如何在 JAVA 中编写仿射密码解密的逻辑？

Question

您好，我正在研究 JAVA 中的仿射密码。我已经成功编写了加密代码，但现在我对解密的逻辑一无所知。

以下是我的加密逻辑：

void encryption()
{
    char character; 
    int plainTextLength=input.length();
    int a=1,b=2,c=3,d=4,e=5,f=6,g=7,h=8,i=9,j=10,k=11,l=12,m=13,n=14,
        o=15,p=16,q=17,r=18,s=19,t=20,u=21,v=22,w=23,x=24,y=25,z=26;

    System.out.print("Cipher text is:" );

    for (int in = 0; in < plainTextLength; in++)
    {
        character = input.charAt(in);

        if (Character.isLetter(character))
        {
            character = (char)((firstKey*(character - 'a') + secondKey) % 26 + 'a');
        }
        System.out.print(character);
    }
    System.out.println();       
}

这是我的加密逻辑： 字符 = (char)((firstKey*(character - 'a') + secondKey) % 26 + 'a');

什么是解密逻辑。我完全糊涂了？

Answer 1

通用仿射密码解密公式很简单：

其中 a 是您的 firstKey，b 是您的 secondKey。

所以加密/解密可以通过以下方式实现：

private static int firstKey = 5;
private static int secondKey = 19;
private static int module = 26;

public static void main(String[] args) {
    String input = "abcdefghijklmnopqrstuvwxyz";
    String cipher = encrypt(input);
    String deciphered = decrypt(cipher);
    System.out.println("Source:    " + input);
    System.out.println("Encrypted: " + cipher);
    System.out.println("Decrypted: " + deciphered);
}

static String encrypt(String input) {
    StringBuilder builder = new StringBuilder();
    for (int in = 0; in < input.length(); in++) {
        char character = input.charAt(in);
        if (Character.isLetter(character)) {
            character = (char) ((firstKey * (character - 'a') + secondKey) % module + 'a');
        }
        builder.append(character);
    }
    return builder.toString();
}

static String decrypt(String input) {
    StringBuilder builder = new StringBuilder();
    // compute firstKey^-1 aka "modular inverse"
    BigInteger inverse = BigInteger.valueOf(firstKey).modInverse(BigInteger.valueOf(module));
    // perform actual decryption
    for (int in = 0; in < input.length(); in++) {
        char character = input.charAt(in);
        if (Character.isLetter(character)) {
            int decoded = inverse.intValue() * (character - 'a' - secondKey + module);
            character = (char) (decoded % module + 'a');
        }
        builder.append(character);
    }
    return builder.toString();
}

输出：

Source:    abcdefghijklmnopqrstuvwxyz
Encrypted: dinsxchmrwbglqvafkpuzejoty
Decrypted: abcdefghijklmnopqrstuvwxyz