Help with De-Encryption?

[redaqen]

Hey guys,

I'm a long time lurker, and first time poster. Please be kind. Anyways, for one of my classes, Principles of Algorithm Design 1, we had an assignment where we had to create a program that would encrypt and then de-encrypt a message by offsetting the characters by a user specified amount as well as inserting random characters. As much as I tried, I found that ultimately... I screwed it up big time. I turned the assignment in for partial credit, but it is still bugging me, so I've decided to come to you kind folk for help. Heres my code:

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
char encrypt(int offset, int random){
        char cur;
        int pos=1;
        FILE* fin=fopen("Input.bak", "r");
        FILE* fout=fopen("Output.bak", "w");

        printf("Please ensure that the message you want to encrypt \n is in file Input.bak\n ");
        while ((fscanf(fin,"%c", &cur))==1) {
                if ((pos%random)==0)
                fprintf(fout,"%c%c ", (rand()%26+'a'), (cur+offset));
                else
                fprintf(fout,"%c", (cur+offset));
                pos++;}
        fclose(fout);
        fclose(fin);
return cur;
}

char decrypt(int offset, int random){
        char cur;
        int pos=1;
        FILE* fin=fopen("Output.bak", "r");
        FILE* fout=fopen("Unencrypted.bak", "w");

        printf("Please ensure that the message you want to decrypt \n is in file Output.bak\n ");
        while ((fscanf(fin,"%c", &cur))==1) {
                if ((pos%(random+5))==0)
                fscanf(fout,"%*c");
                else fprintf(fout,"%c", (cur-offset));
                pos++;
                }
        fclose(fout);
        fclose(fin);
return cur;
}

int main(void){
        char c;
        int offset;
        int random;


        printf("The purpose of this program is to encrypt and decrypt message.\n");
        printf("Please enter e to encrypt or d to decrypt.\n");
                scanf("%c", &c);
                        if ((c == 'e')) printf("You chose to encrypt.\n");
                        else if ((c == 'E')) printf("You chose to encrypt.\n");
                        else if ((c == 'd')) printf("You chose to decrypt.\n");
                        else if ((c == 'D')) printf("You chose to decrypt\n");
                        else {printf("You didn't enter either e or d. Please follow the instructions next time.\n");
                                scanf(" *c");
                                return 101;
                        }
        printf("Please enter the offset.\n");
                scanf("%d", &offset);
        printf("Please enter the number of spaces between random characters.\n");
                scanf("%d", &random);
        printf("Your user-defined offset is %d and the number of spaces between random characters is %d.", offset, random);
                        if ((c == 'e')) encrypt(offset, random);
                        else if ((c == 'E')) encrypt(offset, random);
                        else if ((c == 'd')) decrypt(offset, random);
                        else if  ((c == 'D')) decrypt(offset, random);
                        else printf("Since you didn't choose either e or d, its just a bit worthless.\n");


return 0;
}

Any help in solving this problem would be greatly appreciated. Personally, I think that the decryption isnt removing the random characters correctly as the message comes out somewhat recognizable in some cases.

Thanks again!

[Narue]

>if ((pos%(random+5))==0)
Dare I ask where you got the 5 from?

[redaqen]

To be honest, I'm not sure anymore. This assignment is about three weeks old.

I think, though, that when I went in for help on the assignment, my professor said that my main problem was with finding the random characters that were inserted.

In this case, Random stands for the number of spaces before a 'random' character was inserted into the encrypted message. I was just toying around with it, and 5 was the number that I ended up turning the code in with.

It was asinine, I know, but I was in my final throws of desperation...

P.S. You guys jump on these things pretty fast :o

P.S.S. Awesome Avatar by the way :D

[Narue]

Eh, just a quickie for giggles:

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

void crypt ( const char *msg, char *buffer, int n, int shift, int mix, int decrypt )
{
  int i = 0;

  while ( *msg != '\0' && --n > 0 ) {
    if ( i++ == mix ) {
      i = 0;

      if ( !decrypt )
        *buffer++ = (char)( rand() % 128 );
      else
        ++msg;
    }

    if ( !decrypt )
      *buffer++ = (char)( (unsigned char)*msg++ + shift );
    else
      *buffer++ = (char)( (unsigned char)*msg++ - shift );
  }

  *buffer = '\0';
}

int main ( void )
{
  const char *msg = "this is a test";
  char buffer[20];
  char end[20];

  crypt ( msg, buffer, 20, 1, 2, 0 );
  puts ( buffer );
  crypt ( buffer, end, 20, 1, 2, 1 );
  puts ( end );

  return 0;
}

[redaqen]

Ah, well the thing is, we hadn't learned about arrays when we were working on this assignment...

Anyways, thanks alot for your help! :D

[Narue]

Yea, well, I'm too lazy to set up an example using files. ;)