1. 背景介绍

维吉利亚密码

维吉尼亚密码引入了“密钥”的概念，即根据密钥来决定用哪一行的密表来进行替换，以此来对抗字频统计。假如以上面第一行代表明文字母，左面第一列代表密钥字母，对如下明文加密：

TO BE OR NOT TO BE THAT IS THEQUESTION

当选定RELATIONS作为密钥时，加密过程是：明文一个字母为T，第一个密钥字母为R，因此可以找到在R行中代替T的为K，依此类推，得出对应关系如下：

密钥:RELAT IONSR ELATI ONSRE LATIO NSREL

明文:TOBEO RNOTT OBETH ATIST HEQUE STION

密文:KSMEH ZBBLK SMEMP OGAJX SEJCS FLZSY

2. 需求分析

程序有两种模式：加密模式和解密模式。

加密模式下，提供原文和密钥，即可得到密文；相反，在解密模式下提供密文和密钥即可得到原文。

3. 源代码

代码在Codeblock上运行正常。

密钥用循环链表存储。

#include<stdio.h>
#include<ctype.h>
#define key_size 30
#define name_size 100
#define title "------------------------Life is a Fight-----------------------------------"

typedef struct NODE{
    int a;
    struct NODE* next;
} node;

node* CreateKey(char* key)
{
    node* head, *p;
    head=(node*)malloc(sizeof(node));
    p=head;
    while((*key)!='\0')
    {
        p->next=malloc(sizeof(node));
        p=p->next;
        p->a=tolower(*key)-'a';
        p->next=head->next;
        key++;
    }
    return head;
}


void Encryption(FILE* infp, node* head)
{
    FILE* outfp;
    char ori,enc;
    node* p;
    p=head->next;
    outfp=fopen("Encryption.txt","w");
    while(!feof(infp))
    {
        ori=tolower(fgetc(infp));
        if(ori>='a'&&ori<='z')
        {
            enc=97+(ori-97+p->a)%26;
            p=p->next;
        }
        else
            enc=ori;
        fprintf(outfp,"%c",enc);
    }
    fclose(outfp);
}

void Decryption(FILE* infp, node* head)
{
    FILE* outfp;
    char enc,ori;
    node* p;
    p=head->next;
    outfp=fopen("Decryption.txt","w");
    while(!feof(infp))
    {
        enc=tolower(fgetc(infp));
        if(enc>='a'&&enc<='z')
        {
            ori=97+(enc-97+26-p->a)%26;
            p=p->next;
        }
        else
            ori=enc;
        fprintf(outfp,"%c",ori);
    }
    fclose(outfp);
}

int main(void)
{
    char key[key_size];
    node* h;
    char mode;
    char name[name_size];
    FILE* fp;

    printf("%s\n",title);
    while(1)
    {
        printf("\n\n\nPlease choose mode:\n");
        printf("E---Encryption Mode              D---Decryption Mode              Q---Quit\n");
        scanf("%c",&mode);
        if(mode=='e'||mode=='E')
        {
            printf("Please enter the file name need to be encrypted:");
            scanf("%s",name);
            fp=fopen(name,"r");
            while(fp==NULL)
            {
                fflush(stdin);
                printf("ERR:please re-enter the file name need to be :");
                scanf("%s",name);
                fp=fopen(name,"r");
            }
            printf("Please enter your key word:");
            fflush(stdin);
            gets(key);
            h=CreateKey(key);
            Encryption(fp,h);
            printf("The encryption has been finished.\n");
            fclose(fp);
        }
        else if(mode=='d'||mode=='D')
        {
            printf("Please enter the file name need to be encrypted:");
            scanf("%s",name);
            fp=fopen(name,"r");
            while(fp==NULL)
            {
                fflush(stdin);
                printf("ERR:please re-enter the file name need to be :");
                scanf("%s",name);
                fp=fopen(name,"r");
            }
            printf("Please enter your key word:");
            fflush(stdin);
            gets(key);
            h=CreateKey(key);
            Decryption(fp,h);
            printf("The decryption has been finished.\n");
            fclose(fp);
        }
        else if(mode=='q'||mode=='Q')
            return;
        else
            printf("Err:please select correct mode.");
        fflush(stdin);
    }
    return 0;
}