Encrypt in C++ / Decrypt in x86

Question

I am having a problem with a school assignment. The assignment is to write a metamorphic Hello World program. This program will produce 10 .com files that print "Hello World!" when executed. Each of the 10 .com files must be different from the others. I understand the concept of metamorphic vs oligomorphic vs polymorphic. My program currently creates 10 .com files and then writes the machine code to the files. I began by simply writing only the machine code to print hello world and tested it. It worked just fine. I then tried to add a decryption routine to the beginning of the machine code. Here is my current byte array:

#define ARRAY_SIZE(array) (sizeof((array))/sizeof((array[0])))
BYTE pushCS = 0x0E;
BYTE popDS = 0x1F;
BYTE movDX = 0xBA;
BYTE helloAddr1 = 0x1A;
BYTE helloAddr2 = 0x01;
BYTE movAH = 0xB4;
BYTE nine = 0x09;
BYTE Int = 0xCD;
BYTE tOne = 0x21;
BYTE movAX = 0xB8;
BYTE ret1 = 0x01;
BYTE ret2 = 0x4C;
BYTE movBL = 0xB3;
BYTE keyVal = 0x03; // Encrypt/Decrypt key

typedef unsigned char BYTE;

BYTE data[] = { 0x8D, 0x0E, 0x01, 0xB7, 0x1D, 0xB3, keyVal, 0x30, 0x1C, 0x46, 0xFE, 0xCF, 0x75, 0xF9,
              movDX, helloAddr1, helloAddr2, movAH, nine, Int, tOne, movAX, ret1, ret2, Int, tOne,
               0x48, 0x65, 0x6C, 0x6C, 0x6F, 0x20, 0x57, 0x6F, 0x72, 0x6C, 0x64, 0x21, 0x0D, 0x0D, 0x0A, 0x24 };

The decryption portion of the machine code is the first 14 bytes of "data". This decryption routine would take the obfuscated machine code bytes and decrypt them by xor-ing the bytes with the same key that was used to encrypt them. I am encrypting the bytes in my C++ code with this:

for (int i = 15; i < ARRAY_SIZE(data); i++)
{
    data[i] ^= keyVal;
}

I have verified over and over again that my addressing is correct considering that the code begins at offset 100. What I have noticed is that when keyVal is 0x00, my code runs fine and I get 10 .com files that print Hello World!. However, this does me no good as 0x00 leaves everything unchanged. When I provide an actual key like 0x02, my program no longer works. It simply hangs until I close out DosBox. Any hints as to the cause of this would be a great help. I have some interesting plans for junk insertion (The actual metamorphic part) but I don't want to move on to that until I figure out this encrypt/decrypt issue.

Answer 1

The decryption portion of the machine code is the first 14 bytes of "data".

and

for (int i = 15; i < ARRAY_SIZE(data); i++)

do not match since in C++ array indexes start at 0.

In your array data[15] == helloAddr1 which means you are not encrypting the data[14] == movDX element. Double-check which elements should be encrypted and start at i = 14 if required.