help with encrypting plz

hey, could anyone plz tell me who to encrypt like something using a user defined value with the help of an array, like if they user writes 30, then it puts that too the ascii value of the letter so it changes, and stuff like that, tried to do it, but it kind of I am sillyI am sillyI am sillyI am sillyed up when decrypting, it wouldn't decrypt if the value of the number were over a specifik height and don't know why.

basic basic basic encryption to get you started:

Code:

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

int main(void)
{
  unsigned seed= (unsigned)time(NULL);
  int counter;
  char arr[]="Hello World";
  int length = strlen(arr);
  srand(seed);

  puts(arr);
  for (counter=0; counter<length; counter++)
    arr[counter]+=rand();

  puts(arr); /* Note this may not be a valid C string at this point */
  srand(seed);

  for(counter=0; counter<length; counter++)
    arr[counter]-=rand();
  puts(arr);

  return 0;
}

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If you want to really get encryption you have a long road ahead of you.

Quote:

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Originally Posted by wimpman

hey, could anyone plz tell me who to encrypt like something using a user defined value with the help of an array, like if they user writes 30, then it puts that too the ascii value of the letter so it changes, and stuff like that, tried to do it, but it kind of I am sillyI am sillyI am sillyI am sillyed up when decrypting, it wouldn't decrypt if the value of the number were over a specifik height and don't know why.

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Post what you have

Code:

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void kryptering(int Lang)
{
        //char streng[81];
        char krypt[81];
        int kontrol;
        int i;

        printf("What do you want to encrypt?: ");
        gets(krypt);

        printf("what do you want your encryption key to be: ");
        scanf("%d",&kontrol);

        do{
                for(i=0;i<=81;i++)
                {       
                        krypt[i] = krypt[i] + kontrol;
                }
        }while(i > 255);
}

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it's that kind of extremly simple stuff I'm looking for, then when I try to decrypt it with this code it totally messes up with certain values set as encryption key....

Code:

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void dekryptering(Lang)
{
        char ikrypt[81];
        char dkrypt[81];
        char ckontrol[1];       
        int dkontrol;
        int i;

        printf("what were your encryption key: ");
        gets(ckontrol);
        dkontrol = atoi(ckontrol);
       
        Read_file();
        if(!fp)
        {
                return;
        }

        fgets(ikrypt,81,fp);
       
        Close_file();

        for(i=0;i<=81;i++)
        {
                dkrypt[i] = ikrypt[i] - dkontrol;

                if(ikrypt[i] < 0)
                {
                        dkrypt[i] = 0;
                }
        }
}

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this is not the complete code but just the one I thought would have influence on teh problem, in the read file part, I just extract te result from the encryption to a the variabel ikrypt.

Quote:

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do{
for(i=0;i<=81;i++)
{
krypt[i] = krypt[i] + kontrol;
}
}while(i > 255);

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That do while does nothing since i will always be 82 after the first iteration

Quote:

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gets(krypt);

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see the FAQ on why gets is bad

Quote:

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I am sillyI am sillyI am sillyI am sillys

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no one here cares if you are silly. Stating that fact generally will turn people off

Quote:

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for(i=0;i<=81;i++)

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should be

Code:

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for(i=0;i<81;i++)

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Here is an example using the same basic path you've chosen:

Code:

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#include <stdio.h>
#include <string.h>
void crypt (char *, int);
void decrypt (char *, int);
int main (void)
{
  char arr[81];
  crypt(arr, 81);
  decrypt(arr, 81);
  puts(arr);
  getchar();
  return 0;
}
void crypt(char *arr, int length)
{
  int i, key = 5;
  strcpy (arr, "Encrypt this");
  for(i=0; i<length; i++)
  {
        arr[i] = arr[i] + key;
  }
}
void decrypt(char *arr, int length)
{
  int i, key=5;
  for ( i=0; i<81; i++)
  {
        arr[i] = arr[i] - key;
  }
}

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Quote:

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Originally Posted by Thantos

no one here cares if you are silly. Stating that fact generally will turn people off
should be

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lol. I think "I am silly" is a replacement for characters in banned words

hehe well the I am silly part was becuase I tried to use the F word I guess which this forum didn't like :P well I'll correct it now that I've noticed it :P

heh, thats actually kinda funny :)

Yeah, cryptology is a confusing subject -- you should go to a bookstore and read a book on it (such as Applied Cryptography or Practical Cryptography) and maybe buy it.

Or you could have a look at Handbook of Applied Cryptography here. All of the chapters are available for free.