Columnar Transposition Cipher Algorithm
The Columnar Transposition Cipher Algorithm is a classical encryption technique that involves systematically rearranging the plaintext characters in a predefined manner to form the ciphertext. This method is based on the concept of transposition, where the characters in the plaintext are reordered without altering their actual values. The algorithm is performed using a predetermined keyword or key phrase, which determines the order in which the columns of the plaintext are arranged. The length of the keyword corresponds to the number of columns in the transposition matrix, and the plaintext is written out in rows, filling the matrix column by column. Once the matrix is filled, the ciphertext is formed by reading the characters in the order of the keyword columns.
To decrypt the message, the recipient uses the same keyword to recreate the transposition matrix and reorder the columns back to their original arrangement. The plaintext can then be read row by row from the correctly ordered matrix. One of the main strengths of the Columnar Transposition Cipher is that it obscures the frequency distribution of characters in the plaintext, making it more challenging for cryptanalysts to break the code using frequency analysis. However, advanced cryptanalysis techniques can exploit the patterns formed by the transposition, and the cipher is considered relatively weak by modern standards. Despite its limitations, the Columnar Transposition Cipher provides an excellent introduction to the principles of symmetric encryption and the concept of transposition in cryptography.
package ciphers;
/**
* Columnar Transposition Cipher Encryption and Decryption.
*
* @author <a href="https://github.com/freitzzz">freitzzz</a>
*/
public class ColumnarTranspositionCipher {
private static String keyword;
private static Object[][] table;
private static String abecedarium;
public static final String ABECEDARIUM = "abcdefghijklmnopqrstuvwxyzABCDEFG"
+ "HIJKLMNOPQRSTUVWXYZ0123456789,.;:-@";
private static final String ENCRYPTION_FIELD = "≈";
private static final char ENCRYPTION_FIELD_CHAR = '≈';
/**
* Encrypts a certain String with the Columnar Transposition Cipher Rule
*
* @param word Word being encrypted
* @param keyword String with keyword being used
* @return a String with the word encrypted by the Columnar Transposition
* Cipher Rule
*/
public static String encrpyter(String word, String keyword) {
ColumnarTranspositionCipher.keyword = keyword;
abecedariumBuilder(500);
table = tableBuilder(word);
Object[][] sortedTable = sortTable(table);
String wordEncrypted = "";
for (int i = 0; i < sortedTable[i].length; i++) {
for (int j = 1; j < sortedTable.length; j++) {
wordEncrypted += sortedTable[j][i];
}
}
return wordEncrypted;
}
/**
* Encrypts a certain String with the Columnar Transposition Cipher Rule
*
* @param word Word being encrypted
* @param keyword String with keyword being used
* @param abecedarium String with the abecedarium being used. null for
* default one
* @return a String with the word encrypted by the Columnar Transposition
* Cipher Rule
*/
public static String encrpyter(String word, String keyword,
String abecedarium) {
ColumnarTranspositionCipher.keyword = keyword;
if (abecedarium != null) {
ColumnarTranspositionCipher.abecedarium = abecedarium;
} else {
ColumnarTranspositionCipher.abecedarium = ABECEDARIUM;
}
table = tableBuilder(word);
Object[][] sortedTable = sortTable(table);
String wordEncrypted = "";
for (int i = 0; i < sortedTable[0].length; i++) {
for (int j = 1; j < sortedTable.length; j++) {
wordEncrypted += sortedTable[j][i];
}
}
return wordEncrypted;
}
/**
* Decrypts a certain encrypted String with the Columnar Transposition
* Cipher Rule
*
* @return a String decrypted with the word encrypted by the Columnar
* Transposition Cipher Rule
*/
public static String decrypter() {
String wordDecrypted = "";
for (int i = 1; i < table.length; i++) {
for (Object item : table[i]) {
wordDecrypted += item;
}
}
return wordDecrypted.replaceAll(ENCRYPTION_FIELD, "");
}
/**
* Builds a table with the word to be encrypted in rows by the Columnar
* Transposition Cipher Rule
*
* @return An Object[][] with the word to be encrypted filled in rows and
* columns
*/
private static Object[][] tableBuilder(String word) {
Object[][] table = new Object[numberOfRows(word) + 1][keyword.length()];
char[] wordInChards = word.toCharArray();
//Fils in the respective numbers
table[0] = findElements();
int charElement = 0;
for (int i = 1; i < table.length; i++) {
for (int j = 0; j < table[i].length; j++) {
if (charElement < wordInChards.length) {
table[i][j] = wordInChards[charElement];
charElement++;
} else {
table[i][j] = ENCRYPTION_FIELD_CHAR;
}
}
}
return table;
}
/**
* Determines the number of rows the table should have regarding the
* Columnar Transposition Cipher Rule
*
* @return an int with the number of rows that the table should have in
* order to respect the Columnar Transposition Cipher Rule.
*/
private static int numberOfRows(String word) {
if (word.length() / keyword.length() > word.length() / keyword.length()) {
return (word.length() / keyword.length()) + 1;
} else {
return word.length() / keyword.length();
}
}
/**
*
* @return charValues
*/
private static Object[] findElements() {
Object[] charValues = new Object[keyword.length()];
for (int i = 0; i < charValues.length; i++) {
int charValueIndex = abecedarium.indexOf(keyword.charAt(i));
charValues[i] = charValueIndex > -1 ? charValueIndex : null;
}
return charValues;
}
/**
*
* @param table
* @return tableSorted
*/
private static Object[][] sortTable(Object[][] table) {
Object[][] tableSorted = new Object[table.length][table[0].length];
for (int i = 0; i < tableSorted.length; i++) {
System.arraycopy(table[i], 0, tableSorted[i], 0, tableSorted[i].length);
}
for (int i = 0; i < tableSorted[0].length; i++) {
for (int j = i + 1; j < tableSorted[0].length; j++) {
if ((int) tableSorted[0][i] > (int) table[0][j]) {
Object[] column = getColumn(tableSorted, tableSorted.length, i);
switchColumns(tableSorted, j, i, column);
}
}
}
return tableSorted;
}
/**
*
* @param table
* @param rows
* @param column
* @return columnArray
*/
private static Object[] getColumn(Object[][] table, int rows, int column) {
Object[] columnArray = new Object[rows];
for (int i = 0; i < rows; i++) {
columnArray[i] = table[i][column];
}
return columnArray;
}
/**
*
* @param table
* @param firstColumnIndex
* @param secondColumnIndex
* @param columnToSwitch
*/
private static void switchColumns(Object[][] table, int firstColumnIndex,
int secondColumnIndex, Object[] columnToSwitch) {
for (int i = 0; i < table.length; i++) {
table[i][secondColumnIndex] = table[i][firstColumnIndex];
table[i][firstColumnIndex] = columnToSwitch[i];
}
}
/**
* Creates an abecedarium with a specified ascii inded
*
* @param value Number of characters being used based on the ASCII Table
*/
private static void abecedariumBuilder(int value) {
abecedarium = "";
for (int i = 0; i < value; i++) {
abecedarium += (char) i;
}
}
private static void showTable() {
for (Object[] table1 : table) {
for (Object item : table1) {
System.out.print(item + " ");
}
System.out.println();
}
}
public static void main(String[] args) {
String keywordForExample = "asd215";
String wordBeingEncrypted = "This is a test of the Columnar Transposition Cipher";
System.out.println("### Example of Columnar Transposition Cipher ###\n");
System.out.println("Word being encryped ->>> " + wordBeingEncrypted);
System.out.println("Word encrypted ->>> " + ColumnarTranspositionCipher
.encrpyter(wordBeingEncrypted, keywordForExample));
System.out.println("Word decryped ->>> " + ColumnarTranspositionCipher
.decrypter());
System.out.println("\n### Encrypted Table ###");
showTable();
}
}
