c代码实现密码学仿射变换的加解密

加密：

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

int main() {
    char plain_text[100], cipher_text[100];
    int key_a, key_b, i;

    printf("Enter the plaintext: ");
    gets(plain_text);

    printf("Enter key a: ");
    scanf("%d", &key_a);

    printf("Enter key b: ");
    scanf("%d", &key_b);

    for (i = 0; plain_text[i] != '\0'; i++) {
        if (isalpha(plain_text[i])) {
            if (isupper(plain_text[i])) {
                cipher_text[i] = ((key_a * (plain_text[i] - 'A') + key_b) % 26) + 'A';
            } else {
                cipher_text[i] = ((key_a * (plain_text[i] - 'a') + key_b) % 26) + 'a';
            }
        } else {
            cipher_text[i] = plain_text[i];
        }
    }

    cipher_text[i] = '\0';

    printf("Cipher text: %s\n", cipher_text);

    return 0;
}

解密：

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

int main() {
    char plain_text[100], cipher_text[100];
    int key_a, key_b, i, multiplicative_inverse;

    printf("Enter the ciphertext: ");
    gets(cipher_text);

    printf("Enter key a: ");
    scanf("%d", &key_a);

    printf("Enter key b: ");
    scanf("%d", &key_b);

    for (i = 0; cipher_text[i] != '\0'; i++) {
        if (isalpha(cipher_text[i])) {
            if (isupper(cipher_text[i])) {
                multiplicative_inverse = 1;
                while ((key_a * multiplicative_inverse) % 26 != 1) {
                    multiplicative_inverse++;
                }
                plain_text[i] = ((multiplicative_inverse * (cipher_text[i] - 'A' - key_b + 26)) % 26) + 'A';
            } else {
                multiplicative_inverse = 1;
                while ((key_a * multiplicative_inverse) % 26 != 1) {
                    multiplicative_inverse++;
                }
                plain_text[i] = ((multiplicative_inverse * (cipher_text[i] - 'a' - key_b + 26)) % 26) + 'a';
            }
        } else {
            plain_text[i] = cipher_text[i];
        }
    }

    plain_text[i] = '\0';

    printf("Plain text: %s\n", plain_text);

    return 0;
}