Write a C Program to Implement the Caesar Cipher Substitution Technique

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Aim:

To write a C Program to Implement the Simple Substitution Technique Named Caesar Cipher.

Algorithm:

  1. The Caesar Cipher technique is one of the earliest and simplest method of encryption technique.
  2. 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. For example with a shift of 1,
  3. A would be replaced by B, B would become C, and so on. The method is apparently named after Julius Caesar, who apparently used it to communicate with his officials.
  4. Thus to cipher a given text we need an integer value, known as shift which indicates the number of position each letter of the text has been moved down.
  5. The encryption can be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme,
  6. A = 0, B = 1,…, Z = 25. Encryption of a letter by a shift n can be described mathematically.
  7. End

Program:

#include<stdio.h>
#include<stdlib.h>
char encrpt[100];
char decrpt[100];
int shfit;
void encrypt(char* plain)
{

printf("ENTER THE KEY VALUE : ");
scanf("%d",&shfit);
int i=0;

while(*(plain+i)!='\0')
{
if(plain[i]>='a'&&plain[i]<='z')
{
int ch=plain[i];
encrpt[i]=encrpt[i]+(((ch-97+shfit)%26)+97);

}
else if(plain[i]>='A'&&plain[i]<='Z')
{
int ch=plain[i];
encrpt[i]=encrpt[i]+(((ch-65+shfit)%26)+65);
}
i++;
}
}
void crp()
{
int i=0;
while(*(encrpt+i)!='\0')
{
if(encrpt[i]>='a'&&encrpt[i]<='z')
{
int ch=encrpt[i];
decrpt[i]=decrpt[i]+((ch-97-shfit)%26)+97;
}
else if(encrpt[i]>='A'&&encrpt[i]<='Z')
{
int ch=encrpt[i];
decrpt[i]=decrpt[i]+((ch-65-shfit)%26)+65;
}
i++;
}
}
int main()
{
char plain[100];
printf("ENTER PLAIN TEXT :\n");
scanf ("%s",plain) ;
encrypt(plain);
printf("\nENCRYPTED MESSAGE :%s\n",encrpt);
crp();
printf("DECRYPTED MESSAGE :%s",decrpt);
}

Execution:

Input:
ant 4

Output:

ENTER PLAIN TEXT :
ENTER THE KEY VALUE : 
ENCRYPTED MESSAGE :erx
DECRYPTED MESSAGE :ant

Result:

Thus implementation of the Caesar Cipher program was executed Successfully.

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