程序代码：

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

void jiamis(char *password,int length);
void jiemis(char *password, int length);
int main(void){
    char *password = "321";
    int length = strlen(password);
    jiamis(password,length);
    jiemis(password,length);
    return 0;
}

void jiamis(char *password, int length){
    FILE *fpr, *fpw;
    char *filenamer = "E:\\file\\1.txt";
    char *filenamew = "E:\\file\\jiami.txt";
    char ch;
    int i = 0;
    fpr = fopen(filenamer, "r");
    fpw = fopen(filenamew, "w");
    if (fpr == NULL || fpw == NULL){
        printf("加密失败\n");
        return;
    }
    //循环读取文件
    while (1){
        ch = fgetc(fpr);
        if (feof(fpr)) break;
        ch += password[i++%length];
        fputc(ch, fpw);
    }
    fclose(fpr);
    fclose(fpw);
}
void jiemis(char *password, int length){
    FILE *fpr, *fpw;
    char *filenamer = "E:\\file\\jiami.txt";
    char *filenamew = "E:\\file\\2.txt";
    char ch;
    int i = 0;
    fpr = fopen(filenamer, "r");
    fpw = fopen(filenamew, "w");
    if (fpr == NULL || fpw == NULL){
        printf("解密失败\n");
        return;
    }
    while (1){
        //读取出来
        ch = fgetc(fpr);
        if (feof(fpr))break;
        //解密
        ch -= password[i++%length];
        fputc(ch, fpw);
    }
    fclose(fpr);
    fclose(fpw);
}