This is a short and simple program that will fix the 3 checksums at the top of some games published by EA for the Xbox 360 console. I know that both Dead Space 1 and 3 have the MC02 header, but I think a lot of other EA games do as well.

mc02_fixer.c

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

typedef unsigned long DWORD;
typedef unsigned char BYTE;

typedef struct
{
    DWORD magic;        // 0x4D433032, ASCII = "MC02"
    DWORD fileLength;
    DWORD checksum0Length;
    DWORD checksum1Length;
    DWORD checksum0;
    DWORD checksum1;
    DWORD checksum2;

} MC02_Header;

DWORD table[0x100] =
{
        0x00000000, 0x4C11DB7,  0x9823B6E,  0xD4326D9,  0x130476DC, 0x17C56B6B,
	0x1A864DB2, 0x1E475005, 0x2608EDB8, 0x22C9F00F, 0x2F8AD6D6, 0x2B4BCB61,
	0x350C9B64, 0x31CD86D3, 0x3C8EA00A, 0x384FBDBD, 0x4C11DB70, 0x48D0C6C7,
	0x4593E01E, 0x4152FDA9, 0x5F15ADAC, 0x5BD4B01B, 0x569796C2, 0x52568B75,
	0x6A1936C8, 0x6ED82B7F, 0x639B0DA6, 0x675A1011, 0x791D4014, 0x7DDC5DA3,
	0x709F7B7A, 0x745E66CD, 0x9823B6E0, 0x9CE2AB57, 0x91A18D8E, 0x95609039,
	0x8B27C03C, 0x8FE6DD8B, 0x82A5FB52, 0x8664E6E5, 0xBE2B5B58, 0xBAEA46EF,
	0xB7A96036, 0xB3687D81, 0xAD2F2D84, 0xA9EE3033, 0xA4AD16EA, 0xA06C0B5D,
	0xD4326D90, 0xD0F37027, 0xDDB056FE, 0xD9714B49, 0xC7361B4C, 0xC3F706FB,
	0xCEB42022, 0xCA753D95, 0xF23A8028, 0xF6FB9D9F, 0xFBB8BB46, 0xFF79A6F1,
	0xE13EF6F4, 0xE5FFEB43, 0xE8BCCD9A, 0xEC7DD02D, 0x34867077, 0x30476DC0,
	0x3D044B19, 0x39C556AE, 0x278206AB, 0x23431B1C, 0x2E003DC5, 0x2AC12072,
	0x128E9DCF, 0x164F8078, 0x1B0CA6A1, 0x1FCDBB16, 0x18AEB13,  0x54BF6A4,
	0x808D07D,  0xCC9CDCA,  0x7897AB07, 0x7C56B6B0, 0x71159069, 0x75D48DDE,
	0x6B93DDDB, 0x6F52C06C, 0x6211E6B5, 0x66D0FB02, 0x5E9F46BF, 0x5A5E5B08,
	0x571D7DD1, 0x53DC6066, 0x4D9B3063, 0x495A2DD4, 0x44190B0D, 0x40D816BA,
	0xACA5C697, 0xA864DB20, 0xA527FDF9, 0xA1E6E04E, 0xBFA1B04B, 0xBB60ADFC,
	0xB6238B25, 0xB2E29692, 0x8AAD2B2F, 0x8E6C3698, 0x832F1041, 0x87EE0DF6,
	0x99A95DF3, 0x9D684044, 0x902B669D, 0x94EA7B2A, 0xE0B41DE7, 0xE4750050,
	0xE9362689, 0xEDF73B3E, 0xF3B06B3B, 0xF771768C, 0xFA325055, 0xFEF34DE2,
	0xC6BCF05F, 0xC27DEDE8, 0xCF3ECB31, 0xCBFFD686, 0xD5B88683, 0xD1799B34,
	0xDC3ABDED, 0xD8FBA05A, 0x690CE0EE, 0x6DCDFD59, 0x608EDB80, 0x644FC637,
	0x7A089632, 0x7EC98B85, 0x738AAD5C, 0x774BB0EB, 0x4F040D56, 0x4BC510E1,
	0x46863638, 0x42472B8F, 0x5C007B8A, 0x58C1663D, 0x558240E4, 0x51435D53,
	0x251D3B9E, 0x21DC2629, 0x2C9F00F0, 0x285E1D47, 0x36194D42, 0x32D850F5,
	0x3F9B762C, 0x3B5A6B9B, 0x315D626,  0x7D4CB91,  0xA97ED48,  0xE56F0FF,
	0x1011A0FA, 0x14D0BD4D, 0x19939B94, 0x1D528623, 0xF12F560E, 0xF5EE4BB9,
	0xF8AD6D60, 0xFC6C70D7, 0xE22B20D2, 0xE6EA3D65, 0xEBA91BBC, 0xEF68060B,
	0xD727BBB6, 0xD3E6A601, 0xDEA580D8, 0xDA649D6F, 0xC423CD6A, 0xC0E2D0DD,
	0xCDA1F604, 0xC960EBB3, 0xBD3E8D7E, 0xB9FF90C9, 0xB4BCB610, 0xB07DABA7,
	0xAE3AFBA2, 0xAAFBE615, 0xA7B8C0CC, 0xA379DD7B, 0x9B3660C6, 0x9FF77D71,
	0x92B45BA8, 0x9675461F, 0x8832161A, 0x8CF30BAD, 0x81B02D74, 0x857130C3,
	0x5D8A9099, 0x594B8D2E, 0x5408ABF7, 0x50C9B640, 0x4E8EE645, 0x4A4FFBF2,
	0x470CDD2B, 0x43CDC09C, 0x7B827D21, 0x7F436096, 0x7200464F, 0x76C15BF8,
	0x68860BFD, 0x6C47164A, 0x61043093, 0x65C52D24, 0x119B4BE9, 0x155A565E,
	0x18197087, 0x1CD86D30, 0x29F3D35,  0x65E2082,  0xB1D065B,  0xFDC1BEC,
	0x3793A651, 0x3352BBE6, 0x3E119D3F, 0x3AD08088, 0x2497D08D, 0x2056CD3A,
	0x2D15EBE3, 0x29D4F654, 0xC5A92679, 0xC1683BCE, 0xCC2B1D17, 0xC8EA00A0,
	0xD6AD50A5, 0xD26C4D12, 0xDF2F6BCB, 0xDBEE767C, 0xE3A1CBC1, 0xE760D676,
	0xEA23F0AF, 0xEEE2ED18, 0xF0A5BD1D, 0xF464A0AA, 0xF9278673, 0xFDE69BC4,
	0x89B8FD09, 0x8D79E0BE, 0x803AC667, 0x84FBDBD0, 0x9ABC8BD5, 0x9E7D9662,
	0x933EB0BB, 0x97FFAD0C, 0xAFB010B1, 0xAB710D06, 0xA6322BDF, 0xA2F33668,
	0xBCB4666D, 0xB8757BDA, 0xB5365D03, 0xB1F740B4
};

// I have no clue how this works and understand absolutely none of the math behind it.
// I just reversed it from Dead Space 3.
DWORD MC02(const BYTE *pb, DWORD cb)
{
	if (cb < 4)
		return 0;

	DWORD rotatedThird = (pb[2] << 8) | (pb[2] >> 24);
	DWORD ORedFirstPair = ((pb[0] << 24) | (pb[0] >> 8)) | ((pb[1] << 16) | (pb[1] >> 16));

	DWORD seedValue = ~((rotatedThird | ORedFirstPair) | pb[3]);
	pb += 4;
	cb -= 4;

	for (DWORD i = 0; i < cb; i++)
	{
		DWORD lookedUpValue = table[((seedValue >> 22) & 0x3FC) >> 2];
		DWORD insertedNum = pb[i] | (seedValue << 8);
		seedValue = lookedUpValue ^ insertedNum;
	}

	return ~seedValue;
}

DWORD FlipDword(DWORD dw)
{
    DWORD toReturn = 0;

    toReturn |= (dw >> 24);
    toReturn |= (dw >> 8) & 0xFF00;
    toReturn |= (dw << 8) & 0xFF0000;
    toReturn |= (dw << 24);

    return toReturn;
}

DWORD ReadDword(FILE *file)
{
    DWORD temp;
    fread(&temp, 4, 1, file);

    // flip it because in the file it's big endian
    return FlipDword(temp);
}

void WriteDword(FILE *file, DWORD dw)
{
    // flip it because in the file it's big endian
    dw = FlipDword(dw);
    fwrite(&dw, 4, 1, file);
}

void ReadHeader(FILE *file, MC02_Header *header)
{
    // seek to the beginning of the file
    fseek(file, 0, SEEK_SET);

    header->magic = ReadDword(file);
    header->fileLength = ReadDword(file);
    header->checksum0Length = ReadDword(file);
    header->checksum1Length = ReadDword(file);
    header->checksum0 = ReadDword(file);
    header->checksum1 = ReadDword(file);
    header->checksum2 = ReadDword(file);
}

void WriteHeader(FILE *file, MC02_Header *header)
{
    // seek to the beginning of the file
    fseek(file, 0, SEEK_SET);

    WriteDword(file, header->magic);
    WriteDword(file, header->fileLength);
    WriteDword(file, header->checksum0Length);
    WriteDword(file, header->checksum1Length);
    WriteDword(file, header->checksum0);
    WriteDword(file, header->checksum1);
    WriteDword(file, header->checksum2);
}

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        printf("Usage: mc02_fixer savePath\n");
        return -1;
    }

    BYTE headerBuff[0x18];

    FILE *save = fopen(argv[1], "rb+");

    MC02_Header header;
    ReadHeader(save, &header);

    // verify the magic
    if (header.magic != 0x4D433032)
    {
        printf("Invalid MC02 magic.\n");
        return -1;
    }

    // verify the lengths
    if (header.fileLength != (header.checksum0Length + header.checksum1Length + 0x1C))
    {
        printf("Size mismatch.\n");
        return -1;
    }

    // read in the header as a buffer for hashing later
    fseek(save, 0, SEEK_SET);
    fread(headerBuff, 1, 0x18, save);

    // fix the first checksum
    BYTE *checksum0Buffer = malloc(header.checksum0Length);

    fseek(save, 0x1C, SEEK_SET);
    fread(checksum0Buffer, 1, header.checksum0Length, save);
    header.checksum0 = MC02(checksum0Buffer, header.checksum0Length);

    free(checksum0Buffer);

    // fix the second checksum
    BYTE *checksum1Buffer = malloc(header.checksum1Length);

    fread(checksum1Buffer, 1, header.checksum1Length, save);
    header.checksum1 = MC02(checksum1Buffer, header.checksum1Length);

    free(checksum1Buffer);

    // fix the last checksum
    *(DWORD*)(headerBuff + 0x10) = FlipDword(header.checksum0);
    *(DWORD*)(headerBuff + 0x14) = FlipDword(header.checksum1);

    header.checksum2 = MC02(headerBuff, 0x18);

    WriteHeader(save, &header);

    fflush(save);
    fclose(save);

    printf("Checksums fixed.\n");

    return 0;
}

how can i build this? i'm trying to do a checksum for need for speed game. have all the values i need, but i don't know how to reverse it into a program.