caesar cipher in c program
#include <stdio.h>
#include <ctype.h>
#define MAXSIZE 1024
void encrypt(char*);
void decrypt(char*);
int menu();
int
main(void)
{
char c,
choice[2],
s[MAXSIZE];
while(1)
{
menu();
gets(choice);
if((choice[0]=='e')||(choice[0]=='E'))
{
puts("Input text to encrypt->");
gets(s);
encrypt(s);
}
else if((choice[0]=='d')||(choice[0]=='D'))
{
puts("Input text to decrypt->");
gets(s);
decrypt(s);
}
else
break;
}
return 0;
}
void encrypt(char*str)
{
int n=0;
char *p=str,
q[MAXSIZE];
while(*p)
{
if(islower(*p))
{
if((*p>='a')&&(*p<'x'))
q[n]=toupper(*p + (char)3);
else if(*p=='x')
q[n]='A';
else if(*p=='y')
q[n]='B';
else
q[n]='C';
}
else
{
q[n]=*p;
}
n++; p++;
}
q[n++]='\0';
puts(q);
}
void decrypt(char*str)
{
int n=0;
char *p=str,
q[MAXSIZE];
while(*p)
{
if(isupper(*p))
{
if((*p>='D')&&(*p<='Z'))
q[n]=tolower(*p - (char)3);
else if(*p=='A')
q[n]='x';
else if(*p=='B')
q[n]='y';
else
q[n]='z';
}
else
{
q[n]=*p;
}
n++; p++;
}
q[n++]='\0';
puts(q);
}
int menu()
{
puts("To encrypt, input e or E\n");
puts("To decrypt, input d or D\n");
puts("To exit, input any other letter\n");
puts("Your choice:->\n");
return 0;
}