vigenere密码算法java

以下是 Vigenere 密码算法的 Java 实现：

public class VigenereCipher {
    private String key;

    public VigenereCipher(String key) {
        this.key = key;
    }

    public String encrypt(String plaintext) {
        String ciphertext = "";
        plaintext = plaintext.toUpperCase();
        for (int i = 0, j = 0; i < plaintext.length(); i++) {
            char c = plaintext.charAt(i);
            if (c < 'A' || c > 'Z') {
                continue;
            }
            ciphertext += (char) ((c + key.charAt(j) - 2 * 'A') % 26 + 'A');
            j = ++j % key.length();
        }
        return ciphertext;
    }

    public String decrypt(String ciphertext) {
        String plaintext = "";
        ciphertext = ciphertext.toUpperCase();
        for (int i = 0, j = 0; i < ciphertext.length(); i++) {
            char c = ciphertext.charAt(i);
            if (c < 'A' || c > 'Z') {
                continue;
            }
            plaintext += (char) ((c - key.charAt(j) + 26) % 26 + 'A');
            j = ++j % key.length();
        }
        return plaintext;
    }

    public static void main(String[] args) {
        VigenereCipher cipher = new VigenereCipher("SECRETKEY");
        String plaintext = "HELLO WORLD";
        String ciphertext = cipher.encrypt(plaintext);
        System.out.println(ciphertext);
        System.out.println(cipher.decrypt(ciphertext));
    }
}

其中，构造函数 `VigenereCipher(String key)` 接受一个密钥，`encrypt(String plaintext)` 方法将明文加密为密文，`decrypt(String ciphertext)` 方法将密文解密为明文。在加密和解密过程中，只对大写字母进行操作，忽略其他字符。