///Hàm mã hóa Affine (Affine Encryption)

/// e(x) = (ax + b)(mod m). Note: a & m là số nguyên tố cùng nhau.

public static string AffineEncrypt(string plainText, int a, int b){

string cipherText = "";

char[] chars = plainText.ToUpper().ToCharArray();

foreach (char c in chars){

      int x = Convert.ToInt32(c - 65);

      cipherText += Convert.ToChar((( a * x + b ) % 26) + 65);

}

return cipherText;

}

/// Hàm giải mã Affine (Affine Decryption)

/// d(x) = aInverse * (e(x) − b)(mod m).

public static string AffineDecrypt(string cipherText, int a, int b){

string plainText = "";

int aInverse = MultiplicativeInverse(a); 

char[] chars = cipherText.ToUpper().ToCharArray();

foreach (char c in chars){

    int x = Convert.ToInt32(c - 65);

    if (x - b < 0) x = Convert.ToInt32(x) + 26;

    plainText += Convert.ToChar(((aInverse * (x - b)) % 26) + 65);

} 

    return plainText;

}

public static int MultiplicativeInverse(int a){

for (int x = 1; x < 27; x++){

    if ((a * x) % 26 == 1)

    return x;

}

throw new Exception("No multiplicative inverse found!");

}

Tag: C, C++, C#, mã hóa cổ điển, Affine, Affine Cipher, an toàn, bảo mật, Security