使用C语言实现仿射密码和Vigenere密码

仿射密码：

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int gcd(int a, int b) {
    if (b == 0) {
        return a;
    }
    return gcd(b, a % b);
}

int inverse(int a, int m) {
    a = a % m;
    for (int x = 1; x < m; x++) {
        if ((a * x) % m == 1) {
            return x;
        }
    }
}

void affine_encrypt(char* plaintext, int key_a, int key_b) {
    int len = strlen(plaintext);
    for (int i = 0; i < len; i++) {
        if (isalpha(plaintext[i])) {
            plaintext[i] = toupper(plaintext[i]);
            plaintext[i] = ((key_a * (plaintext[i] - 'A') + key_b) % 26) + 'A';
        }
    }
}

void affine_decrypt(char* ciphertext, int key_a, int key_b) {
    int len = strlen(ciphertext);
    int a_inverse = inverse(key_a, 26);
    for (int i = 0; i < len; i++) {
        if (isalpha(ciphertext[i])) {
            ciphertext[i] = toupper(ciphertext[i]);
            ciphertext[i] = (a_inverse * (ciphertext[i] - 'A' - key_b + 26)) % 26 + 'A';
        }
    }
}

int main() {
    char plaintext[] = "HELLO WORLD";
    int key_a = 5;
    int key_b = 8;

    printf("Plaintext: %s\n", plaintext);

    affine_encrypt(plaintext, key_a, key_b);
    printf("Encrypted Text: %s\n", plaintext);

    affine_decrypt(plaintext, key_a, key_b);
    printf("Decrypted Text: %s\n", plaintext);

    return 0;
}

Vigenere密码：

#include <stdio.h>
#include <string.h>
#include <ctype.h>

void vigenere_encrypt(char* plaintext, char* key) {
    int len = strlen(plaintext);
    int key_len = strlen(key);

    for (int i = 0, j = 0; i < len; i++, j++) {
        if (j == key_len) {
            j = 0;
        }
        if (isalpha(plaintext[i])) {
            plaintext[i] = toupper(plaintext[i]);
            key[j] = toupper(key[j]);
            plaintext[i] = ((plaintext[i] - 'A') + (key[j] - 'A')) % 26 + 'A';
        }
    }
}

void vigenere_decrypt(char* ciphertext, char* key) {
    int len = strlen(ciphertext);
    int key_len = strlen(key);

    for (int i = 0, j = 0; i < len; i++, j++) {
        if (j == key_len) {
            j = 0;
        }
        if (isalpha(ciphertext[i])) {
            ciphertext[i] = toupper(ciphertext[i]);
            key[j] = toupper(key[j]);
            ciphertext[i] = ((ciphertext[i] - 'A') - (key[j] - 'A') + 26) % 26 + 'A';
        }
    }
}

int main() {
    char plaintext[] = "HELLO WORLD";
    char key[] = "KEY";

    printf("Plaintext: %s\n", plaintext);

    vigenere_encrypt(plaintext, key);
    printf("Encrypted Text: %s\n", plaintext);

    vigenere_decrypt(plaintext, key);
    printf("Decrypted Text: %s\n", plaintext);

    return 0;
}