#include<stdio.h>
#include<string.h>
#define MAX 81

char *Vigenere_encode(char *Plaintext,char *Key) 
{
	int k=0,i=0,j=0;
	int lenp,lenk;       
	char temp;
	lenp=strlen(Plaintext);
	lenk=strlen(Key);
	for(i=0;i<lenp;i++)
	{
		j=j%lenk;
        if(Key[i]>='A'&&Key[j]<='Z')  
			temp='A';
	    else if(Key[i]>='a'&&Key[j]<='z')
		    temp='a';
	    if(Plaintext[i]>='A'&&Plaintext[i]<='Z')  
		    Plaintext[i]=((Plaintext[i]-'A'+Key[j]-temp)%26+'a');  
	    else if(Plaintext[i]>='a'&&Plaintext[i]<='z')
		    Plaintext[i]=((Plaintext[i]-'a'+Key[j]-temp)%26+'A');
	    else
	       Plaintext[i]=Plaintext[i];
		j++;
	}
	return Plaintext;
}

int Judge_char(char Key[MAX])   
{
	int Lenk; 
	Lenk=strlen(Key);
	int a=0;
	for(a=0;a<Lenk;a++)
	{
		if(Key[a]>='A'&&Key[a]<='Z'||Key[a]>='a'&&Key[a]<='z');
		else 
			return -1;
	}
	return 0;
}

void main()
{
	int k=0;
	char P[MAX],K[MAX],*C;       

	printf("------------------------Welcome to the Vigenere Encode!---------------------\n");
    printf("Please input the Plaintext:  ");
	gets(P);
	printf("Input the Key:  ");
	gets(K);  
	
	k=Judge_char(K);
	while(k==-1)  
	{
		printf("\n Wrong!  Please Input the Key again:  ");
		gets(K);
        k=Judge_char(K);
	}
	
	printf("-------------------------------------------------------------------\n");
	printf("The Ciphertext is:  ");		
	C=Vigenere_encode(P,K);
	puts(C);
	printf("--------------------------------------------------------------------\n");
}