Thread: Encrypt & Decrypt Input String [Help Please]

Hi guys, I'm new here and also new to C programming. So i got an assignment on C about encrypt and decrypt the string input. I need to use all what I've learn so far which the basic, array, function, pointer, string. I've already learn file processing but I'm not sure if I can include it in this assignment.


Basically, user need to input a string so that the program will decrypt it into other character(in my case I'm encrypting the string into alphanumeric char so that it will be easy to read by user.). Then the program will decrypt the encrypted string back to it's origin.


Here's how the flow should works :


Input (user input string) => gh6T0 (encrypted string)=> Input (decrypt to original string)


Here I've write some code but it's a little bit messy but hope you guys can read it. The problem I faced here's that I can't decrypt the value back to origin since I'm using modulo in the encrypted part (modulo is irreversible calculation), so that means impossible for me to decrypt back to origin string. Anyone please help me overcome this please :'( Thanks in advance, and pardon for my bad english.

Code:

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


#define SIZE 10 //string max size


void encrypt(char [], int []);    //function prototype for encrypt
void decrypt(char [], int []);    //function prototype for decrypt


int main()    //main function/body
{
    char password[SIZE];    //input array
    int randNO[SIZE];        //random number array to be used in both enc&decrypt function
    int j=0, count;
    srand(time(NULL));


    puts("Enter your password: ");
    gets(password);        //get input string from user
    
    count=strlen(password);
     
     for(j=0;j<count;j++)
     randNO[j]=10+rand()%(10000-10+1);    //generate random number to be used in enc&decrypt function
    
    encrypt(password, randNO);        //encrypt function call


    decrypt(password, randNO);        //decrypt function call
    
    system("pause");
    return 0;
}


void encrypt(char *PASSWORD, int *randNO)        //encrypt function body
{
     int i=0, j=0, k=0;
     int count=0;
     char alphanum[] ="0123456789""abcdefghijklmnopqrstuvwxyz""ABCDEFGHIJKLMNOPQRSTUVWXYZ";
     
     count=strlen(PASSWORD);
     
     for(j=0;j<count;j++)
     randNO[j]=10+rand()%(10000-10+1);


     printf("Your password is : ");
     puts(PASSWORD);
     printf("Encryption in progress...\n");
     
     for(i=0;i<count;i++)
     {
        PASSWORD[i] = alphanum[randNO[i]%(sizeof(alphanum) - 1)];    //encrypting
        
      
     }
     PASSWORD[count]='\0';
     puts(PASSWORD);        //encrypted value will be printed here
     
     return ;
}


void decrypt(char *PASSWORD, int *randNO)        //decrypt function body
{
     int i=0, j=0, k=0;
     int count=0;
     char alphanum[] ="0123456789""abcdefghijklmnopqrstuvwxyz""ABCDEFGHIJKLMNOPQRSTUVWXYZ";
     
     count=strlen(PASSWORD);
     printf("Your encrypted password is : ");
     puts(PASSWORD);
     
     printf("Decryption in progress...\n");
     
     for(i=0;i<count;i++)
     {
        PASSWORD[i] = alphanum[randNO[i]%(sizeof(alphanum) + 1)];
        
      
     }
     PASSWORD[count]='\0';
     puts(PASSWORD);
     
     return ;
}

If the encryption is irreversible, then it was wrong to use it in the first place. In this case, the irreversibility comes not from the modulo operator, but from the fact that you can't retrieve those random numbers again.

@tabstop, that's why im putting the random number array in the main function. Once the program generate the rand number and save it in array, i can call it in both in the encrypt and decrypt . Which means the rand only generated once, and can be retrieve in decrypt function. So that means the modulo part become the problem here. If only i can generate random without using modulo (other equation to get the rand), then it'll be great. By the way, do you know how i could change it according to your idea which is not to use the rand

and @oogabooga, hmm, pardon me man, i really thought by generating random character like this is part of encryption. I found one example about encrypt and decrypt on C Program Examples: C program to Encrypt and Decrypt a password but the encryption generated was some unreadable character by human. Here's the example they put on the website

Code:

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

void encrypt(char password[],int key)
{
    unsigned int i;
    for(i=0;i<strlen(password);++i)
    {
        password[i] = password[i] - key;
    }
}

void decrypt(char password[],int key)
{
    unsigned int i;
    for(i=0;i<strlen(password);++i)
    {
        password[i] = password[i] + key;
    }
}
int main()
{
    char password[20] ;
    printf("Enter the password: \n ");
    scanf("%s",password);
    printf("Passwrod     = %s\n",password);
    encrypt(password,0xFACA);
    printf("Encrypted value = %s\n",password);
    decrypt(password,0xFACA);
    printf("Decrypted value = %s\n",password);
    return 0;
}

About the 0xFACA he use as the key, is it some hexadecimal? He stated in the comment there that it can be change with other value, but how can I change it? Btw thanks to you guys for responding to my thread. I really appreaciate that

Ahh, I see. So i need to do at least 2 or more level of encryption right, and here comes the file i/o part where I save the encrypted code, then later I will make new program and open the encrypted code back and decrypt it to the original string . Ahhh, so that's the real meaning of encrypt/decrypt thingy, now I understand what oogabooga trying to say. Pardon me for my misunderstanding.

Oops, I just realized that i didnt even use the original string the user input in the first place. Thank you so much for enlighten me up. I'll try work out something on that one like you mentioned. Thanks.

And by the way, can you explain a little the example i get from the other website that I pasted above. The key value which is 0xFACA really bother me somehow.

Using 0xFACA is a bit weird, as it is a 16-bit number and therefore larger than a char. The operations happen as an int (either 16- or 32-bit depending on the hardware I suppose), but then when it gets stored back as a character only the last eight bits are kept. At the moment I can't see any difference with just using 0xCA instead (I don't think there's any difference that will happen with signed/unsigned char, but I might have missed something). If someone managed to get 0xCA (which for me on this Windows system is ╩) into the string (by hitting Alt-202 or perhaps getting UTF8 into the input, although that's in a weird section of the Unicode table) then the program will give erroneous results (as that will encrypt to a NUL character and therefore prematurely end the string).

I see, Thanks for the explaination. Now I need to work out something on my code. Thank you

Code:

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That's pretty funny.