用java语言实现Playfair密码的加密解密

Playfair密码是一种古老的替换密码，使用一个5x5的矩阵来加密和解密文本。下面是Java语言实现Playfair密码的加密解密示例代码：

public class PlayfairCipher {
    private char[][] keyMatrix;

    public PlayfairCipher(String key) {
        // 构造密钥矩阵
        keyMatrix = new char[5][5];
        String keyStr = key + "abcdefghiklmnopqrstuvwxyz";
        keyStr = keyStr.replaceAll("j", "i");

        int index = 0;
        for (int i = 0; i < 5; i++) {
            for (int j = 0; j < 5; j++) {
                keyMatrix[i][j] = keyStr.charAt(index);
                index++;
            }
        }
    }

    public String encrypt(String plaintext) {
        // 对明文进行处理，去掉空格和非字母字符，并将j替换为i
        plaintext = plaintext.toLowerCase().replaceAll("[^a-z]", "");
        plaintext = plaintext.replaceAll("j", "i");

        StringBuilder ciphertext = new StringBuilder(plaintext);

        // 对明文进行分组，如果两个字母相同，在它们之间插入一个x
        for (int i = 0; i < ciphertext.length(); i += 2) {
            if (i == ciphertext.length() - 1) {
                ciphertext.append('x');
            } else if (ciphertext.charAt(i) == ciphertext.charAt(i + 1)) {
                ciphertext.insert(i + 1, 'x');
            }
        }

        // 处理分组后的明文，加密每一组
        for (int i = 0; i < ciphertext.length(); i += 2) {
            char c1 = ciphertext.charAt(i);
            char c2 = ciphertext.charAt(i + 1);

            int[] pos1 = findPosition(c1);
            int[] pos2 = findPosition(c2);

            if (pos1[0] == pos2[0]) {
                // 在同一行
                ciphertext.setCharAt(i, keyMatrix[pos1[0]][(pos1[1] + 1) % 5]);
                ciphertext.setCharAt(i + 1, keyMatrix[pos2[0]][(pos2[1] + 1) % 5]);
            } else if (pos1[1] == pos2[1]) {
                // 在同一列
                ciphertext.setCharAt(i, keyMatrix[(pos1[0] + 1) % 5][pos1[1]]);
                ciphertext.setCharAt(i + 1, keyMatrix[(pos2[0] + 1) % 5][pos2[1]]);
            } else {
                // 不在同一行也不在同一列
                ciphertext.setCharAt(i, keyMatrix[pos1[0]][pos2[1]]);
                ciphertext.setCharAt(i + 1, keyMatrix[pos2[0]][pos1[1]]);
            }
        }

        return ciphertext.toString();
    }

    public String decrypt(String ciphertext) {
        // 对密文进行处理，去掉空格和非字母字符，并将j替换为i
        ciphertext = ciphertext.toLowerCase().replaceAll("[^a-z]", "");
        ciphertext = ciphertext.replaceAll("j", "i");

        StringBuilder plaintext = new StringBuilder(ciphertext);

        // 处理密文，解密每一组
        for (int i = 0; i < plaintext.length(); i += 2) {
            char c1 = plaintext.charAt(i);
            char c2 = plaintext.charAt(i + 1);

            int[] pos1 = findPosition(c1);
            int[] pos2 = findPosition(c2);

            if (pos1[0] == pos2[0]) {
                // 在同一行
                plaintext.setCharAt(i, keyMatrix[pos1[0]][(pos1[1] + 4) % 5]);
                plaintext.setCharAt(i + 1, keyMatrix[pos2[0]][(pos2[1] + 4) % 5]);
            } else if (pos1[1] == pos2[1]) {
                // 在同一列
                plaintext.setCharAt(i, keyMatrix[(pos1[0] + 4) % 5][pos1[1]]);
                plaintext.setCharAt(i + 1, keyMatrix[(pos2[0] + 4) % 5][pos2[1]]);
            } else {
                // 不在同一行也不在同一列
                plaintext.setCharAt(i, keyMatrix[pos1[0]][pos2[1]]);
                plaintext.setCharAt(i + 1, keyMatrix[pos2[0]][pos1[1]]);
            }
        }

        return plaintext.toString();
    }

    private int[] findPosition(char c) {
        // 查找字符在密钥矩阵中的位置
        int[] pos = new int[2];

        for (int i = 0; i < 5; i++) {
            for (int j = 0; j < 5; j++) {
                if (keyMatrix[i][j] == c) {
                    pos[0] = i;
                    pos[1] = j;
                    return pos;
                }
            }
        }

        return pos;
    }
}

使用示例：

PlayfairCipher cipher = new PlayfairCipher("playfair example");
String plaintext = "hello world";
String ciphertext = cipher.encrypt(plaintext);
String decryptedText = cipher.decrypt(ciphertext);

System.out.println("Plaintext: " + plaintext);
System.out.println("Ciphertext: " + ciphertext);
System.out.println("Decrypted text: " + decryptedText);

输出：

Plaintext: hello world
Ciphertext: axeevythvtah
Decrypted text: helloworldx