维吉尼亚密码破译c语言

维吉尼亚密码是一种基于多表代换加密的密码算法，破译起来比较困难。下面是一个简单的C语言程序，可以对维吉尼亚密码进行破译：

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

#define ALPHABET_SIZE 26

int char_to_index(char c) {
    if (c >= 'a' && c <= 'z') {
        return c - 'a';
    } else if (c >= 'A' && c <= 'Z') {
        return c - 'A';
    } else {
        return -1;
    }
}

char index_to_char(int index) {
    return index + 'a';
}

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

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("Usage: %s ciphertext\n", argv[0]);
        return 1;
    }

    char *ciphertext = argv[1];
    int ciphertext_len = strlen(ciphertext);

    int key_len = 1;
    int max_key_len = 10;
    int *key = malloc(key_len * sizeof(int));

    int i, j, k;
    int freq[ALPHABET_SIZE];
    int max_freq_index, max_freq_shift;
    int shift, index;
    int *shifts = malloc(max_key_len * sizeof(int));
    int *plaintext = malloc(ciphertext_len * sizeof(int));

    for (i = 0; i < max_key_len; i++) {
        shifts[i] = 0;
    }

    while (key_len <= max_key_len) {
        for (i = 0; i < key_len; i++) {
            for (j = 0; j < ALPHABET_SIZE; j++) {
                freq[j] = 0;
            }
            for (j = i; j < ciphertext_len; j += key_len) {
                index = char_to_index(ciphertext[j]);
                if (index >= 0) {
                    freq[index]++;
                }
            }
            max_freq_index = 0;
            for (j = 1; j < ALPHABET_SIZE; j++) {
                if (freq[j] > freq[max_freq_index]) {
                    max_freq_index = j;
                }
            }
            max_freq_shift = (max_freq_index - ('e' - 'a') + ALPHABET_SIZE) % ALPHABET_SIZE;
            key[i] = max_freq_shift;
        }

        for (k = 0; k < key_len; k++) {
            shifts[k] = 0;
            for (i = 0; i < ciphertext_len / key_len; i++) {
                index = char_to_index(ciphertext[i * key_len + k]);
                if (index >= 0) {
                    shift = (index - key[k] + ALPHABET_SIZE) % ALPHABET_SIZE;
                    freq[shift]++;
                }
            }
            max_freq_index = 0;
            for (i = 1; i < ALPHABET_SIZE; i++) {
                if (freq[i] > freq[max_freq_index]) {
                    max_freq_index = i;
                }
            }
            shifts[k] = (max_freq_index - ('e' - 'a') + ALPHABET_SIZE) % ALPHABET_SIZE;
        }

        int key_gcd = shifts[0];
        for (k = 1; k < key_len; k++) {
            key_gcd = gcd(key_gcd, shifts[k]);
        }

        if (key_gcd == 1) {
            printf("Key: ");
            for (i = 0; i < key_len; i++) {
                printf("%c", index_to_char(key[i]));
            }
            printf("\n");

            for (i = 0; i < ciphertext_len; i++) {
                index = char_to_index(ciphertext[i]);
                if (index >= 0) {
                    plaintext[i] = (index - key[i % key_len] + ALPHABET_SIZE) % ALPHABET_SIZE;
                } else {
                    plaintext[i] = -1;
                }
            }

            printf("Plaintext: ");
            for (i = 0; i < ciphertext_len; i++) {
                if (plaintext[i] >= 0) {
                    printf("%c", index_to_char(plaintext[i]));
                } else {
                    printf("%c", ciphertext[i]);
                }
            }
            printf("\n");

            break;
        }

        key_len++;
        key = realloc(key, key_len * sizeof(int));
    }

    free(key);
    free(shifts);
    free(plaintext);

    return 0;
}

该程序可以对一个给定的密文进行破译，并输出对应的明文和密钥。需要注意的是，该程序只针对长度不超过10的密钥进行破译，如果密钥长度更长，则需要进行一些修改。此外，该程序还需要一些辅助函数来完成字符和索引之间的转换、计算最大公约数等操作。