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Ekey, posted Tue Jul 07, 2020 3:04 pm (57483)

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Hey guys. I have a specific project for which it is necessary to encrypt some files back after decryption. The algorithm is simple (XOR) and I was hoping that it would work both ways (decrypt and encrypt data) but the function works as it turned out only in one direction - decrypt only. I need expert help and teach the function to correctly encrypt data back

Code:

uint8_t kBox1[20] = {
   0xEB, 0x57, 0x50, 0x86, 0x32, 0x0D, 0x27, 0x60, 0xF4, 0xDE,
   0xF2, 0x4F, 0x6F, 0x13, 0xDA, 0x7B, 0x29, 0x3F, 0xF1, 0x41};

uint8_t kBox2[20] = {
   0x2D, 0x50, 0xD7, 0x64, 0x17, 0xAB, 0x25, 0x0B, 0xC0, 0x3E,
   0x61, 0x49, 0x20, 0x7A, 0x8E, 0x78, 0xA0, 0x67, 0x8D, 0x29};
   
uint32_t Var4 = 0;
int32_t Var8 = (kBox2[16] * kBox1[12]) * dwSize;

for(int32_t i = 0; i {
    uint32_t j = int32_t(Var8 & (Var4 > (kBox2[19-(i % 20)] % 7);
    lpBuffer[i] ^= kBox1[j % 20] ^ (j   j / 255) ^ kBox2[i % 20];
    Var4  = dwSize;
}

Files for testing are attached. I will be glad to any help and advice.

Thanks in advance

aluigi, posted Tue Jul 07, 2020 9:44 pm (57495)

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The function works with both encryption and decryption, it's the same.

The only difference is that the encrypted file has an additional footer at the end "TGCA\x14\x10\x06\x15" so when you decrypt it you must remove the 8 bytes from dwSize.

Ekey, posted Wed Jul 08, 2020 9:55 am (57503)

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Thanks Luigi for answer.

Unfortunately, I already tried to encrypt but the data is different, except for the 1st byte. In addition, the encrypted file is not decrypted by this function - the output data is like garbage

Comparison:

First 0x200 bytes of original:



And first 0x200 of new file



Therefore, I wonder what could be the solution

Ekey, posted Wed Jul 08, 2020 10:11 am (57504)

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Ah, i'm idiot