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Add Playfair Cipher (TheAlgorithms#4988)
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‎DIRECTORY.md

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* [NonRepeatingNumberFinder](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/bitmanipulation/NonRepeatingNumberFinder.java)
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* [NumbersDifferentSigns](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/bitmanipulation/NumbersDifferentSigns.java)
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* [ReverseBits](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/bitmanipulation/ReverseBits.java)
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* [SetKthBit](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/bitmanipulation/SetKthBit.java)
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* [SingleBitOperations](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/bitmanipulation/SingleBitOperations.java)
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* ciphers
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* a5
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* [ColumnarTranspositionCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/ColumnarTranspositionCipher.java)
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* [DES](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/DES.java)
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* [HillCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/HillCipher.java)
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* [PlayfairCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/PlayfairCipher.java)
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* [Polybius](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/Polybius.java)
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* [ProductCipher](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/ProductCipher.java)
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* [RSA](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/ciphers/RSA.java)
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* [NonRepeatingNumberFinderTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/bitmanipulation/NonRepeatingNumberFinderTest.java)
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* [NumbersDifferentSignsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/bitmanipulation/NumbersDifferentSignsTest.java)
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* [ReverseBitsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/bitmanipulation/ReverseBitsTest.java)
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* [SetKthBitTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/bitmanipulation/SetKthBitTest.java)
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* [SingleBitOperationsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/bitmanipulation/SingleBitOperationsTest.java)
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* ciphers
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* a5
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* [LFSRTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/a5/LFSRTest.java)
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* [BlowfishTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/BlowfishTest.java)
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* [CaesarTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/CaesarTest.java)
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* [DESTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/DESTest.java)
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* [PlayfairTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/PlayfairTest.java)
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* [PolybiusTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/PolybiusTest.java)
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* [RSATest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/RSATest.java)
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* [SimpleSubCipherTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/ciphers/SimpleSubCipherTest.java)

‎src/main/java/com/thealgorithms/ciphers/PlayfairCipher.java

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package com.thealgorithms.ciphers;
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public class PlayfairCipher {
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private char[][] matrix;
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private String key;
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public PlayfairCipher(String key) {
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this.key = key;
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generateMatrix();
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}
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public String encrypt(String plaintext) {
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plaintext = prepareText(plaintext.replace("J", "I"));
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StringBuilder ciphertext = new StringBuilder();
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for (int i = 0; i < plaintext.length(); i += 2) {
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char char1 = plaintext.charAt(i);
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char char2 = plaintext.charAt(i + 1);
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int[] pos1 = findPosition(char1);
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int[] pos2 = findPosition(char2);
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int row1 = pos1[0];
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int col1 = pos1[1];
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int row2 = pos2[0];
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int col2 = pos2[1];
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if (row1 == row2) {
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ciphertext.append(matrix[row1][(col1 + 1) % 5]);
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ciphertext.append(matrix[row2][(col2 + 1) % 5]);
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} else if (col1 == col2) {
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ciphertext.append(matrix[(row1 + 1) % 5][col1]);
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ciphertext.append(matrix[(row2 + 1) % 5][col2]);
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} else {
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ciphertext.append(matrix[row1][col2]);
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ciphertext.append(matrix[row2][col1]);
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}
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}
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return ciphertext.toString();
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}
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public String decrypt(String ciphertext) {
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StringBuilder plaintext = new StringBuilder();
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for (int i = 0; i < ciphertext.length(); i += 2) {
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char char1 = ciphertext.charAt(i);
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char char2 = ciphertext.charAt(i + 1);
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int[] pos1 = findPosition(char1);
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int[] pos2 = findPosition(char2);
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int row1 = pos1[0];
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int col1 = pos1[1];
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int row2 = pos2[0];
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int col2 = pos2[1];
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if (row1 == row2) {
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plaintext.append(matrix[row1][(col1 + 4) % 5]);
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plaintext.append(matrix[row2][(col2 + 4) % 5]);
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} else if (col1 == col2) {
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plaintext.append(matrix[(row1 + 4) % 5][col1]);
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plaintext.append(matrix[(row2 + 4) % 5][col2]);
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} else {
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plaintext.append(matrix[row1][col2]);
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plaintext.append(matrix[row2][col1]);
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}
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}
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return plaintext.toString();
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}
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private void generateMatrix() {
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String keyWithoutDuplicates = removeDuplicateChars(key + "ABCDEFGHIKLMNOPQRSTUVWXYZ");
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matrix = new char[5][5];
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int index = 0;
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for (int i = 0; i < 5; i++) {
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for (int j = 0; j < 5; j++) {
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matrix[i][j] = keyWithoutDuplicates.charAt(index);
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index++;
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}
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}
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}
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private String removeDuplicateChars(String str) {
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StringBuilder result = new StringBuilder();
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for (int i = 0; i < str.length(); i++) {
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if (result.indexOf(String.valueOf(str.charAt(i))) == -1) {
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result.append(str.charAt(i));
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}
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}
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return result.toString();
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}
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private String prepareText(String text) {
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text = text.toUpperCase().replaceAll("[^A-Z]", "");
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StringBuilder preparedText = new StringBuilder();
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char prevChar = '\0';
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for (char c : text.toCharArray()) {
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if (c != prevChar) {
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preparedText.append(c);
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prevChar = c;
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} else {
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preparedText.append('X').append(c);
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prevChar = '\0';
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}
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}
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if (preparedText.length() % 2 != 0) {
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preparedText.append('X');
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}
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return preparedText.toString();
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}
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private int[] findPosition(char c) {
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int[] pos = new int[2];
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for (int i = 0; i < 5; i++) {
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for (int j = 0; j < 5; j++) {
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if (matrix[i][j] == c) {
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pos[0] = i;
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pos[1] = j;
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return pos;
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}
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}
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}
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return pos;
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}
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public void printMatrix() {
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System.out.println("\nPlayfair Cipher Matrix:");
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for (int i = 0; i < 5; i++) {
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for (int j = 0; j < 5; j++) {
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System.out.print(matrix[i][j] + " ");
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}
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System.out.println();
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}
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}
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}

‎src/test/java/com/thealgorithms/ciphers/PlayfairTest.java

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package com.thealgorithms.ciphers;
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import static org.junit.jupiter.api.Assertions.*;
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import org.junit.jupiter.api.Test;
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public class PlayfairTest {
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@Test
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public void testEncryption() {
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PlayfairCipher playfairCipher = new PlayfairCipher("KEYWORD");
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String plaintext = "HELLO";
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String encryptedText = playfairCipher.encrypt(plaintext);
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assertEquals("GYIZSC", encryptedText);
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}
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@Test
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public void testDecryption() {
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PlayfairCipher playfairCipher = new PlayfairCipher("KEYWORD");
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String encryptedText = "UDRIYP";
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String decryptedText = playfairCipher.decrypt(encryptedText);
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assertEquals("NEBFVH", decryptedText);
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}
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@Test
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public void testEncryptionAndDecryption() {
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PlayfairCipher playfairCipher = new PlayfairCipher("KEYWORD");
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String plaintext = "PLAYFAIR";
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String encryptedText = playfairCipher.encrypt(plaintext);
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String decryptedText = playfairCipher.decrypt(encryptedText);
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assertEquals(plaintext, decryptedText);
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}
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}

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