Problems with Xor encryption

Quote:

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Originally Posted by lala123

So.... Does anyone know why my code ain't working? The executable runs fine but the file remains unchanged.

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Quote:

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When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for "update"). However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired.

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So check the return values of th efunction called

vart

Amazing! It worked! Thank you =)

Here's the code in case someone wants it:

Code:

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

int main(int argc, char *argv[])
{
    FILE *fp;
    long fSize;
    size_t result;
    char key[] = "123", *buffer;

    fp = fopen ("xor.txt", "rb+");
   
    if (fp == NULL)
      {fputs("File error",stderr); exit(1);}
     
    fseek (fp, 0, SEEK_END);
    fSize = ftell (fp);
    rewind (fp);
   
    buffer = malloc (sizeof(char)*fSize);
   
    if (buffer == NULL)
      {fputs ("Memory error",stderr); exit (2);}
     
    result = fread (buffer, 1, fSize, fp);
   
    if (result != fSize)
      {fputs ("Reading error",stderr); exit (3);}
     
    XorEncrypt(buffer, key);
   
    rewind (fp);
    fwrite (buffer, 1, fSize, fp);
   
    fclose(fp);
    free(buffer);
    getch();
    return 0;
}

int XorEncrypt (char *string, char *key)
{
    int i, j, mod;
   
    int StringLen = strlen(string); // 20
    int KeyLen = strlen(key); // 6

    for (i = 0; i < (StringLen / KeyLen); i++)
    {
        for (j = 0; j < KeyLen; j++)
        {
            string[i*KeyLen + j] ^= key[j];
        }
    }
   
    mod = StringLen &#37; KeyLen;
   
    if (mod != 0)
    {
        for (i = 0; i < mod; i++)
        {
            string[StringLen - mod + i] ^= key[i];
        }
    }
   
    return 0;
}

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Well actually not really.....

When I use small strings in the file it works finely, but with larger texts and files...

Example: (using key "test43")

"This is a test!!!" -> " ZET@VRU" -> "This is a test!!!"

But when I placed the code (the same I just posted) and then passed the output I got:

Code:

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#includ SHGGZ\
yoPZP]QZ
My>>~ ^\]] ESPXEUATOSE/8Zy>HyoST2,?1Hy>TES[]E']I^~~TE NV+SQ@
        O99TESTW[SQJ/8SIEW@V^RV^~~99TESTRCTXS[CS\K] LGVISVFQ_GZO99TEST99TEST]UTMIE=!x]hyTTESTOU2VFA]^SLZ MB]NyoSTTESy>TESGVS\RCXECX`1 8+q}0LHy>TESgZ SIU  ZO99TESTFVZO99TEST99TESTVFTX\E[]I
\W[Z^R`]>~EST>~ESTZE[AU
T        T+&8xyoSTTESRC
 T9 FJT
[AVI  PVZOVT
]^y>TESTyoST 
XGTXSFV
S\VFEISgZ _TRC]^~~TE~~TE 
XGTDNTR`]99TESTTAGE[VfV
S
FXQALHTQKS\O~~TEST>~ESTk6WA
\VF
]>~EST>~ESTAP\]>~ESTU Q\RVISE6QT]>~EST>~ESTU        QZO99TESTRA [AU
]>~ESTTOhyTT AAECO99        hyy>ZS,[A1MC E[\REY@AXP
TXZy>HyoSTT]T_TY\^~~TE~~TE@'
ZT8 T        
Q]\]]LHTTWCy>TESZGT.
xVENTGG        XZO[EEy>>~ESTUS\]IECOZTYS\gGxVE\TV
)TX_>~ESTHyoSTTESTR\E[TUHT^HE8MHT^_L~~TESThyTTESTTES@A/]?
8Q]TNSi*XSQJ/.O99TESTTEy>TES        99TEST99TESTY\ENTgGxVEVTV
)>~EST>~ESTZE[[WTDNTyoSThyTTEST

TZTXSDEOTY\^S]hyTTESThyTTESTTESTGGo` SSY^
S_Z)E-IX(iyoSTTESTNyoST        hyTThyTT AAECO99

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=S There must be something wrong with the algo right?

Code:

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#include
test43test43

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What value do you get when you exclusive-or a letter with itself?
What do you think strlen() is going to do when it gets to that value?

Quote:

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Originally Posted by Salem

Code:

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#include
test43test43

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What value do you get when you exclusive-or a letter with itself?
What do you think strlen() is going to do when it gets to that value?

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If lala123 had been paying attention, he/she would have seen that I already pointed this out :rolleyes:

Oh, thanks for that. The code is working now.

Code:

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int XorEncrypt (char *string, char *key, long sLen)
{
    int i, j, mod;
   
    int KeyLen = strlen(key); // 6

    for (i = 0; i < (sLen / KeyLen); i++)
    {
        for (j = 0; j < KeyLen; j++)
        {
            string[i*KeyLen + j] ^= key[j];
        }
    }
   
    mod = sLen &#37; KeyLen;
   
    if (mod != 0)
    {
        for (i = 0; i < mod; i++)
        {
            string[sLen - mod + i] ^= key[i];
        }
    }
   
    return 0;
}

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Oh and BTW Im a man...

Quote:

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Originally Posted by lala123

Oh and BTW Im a man...

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Not like it matters, I just don't want to mistakenly refer to a woman as "he" or a man as "she" so I just use both :)

Ok :)