C Source Code Example for Ceaser Cipher

#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;
}