EX. NO: 1 : IMPLEMENTATION OF CAESAR CIPHER

AIM:

To implement the simple substitution technique named Caesar cipher using C language.

DESCRIPTION:

To encrypt a message with a Caesar cipher, each letter in the message is changed using a simple rule: shift by three. Each letter is replaced by the letter three letters ahead in the alphabet. A becomes D, B becomes E, and so on. For the last letters, we can think of the alphabet as a circle and "wrap around". W becomes Z, X becomes A, Y bec mes B, and Z becomes C. To change a message back, each letter is replaced by the one three before it.

EXAMPLE:

ALGORITHM:

STEP-1: Read the plain text from the user.

STEP-2: Read the key value from the user.

STEP-3: If the key is positive then encrypt the text by adding the key with each character in the plain text.

STEP-4: Else subtract the key from the plain text.

STEP-5: Display the cipher text obtained above.

PROGRAM :-

NAME: LOGA MITHRA R
REGISTER NUMBER: 212223100027

#include <stdio.h> 
#include <string.h> 
 
#include <ctype.h> 
void main() 
 
{ 
    char plain[10],cipher[10]; 
    int key,i,length; 
    int result; 
    printf("\n Enter the plain text:"); 
    scanf("%s", plain); 
    printf("\n Enter the key value:"); 
    scanf("%d", &key); 
    printf("\n \n \t PLAIN TEXt: %s", plain); 
    printf("\n \n \t ENCRYPTED TEXT:"); 
    for(i=0, length = strlen(plain); i<length; i++) 
    { 
         
        cipher[i]=plain[i] + key; 
        if (isupper(plain[i]) && (cipher[i] > 'Z')) 
        cipher[i] = cipher[i] - 26; 
        if (islower(plain[i]) && (cipher[i] > 'z')) 
        cipher[i] = cipher[i] - 26; 
        printf("%c", cipher[i]); 
 
    } 
    printf("\n \n \t AFTER DECRYPTION : "); 
    for(i=0;i<length;i++) 
    { 
         
        plain[i]=cipher[i]-key; 
        if(isupper(cipher[i])&&(plain[i]<'A')) 
        plain[i]=plain[i]+26; 
        if(islower(cipher[i])&&(plain[i]<'a')) 
        plain[i]=plain[i]+26; 
        printf("%c",plain[i]); 
    }    
}

OUTPUT :-