Question

Program 1: For this program you will be implementing a simple rotation encryption to be used on a text file. Your program will need to read in text from a text file and encrypt or decrypt it (have the user select which) using the encryption key entered by the user, then write the results out to another file. The user will enter the names of the source and destination files, the encryption key, and will select whether they are encrypting or decrypting the file. You can use any text file of choice.

The program should have these 4 functions:

• A function that reads from a file (takes a single string parameter for the filename and returns a string).
• A function that writes to a file (takes 2 string parameters, one for the filename and one for the contents to be written, return type of void).
• A function that performs encryption (takes a string parameter for the string to be encrypted and an int parameter for the encryption key, returns the encrypted result as a string).
• A function that performs decryption (takes a string parameter for the string to be encrypted and an int parameter for the encryption key, returns the decrypted result as a string).
• Hint: Encryption and decryption are very similar – you might want to have your encryption function call your decryption function or vice versa, or write a third function that encryption and decryption both call in order to avoid repetition.
• Note: The Main should handle all user input. The encrypted or decrypted result is written to a file. Acceptable encryption keys are in the range 1-25 inclusive, you must validate the user’s entered encryption key and if it is out of range require that they re-enter it.

Answer 1

C CODE WITH ALL FOUR FUNCTIONS :-

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
int main()
{
encrypt();
decrypt();
read();
write();
}
void read()
{
   char c[1000];
   FILE *fp;
   if((fp=fopen("filename.txt", "r"))==NULL)
   {
       printf("Error in opening file..!!");
       getch();
       exit(1);
   }
   fscanf(fp, "%s", c);
   printf("Data in file = %s",c);
   fclose(fp);
   getch();
}
void write()
{
   FILE *fp;
   char s[100], fname[20];
   printf("Enter a file name with extension (like file.txt) to create a file : ");
   gets(fname);
   fp=fopen(fname, "w");
   if(fp==NULL)
   {
       printf("Error in opening file..!!");
       getch();
       exit(1);
   }
   printf("\nEnter few lines of text :\n");
   while(strlen(gets(s))>0)
   {
       fputs(s, fp);
       fputs("\n",fp);
   }
   fclose(fp);
   getch();
}
void encrypt()
{
   char fname[20], ch, choice;
   FILE *fps, *fpt;
   printf("Enter file name (with extension like file.txt) to encrypt : ");
   gets(fname);
   fps=fopen(fname, "r");
   if(fps==NULL)
   {
       printf("Error in opening file..!!");
       printf("\nPress any key to exit...");
       getch();
       exit(1);
   }
   fpt=fopen("temp.txt", "w");
   if(fpt==NULL)
   {
       printf("Error in creating temp.txt file..!!");
       fclose(fps);
       printf("\nPress any key to exit...");
       getch();
       exit(2);
   }
   while(1)
   {
       ch=fgetc(fps);
       if(ch==EOF)
       {
           break;
       }
       else
       {
           ch=ch+100;
           fputc(ch, fpt);
       }
   }
   fclose(fps);
   fclose(fpt);
   fps=fopen(fname, "w");
   if(fps==NULL)
   {
       printf("Error in opening source file..!!");
       printf("\nPress any key to exit...");
       getch();
       exit(3);
   }
   fpt=fopen("temp.txt", "r");
   if(fpt==NULL)
   {
       printf("Error in opening temp.txt file...!!");
       fclose(fps);
       printf("\nPress any key to exit...");
       getch();
       exit(4);
   }
   while(1)
   {
       ch=fgetc(fpt);
       if(ch==EOF)
       {
           break;
       }
       else
       {
           fputc(ch, fps);
       }
   }
   printf("File %s encrypted successfully..!!", fname);
   printf("\nPress any key to exit...");
   fclose(fps);
   fclose(fpt);
   getch();
}

void decrypt()
{
   char ch, choice, fname[20];
   FILE *fps, *fpt;
   printf("Enter file name (with extension like file.txt) which you have encrypted earlier to decrypt : ");
   gets(fname);
   fps=fopen(fname, "w");
   if(fps==NULL)
   {
       printf("Error in opening source file..!!");
       printf("\nPress any key to exit...");
       getch();
       exit(7);
   }
   fpt=fopen("temp.txt", "r");
   if(fpt==NULL)
   {
       printf("Error in opening temp.txt file..!!");
       fclose(fps);
       printf("\nPress any key to exit...");
       getch();
       exit(9);
   }
   while(1)
   {
       ch=fgetc(fpt);
       if(ch==EOF)
       {
           break;
       }
       else
       {
           ch=ch-100;
           fputc(ch, fps);
       }
   }
   printf("File %s decrypted successfully..!!",fname);
   printf("\nPress any key to exit...");
   fclose(fps);
   fclose(fpt);
   getch();
}

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