Implement Ceasar cipher encryption-decryption in c.

Introduction:

The Caesar Cipher technique is one of the earliest and simplest method of encryption technique. It’s simply a type of substitution cipher, i.e., each letter of a given text is replaced by a letter some fixed number of positions down the alphabet. The method is apparently named after Julius Caesar, who apparently used it to communicate with his officials.

Formula:

Ciphertext(C): E(k,p) = (p+k)mod26

Plaintext(p): D(k,C) = (C-k)mod26

Where k takes a value in the range 1 to 25.

Let us assign a numerical equivalent to each letter:

a	b	c	d	E	F	g	h	i	J	k	l	m
0	1	2	3	4	5	6	7	8	9	10	11	12

n	o	P	q	R	S	T	u	v	W	x	y	z
13	14	15	16	17	18	19	20	21	22	23	24	25

Example:

Key=3

Plain text: hello how are you

Cipher text: KHOOR KRZ DUH BRX

C code:

//CEASER-CIPHER

#include<stdio.h>

#include<conio.h>

void main()

{

            char pt[20]={'\0'},ct[20]={'\0'},rt[20]={'\0'},st[20]={'\0'};

            int key,i;

            clrscr();

            printf("\n enter the plain text:");

            gets(pt);

            printf("\n enter teh key value in numeric (0 to 25):");

            scanf("\n %d",&key);

            //converting plain text into cipher text (encryption)

            for(i=0;i<strlen(pt);i++)

            {

                        ct[i]=((((pt[i]-'a')+key)%26)+'a');

            }

            printf("\n cipher text is: %s",ct);

            //converting cipher text into plain text (decryption)

            for(i=0;i<strlen(ct);i++)

            {

            if(ct[i]<pt[i])

            {

                        rt[i]=(26+((ct[i]-97)-key)+97);

            }

            else

            {

                        rt[i]=((((ct[i]-97)-key)%26)+97);

            }

            }

            printf("\n \n plain text is: %s",rt);

            getch();

}

 
Output:
 

THANK YOU!!!!