Encryption and Decryption

I have a school assignment in which asks me to create a program that does the following on encryption and decryption:

Write a program that encrypts and decrypts a text (string). The program reads a text into a character pointer: char * originaltext. Write a function that encrypts originaltext and store the encrypted text into: char * ciphertext. The encryption algorithm is replacing each character by a character having 10 added to its ASCII code. Write another function that decrypts ciphertext and store the result into: char * deciphertext. The decryption algorithm is replacing each character by a character having 10 subtracted from its ASCII code. Display the three strings originaltext, ciphertext,and deciphertext.

It then gives the following hints to start the program:

• Use malloc or calloc functions to allocate dynamically a memory space for strings.
• Assume that the text has a maximum of 100 characters.
• Don’t forget to deallocate the memory using free function.

Could somebody help me to write the program, I'm really stuck and don't know how to start.

Quote:

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Originally Posted by dbz8gokugohan

Could somebody help me to write the program, I'm really stuck and don't know how to start.

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We don't do people's homework for them. You must at least know how to create a main() function. And I doubt the instructor would have you using dynamic memory allocation without going over it first. Come on, at least try.

The good news is that even as an utmost beginner this shouldn't take you more than 2 hours to write, debug and test.

So far this is what I have:

Code:

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


int main (void)
{
    char *originaltext;
    char *ciphertext;
    char *deciphertext;
   
    printf("Type In Your String:");
    scanf("%100s",originaltext);
   
    int encryption ();
    {
        *ciphertext = *originaltext + 10;
        return 1;
    };
   
    int decryption ();
    {
        *deciphertext = *ciphertext - 10;
        return 2;
    };

   
   
    if ((originaltext=(char *)malloc(100))==NULL)
    {
                        printf("Memory allocation error !\n");
                        exit(1);
    }
   
    printf("Original String:\n %s , \nLength of Original String: %i\n",originaltext, strlen(originaltext));
    printf("Encrypted String:\n %s , \nLength of Encrypted String: %i\n",ciphertext, strlen(ciphertext));
    printf("Decrypted String:\n %s , \nLength of Decrypted String: %i\n",deciphertext, strlen(deciphertext));
    free(originaltext);
   
    getchar();
    getchar();
   
    return 0;
}

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It's most likely wrong since it gives me a blank window screen

1) originalString is a pointer pointing to neverland. It is never initialized to point to a memory location that has enough space to hold 100 characters. Therefore you can't read in it. Also, don't use scanf to read strings. Use fgets().

2) don't cast malloc

3) other functions don't belong inside the main function, they belong above.

4) *ciphertext = *originaltext + 10;

This is wrong, I'll let you figure out what *ciphertext and *originaltext really are. (Hint: What is their type?)

"It's most likely wrong since it gives me a blank window screen"

This is not a trial and error exercise. You have to tell us what is wrong by COMPILING THE CODE. If you were a bridge engineer you wouldn't go: try to drive over, although this is most likely wrong and you will die trying.

You cant nest functions like that in C. Move them out of main.

Ok so I modified the program slightly and got it to display the original string I type, but I can't get the encrypt and decrypt functions

Code:

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#include <stdio.h>    // Calls for the libraries
#include <string.h>
#include <stdlib.h>

int encrypt (char * originaltext)    // Function to encrypt a string
{
    char * ciphertext;
   
    if (ciphertext) 
    {
        ciphertext = (char *)malloc(100);
        *ciphertext = *originaltext + 10;
        return *originaltext + 10; // Exits encrypt function
    }
}
   
int decrypt (char * originaltext)    // Function to decrypt a string
{
    char * ciphertext;
    char * deciphertext;
    if (deciphertext) 
    {
        deciphertext = (char *)malloc(100);
        *ciphertext = *originaltext + 10;
        *deciphertext = *ciphertext - 10;
          return *ciphertext - 10; // Exits decrypt function 
    }
}

int main (void)    // Start of program
{
    char * originaltext;    // Defines pointers
    char * ciphertext;
    char * deciphertext;
   
    if ((originaltext=(char *)malloc(100))==NULL)  // Sets the maximum number of characters for string
    {
        printf("\nMemory allocation error!\n");
        exit(1);
    }

    printf("Type In Your String:\n\n");        // Asks for the string to be typed
    scanf("%100s",originaltext);              // Allows user to type string
   
    encrypt (originaltext);    // Calls for the encrypt function
    decrypt (originaltext);    // Calls for the decrypt function
   
    printf("\n\nOriginal String:\n\n %s \n\nLength Of Original String: %i",originaltext, strlen(originaltext));    // Displays the original string
    printf("\n\nEncrypted String:\n\n %s \n\nLength Of Encrypted String: %i",ciphertext, strlen(ciphertext));      // Displays the encrypted string
    printf("\n\nDecrypted String:\n\n %s \n\nLength Of Decrypted String: %i",deciphertext, strlen(deciphertext));  // Displays the decrypted string

    free(originaltext);              // Frees the original string
   
    getchar();
    getchar();
   
    return 0;          // End of program
}

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I'm using *ciphertext = *originaltext + 10; for encrypt because it's supposed to lead to each character of a string replaced by a character having 10 added, and I'm using *deciphertext = *ciphertext - 10: for decrypt as it's supposed to be the previous *ciphertext subtracted by 10 (I would think that this means *deciphertext points the same as *originaltext)

Code:

---

int encrypt (char * originaltext)
{
    char * ciphertext;
   
    if (ciphertext) 
    {
        ciphertext = (char *)malloc(100);
        *ciphertext = *originaltext + 10;
        return *originaltext + 10;
    }
}

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Your if statement doesn't make sense; not where it is at. If you're checking to see if your malloc() succeeded, you should do something like this.

Code:

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char* string_ptr;
string_ptr = malloc( sizeof( *string_ptr ) * string_length );      // no casting malloc(), m'kay?
if( string_ptr == NULL )
{
    fprintf( stderr, "malloc() for string_ptr failed\n" );
    exit( EXIT_FAILURE );
}

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You're not doing what you think you are, here.

Code:

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*ciphertext = *originaltext + 10;

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What this will do is assign the decimal char value of the first character in originaltext string + 10 to the memory location pointed to by ciphertext. Observe what the following code does.

Code:

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

int
main( void )
{
    char* string_ptr;
    string_ptr = malloc( sizeof( *string_ptr ) * 100 );
    strncpy( string_ptr, "Quick red fox jumps over lazy brown dog.", 100 );

    fprintf( stdout, "Printing original string:\n%s\n\n", string_ptr );

    *string_ptr = *string_ptr + 10;    // 'Q' just became '['
    fprintf( stdout, "Printing modified string:\n%s\n\n", string_ptr );

    return 0;
}

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What you don't seem to realize is that originaltext and ciphertext are both pointers to the beginning of the block you malloc()'ed.