Question

1. The mathematical expression of the encryption and decryption process of a Caesar cipher algorithm is expressed respectively as:

c=Ep, k=p+k%26                                                                                                                         (1)

p=Dc,k=c-k%26                                                                                                                         (2)

Please do the following:

1. Write at least two paragraphs to explain the principle of operation of the algorithm. For a full credit, your explanation must show the architectural diagram of the encryption and decryption process.
2. Write a program to implement the Caesar algorithm
   1. Code must have two functions; encryption and decryption
   2. Test your codes with p as ‘cryptography’ using k =5

Answer 1

1 )


II) C program for implementing Caesar cipher algorithm:

#include<stdio.h>
#include<stdlib.h>

char data[50], temp;
int key, count;

void getmessage()
{
printf("Enter a String:\t");
scanf("%[^\n]s", data);
}

void key_input()
{
printf("Enter a Key:\t");
scanf("%d", &key);
}

void encryption() /* function for encryption */
{
for(count = 0; data[count] != '\0'; count++)
{
temp = data[count];
if(temp >= 'a' && temp <= 'z')
{
temp = temp + key;
if(temp > 'z')
{
temp = temp - 'z' + 'a' - 1;
}
data[count] = temp;
}
else if(temp >= 'A' && temp <= 'Z')
{
temp = temp + key;
if(temp > 'Z')
{
temp = temp - 'Z' + 'A' - 1;
}
data[count] = temp;
}
}
printf("\nEncrypted Message:\t%s\n", data);
}

void decryption() /* function for decryption */
{
for(count = 0; data[count] != '\0'; count++)
{
temp = data[count];
if(temp >= 'a' && temp <= 'z')
{
temp = temp - key;
if(temp < 'a')
{
temp = temp + 'z' - 'a' + 1;
}
data[count] = temp;
}
else if(temp >= 'A' && temp <= 'Z')
{
temp = temp - key;
if(temp < 'A')
{
temp = temp + 'Z' - 'A' + 1;
}
data[count] = temp;
}
}
printf("\nDecrypted Message:\t%s\n", data);
}

int main()
{
int choice;
getmessage();
key_input();
while(1)
{
printf("\n1. Encryption\n2. Decryption\n3. Exit\n");
printf("\nEnter You Choice:\t");
scanf("%d", &choice);
switch(choice)
{
case 1: encryption();
break;
case 2: decryption();
break;
case 3: exit(0);
default: printf("\nPlease select a correct option:\n");
}
}
printf("\n");
return 0;
}

output :
Enter a String:
' cryptography '
Enter a Key :
5
1.Encryption
2.Decryption
3.Exit
Enter Your Choice:
1
Encrypted Message: hwduytlwfumd

1.Encryption
2.Decryption
3.Exit
Enter Your Choice:
2
Decrypted Message : cryptography

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