Question

Your assignment: 1) Ask the user if we are encrypting or decrypting. 2) Ask the user to enter a sentence to be transformed. 3) Ask the user to enter a sentence that will be used as the encryption or decryption key. 4) The sentences (array of characters) should end with a NULL terminator '\0'. 5) The range of values will be all printable characters on the ASCII chart starting with a SPACE - Value 32, up to and including a ~ - Value 126. Therefore, the number of symbols is 95 (0 to 94), i.e. Mod 95. 6) If the key is shorter than the message, make sure the key repeats until it matches the length of the text to be transformed. 7) Print out the transformed (encrypted or decrypted) message. has to be written in c

Answer 1

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

// Function to encrypt the string
void encrypt(char *str, char *key, int size)
{
   int i;
   char ch;
   for(i=0; i<size; i++)
   {
       // XOR characters from str with the key
       ch = str[i] ^ key[i];
       str[i] = ch;
   }
   //Write a null byte at end of the string to make printing of strings proper
   str[size] = '\0';
}

// Function to decrypt the string
void decrypt(char *str, char *key, int size)
{
   int i;
   char ch;
   for(i=0; i<size; i++)
   {
       // XOR characters from str with the key
       ch = str[i] ^ key[i];
       str[i] = ch;
   }
   //Write a null byte at end of the string to make printing of strings proper
   str[size] = '\0';
}

int main()
{
   char str[100], key[100];
   int choice, size;

   //Ask user for input.
   printf("Enter a string: ");
   fgets(str, sizeof(str), stdin);
   printf("Enter key : ");
   fgets(key, sizeof(key), stdin);

   //fgets() reads newline from input, eliminate newline present at the end
   str[strlen(str)-1] = '\0';
   key[strlen(key)-1] = '\0';

   // If key is shorter than the string, keep appending the key to itself
   while(strlen(str) > strlen(key))
   {
       size = strlen(str) - strlen(key);
       if(size > strlen(key))
           size = strlen(key);
       strncat(key, key, size);
   }

   // Read input from user for encrypt/decrypt
   printf("1.Encrypt 2.Decrypt\nEnter your choice: ");
   scanf("%d", &choice);

   size = strlen(str);
   if(choice == 1)
   {
       //Calling encrypt function
       encrypt(str, key, size);
       printf("Encrypted String: %s\n", str);
// uncomment below 2 lines. in sample output, I have added the decrypt function in the encrypt function, because some ascii characters within range 32-126 are not displayed in linux terminal. So calling decrypt function immediately to check whether encryption is proper or not. The decrypt function takes the encrypted string and key as argument and returns the original string.
       decrypt(str, key, size);
       printf("Decrypted String: %s\n", str);
   }
   else if(choice == 2)
   {
       //Calling decrypt function
       decrypt(str, key, size);
       printf("Decrypted String: %s\n", str);
   }
   else
       printf("Error, Please input a valid chioce.\n");

}

oot@Ma1 ware : /mnt/d/Programs # .oot@Malware : /mnt/d/Programs # ./a.out Enter a string: the quick brown fox jumps cover alazy dog nter key: hiddenSecretKey123 L.Encrypt 2.Decrypt nter your choice: 1 ncrypted String: 000DOROSOOOT OIOO EO ecrypted String: the quick brown fox jumps over a lazy dog .oot@Ma1 ware : /mnt/d/Programs # oot@Malware : /mnt/d/Programs# oot@Malware : /mnt/d/Programs# ence