XOR Encryption Packets not decrypting properly

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I have made a simple XOR encryption to encrypt some packets, but decrypting does not work well... since i am a beginner in C++ please help me...

unsigned char cheimagice[] = { 0xCF, 0x10, 0x4E, 0x3A, 0xC2, 0xD8, 0x5F, 0xAD, 0xE4 };
    unsigned char test[] = { 0xC3, 0x18, 0xC7, 0xBE, 0x08, 0x1B, 0x25, 0xFF, 0x81, 0x55, 0xE0, 0xB5 };
    printf("[Criptare]: \n");
    printf("[INAINTE]: 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX \n", (unsigned char)test[0], (unsigned char)test[1], (unsigned char)test[2], (unsigned char)test[3], (unsigned char)test[4], (unsigned char)test[5], (unsigned char)test[6], (unsigned char)test[7], (unsigned char)test[8], (unsigned char)test[9], (unsigned char)test[10], (unsigned char)test[11]);
    unsigned char *test1 = new unsigned char[sizeof(test)];
    unsigned char *test2 = new unsigned char[sizeof(test)];
    // criptare :)
    for (unsigned int i = 0; i < sizeof(test); ++i)
    {
        test1[i] ^= test[i - 1] ^ cheimagice[i % 9];
    }
    printf("[DUPA]: 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX \n", (unsigned char)test1[0], (unsigned char)test1[1], (unsigned char)test1[2], (unsigned char)test1[3], (unsigned char)test1[4], (unsigned char)test1[5], (unsigned char)test1[6], (unsigned char)test1[7], (unsigned char)test1[8], (unsigned char)test1[9], (unsigned char)test1[10], (unsigned char)test1[11]);
    // decriptare :)
    printf("[Decriptare]: \n");
    printf("[INAINTE]: 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX \n", (unsigned char)test1[0], (unsigned char)test1[1], (unsigned char)test1[2], (unsigned char)test1[3], (unsigned char)test1[4], (unsigned char)test1[5], (unsigned char)test1[6], (unsigned char)test1[7], (unsigned char)test1[8], (unsigned char)test1[9], (unsigned char)test1[10], (unsigned char)test1[11]);
    for (unsigned int i = 0; i < sizeof(test1); ++i)
    {
        test2[i] ^= test1[i - 1] ^ cheimagice[i % 9];
    }
    printf("[DUPA]: 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX \n", (unsigned char)test2[0], (unsigned char)test2[1], (unsigned char)test2[2], (unsigned char)test2[3], (unsigned char)test2[4], (unsigned char)test2[5], (unsigned char)test2[6], (unsigned char)test2[7], (unsigned char)test2[8], (unsigned char)test2[9], (unsigned char)test2[10], (unsigned char)test2[11]);

The Output:

[Criptare]:
[INAINTE]: 0xC3 0x18 0xC7 0xBE 0x08 0x1B 0x25 0xFF 0x81 0x55 0xE0 0xB5
[DUPA]: 0xE3 0xD3 0x6C 0xFD 0x11 0xD0 0x29 0x88 0x21 0x4E 0x36 0xAE
[Decriptare]:
[INAINTE]: 0xE3 0xD3 0x6C 0xFD 0x11 0xD0 0x29 0x88 0x21 0x4E 0x36 0xAE
[DUPA]: 0x6C 0xF3 0x9D 0x56 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

Posted 20-May-15 2:22am

User 9994025

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Comments

Jochen Arndt 20-May-15 8:48am

You have access violations in test[i-1] and test1[i-1]. I don't know your algorithm but I assume that zero should be used instead when i is zero.

Decryption must be the reverse operation of encryption. I don't think that your code is correct because you are using identical operations and uses looping from zero at decryption. Because the previous value is used during encryption it might be necessary to start decryption at the last value. But again, I don't know your algorithm.

Frankie-C 20-May-15 9:02am

It's a simple xoring of values with a salt, in this case of the same lenght of the sequence to encrypt.
So simply exoring the result wit the salt will give back the value:
10101 xor 01011 = 11110
11110 xor 01011 = 10101

Jochen Arndt 20-May-15 9:07am

But not when x-oring also with a previous (already encrypted) value.

Frankie-C 20-May-15 10:01am

You're right! I have not noticed it!
I made a fast test in debug mode, where the array test1 was filled with 0's...

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Frankie-C · Accepted Answer · 2015-05-20T02:46:00

Why are you trying to address values outside the lower bound of the input array?
Here

C++

for (unsigned int i = 0; i < sizeof(test); ++i)
{
    test1[i] ^= test[i - 1] ^ cheimagice[i % 9];
}

and here

C++

for (unsigned int i = 0; i < sizeof(test1); ++i)
{
    test2[i] ^= test1[i - 1] ^ cheimagice[i % 9];
}

What should have been the scope? This should have also triggered a memory violation exception. Moreover Xoring contents of destination doesn't help to get the result.
[EDIT] I fixed the code:

C++

unsigned char cheimagice[] = { 0xCF, 0x10, 0x4E, 0x3A, 0xC2, 0xD8, 0x5F, 0xAD, 0xE4 };
		unsigned char test[] = { 0xC3, 0x18, 0xC7, 0xBE, 0x08, 0x1B, 0x25, 0xFF, 0x81, 0x55, 0xE0, 0xB5 };
		printf("[Criptare]: \n");
		printf("[INAINTE]: 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX \n", (unsigned char)test[0], (unsigned char)test[1], (unsigned char)test[2], (unsigned char)test[3], (unsigned char)test[4], (unsigned char)test[5], (unsigned char)test[6], (unsigned char)test[7], (unsigned char)test[8], (unsigned char)test[9], (unsigned char)test[10], (unsigned char)test[11]);
		unsigned char *test1 = new unsigned char[sizeof(test)];
		unsigned char *test2 = new unsigned char[sizeof(test)];
	    // criptare :)
		for (unsigned int i = 0; i < sizeof(test); ++i)
		{
			test1[i] = test[i] ^ cheimagice[i % 9];
		}
		printf("[DUPA]: 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX \n", (unsigned char)test1[0], (unsigned char)test1[1], (unsigned char)test1[2], (unsigned char)test1[3], (unsigned char)test1[4], (unsigned char)test1[5], (unsigned char)test1[6], (unsigned char)test1[7], (unsigned char)test1[8], (unsigned char)test1[9], (unsigned char)test1[10], (unsigned char)test1[11]);
	    // decriptare :)
		printf("[Decriptare]: \n");
		printf("[INAINTE]: 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX \n", (unsigned char)test1[0], (unsigned char)test1[1], (unsigned char)test1[2], (unsigned char)test1[3], (unsigned char)test1[4], (unsigned char)test1[5], (unsigned char)test1[6], (unsigned char)test1[7], (unsigned char)test1[8], (unsigned char)test1[9], (unsigned char)test1[10], (unsigned char)test1[11]);
		for (unsigned int i = 0; i < sizeof(test1); ++i)
		{
			test2[i] = test1[i] ^ cheimagice[i % 9];
		}
		printf("[DUPA]: 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX 0x%02hX \n", (unsigned char)test2[0], (unsigned char)test2[1], (unsigned char)test2[2], (unsigned char)test2[3], (unsigned char)test2[4], (unsigned char)test2[5], (unsigned char)test2[6], (unsigned char)test2[7], (unsigned char)test2[8], (unsigned char)test2[9], (unsigned char)test2[10], (unsigned char)test2[11]);