railText[] = new String[rails];
        int charDivision = 2 + 2 * (rails - 2);
        int charsPerRail = text.length() / charDivision;
        int remainderChars = text.length() % charDivision;

        int textIndex = 0;
        for (int rail = 0; rail < rails; rail++) {
            int railCharCount;

            if (rail == 0 || rail == rails - 1) {
                railCharCount = charsPerRail + (remainderChars > rail ? 1 : 0);
            } else {
                railCharCount = charsPerRail * 2 + (remainderChars > rail ? 1 : 0) + (remainderChars > rails + (rails - 1 - rail) ? 1 : 0);
            }

            railText[rail] = text.substring(textIndex, textIndex + railCharCount);
            textIndex += railCharCount;
        }

        int outerIndex = -1;
        int innerIndex = -1;
        int direction = -1;
        int railIndex = 0;

        StringBuilder decodedText = new StringBuilder();
        while (decodedText.length() < text.length()) {
            if (railIndex == 0) {
                direction *= -1;
                outerIndex++;
                innerIndex++;
            }

            if (railIndex == rails - 1) {
                direction *= -1;
                innerIndex++;
            }

            decodedText.append(railText[railIndex].charAt(railIndex == 0 || railIndex == rails - 1 ? outerIndex : innerIndex));
            railIndex += direction;
        }

        return decodedText.toString();
    }
}

Community comments

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elrolfe's Reflection

The encoding isn't bad, but the whole thing seems very hacky to me, especially the decoding. It works though. I plan to revisit it later to see if I can clean it up.