x96 Windows Shellcode: one payload able to used in both 32-bit & 64-bit

x96shell_msgbox.asm

; x96 shellcode (x32+x64) by [email protected]
; yasm -f bin -o x96shell_msgbox x96shell_msgbox.asm
section .text
bits 32
_main:
call entry
entry:
mov ax, cs
sub ax, 0x23
jz retTo32b
nop

retTo64b:
add dword [esp], b64_shellcode-entry
ret
retTo32b:
add dword [esp], b32_shellcode-entry
ret

; 64 bit shellcode - FatalAppExitA(0, "64bit Hello!")
b64_shellcode:
db 0xE9, 0x2B, 0x01, 0x00, 0x00, 0x90, 0x4C, 0x8D, 0x41, 0x02, 0x31, 0xC0, 0x66, 0x83, 0x39, 0x00, 0x74, 0x1E, 0x41, 0x0F, 0xB7, 0x08, 0x49, 0x83, 0xC0, 0x02, 0x89, 0xCA, 0x83, 0xCA, 0x20, 0x0F, 0xB7, 0xD2, 0x01, 0xD0, 0xC1, 0xC8, 0x08, 0x66, 0x85, 0xC9, 0x75, 0xE6, 0xC3, 0x0F, 0x1F, 0x00, 0xC3, 0x4C, 0x8D, 0x41, 0x01, 0x31, 0xC0, 0x80, 0x39, 0x00, 0x74, 0x24, 0x0F, 0x1F, 0x40, 0x00, 0x41, 0x0F, 0xB6, 0x08, 0x49, 0x83, 0xC0, 0x01, 0x89, 0xCA, 0x83, 0xCA, 0x20, 0x0F, 0xBE, 0xD2, 0x01, 0xD0, 0xC1, 0xC8, 0x08, 0x84, 0xC9, 0x75, 0xE7, 0xC3, 0x66, 0x0F, 0x1F, 0x44, 0x00, 0x00, 0xC3, 0x65, 0x48, 0x8B, 0x04, 0x25, 0x60, 0x00, 0x00, 0x00, 0x48, 0x8B, 0x40, 0x18, 0x4C, 0x8B, 0x48, 0x20, 0x4C, 0x8D, 0x50, 0x20, 0x4D, 0x39, 0xD1, 0x74, 0x2F, 0x48, 0x83, 0xEC, 0x28, 0x41, 0x89, 0xCB, 0xEB, 0x08, 0x4D, 0x8B, 0x09, 0x4D, 0x39, 0xD1, 0x74, 0x17, 0x49, 0x8B, 0x49, 0x50, 0xE8, 0x71, 0xFF, 0xFF, 0xFF, 0x44, 0x39, 0xD8, 0x75, 0xEA, 0x49, 0x8B, 0x41, 0x20, 0x48, 0x83, 0xC4, 0x28, 0xC3, 0x31, 0xC0, 0x48, 0x83, 0xC4, 0x28, 0xC3, 0x31, 0xC0, 0xC3, 0x57, 0x56, 0x53, 0x48, 0x83, 0xEC, 0x20, 0x48, 0x63, 0x41, 0x3C, 0x8B, 0xB4, 0x01, 0x88, 0x00, 0x00, 0x00, 0x85, 0xF6, 0x74, 0x42, 0x48, 0x01, 0xCE, 0x8B, 0x46, 0x18, 0x85, 0xC0, 0x74, 0x38, 0x44, 0x8B, 0x4E, 0x20, 0x89, 0xD7, 0x49, 0x89, 0xCB, 0x45, 0x31, 0xD2, 0x8D, 0x58, 0xFF, 0x49, 0x01, 0xC9, 0xEB, 0x03, 0x4D, 0x89, 0xC2, 0x4D, 0x85, 0xC9, 0x74, 0x0F, 0x41, 0x8B, 0x09, 0x4C, 0x01, 0xD9, 0xE8, 0x3D, 0xFF, 0xFF, 0xFF, 0x39, 0xF8, 0x74, 0x18, 0x4D, 0x8D, 0x42, 0x01, 0x49, 0x83, 0xC1, 0x04, 0x4C, 0x39, 0xD3, 0x75, 0xDC, 0x48, 0x83, 0xC4, 0x20, 0x31, 0xC0, 0x5B, 0x5E, 0x5F, 0xC3, 0x90, 0x8B, 0x46, 0x24, 0x4B, 0x8D, 0x14, 0x53, 0x0F, 0xB7, 0x14, 0x02, 0x8B, 0x46, 0x1C, 0x49, 0x8D, 0x14, 0x93, 0x8B, 0x04, 0x02, 0x48, 0x83, 0xC4, 0x20, 0x5B, 0x5E, 0x5F, 0x4C, 0x01, 0xD8, 0xC3, 0x48, 0xB8, 0x46, 0x61, 0x74, 0x61, 0x6C, 0x41, 0x70, 0x70, 0x57, 0x56, 0x53, 0x48, 0x83, 0xEC, 0x40, 0x48, 0x89, 0x44, 0x24, 0x32, 0x48, 0x8D, 0x4C, 0x24, 0x32, 0xB8, 0x41, 0x00, 0x00, 0x00, 0xC7, 0x44, 0x24, 0x3A, 0x45, 0x78, 0x69, 0x74, 0x66, 0x89, 0x44, 0x24, 0x3E, 0xE8, 0xCF, 0xFE, 0xFF, 0xFF, 0x89, 0xC7, 0x65, 0x48, 0x8B, 0x04, 0x25, 0x60, 0x00, 0x00, 0x00, 0x48, 0x8B, 0x40, 0x18, 0x48, 0x8B, 0x58, 0x20, 0x48, 0x8D, 0x70, 0x20, 0x48, 0x39, 0xDE, 0x75, 0x0A, 0xEB, 0x45, 0x48, 0x8B, 0x1B, 0x48, 0x39, 0xDE, 0x74, 0x10, 0x48, 0x8B, 0x4B, 0x20, 0x89, 0xFA, 0xE8, 0x1A, 0xFF, 0xFF, 0xFF, 0x48, 0x85, 0xC0, 0x74, 0xE8, 0xC7, 0x44, 0x24, 0x2D, 0x6C, 0x6C, 0x6F, 0x21, 0x48, 0x8D, 0x54, 0x24, 0x25, 0x31, 0xC9, 0x48, 0xBF, 0x36, 0x34, 0x62, 0x69, 0x74, 0x20, 0x48, 0x65, 0x48, 0x89, 0x7C, 0x24, 0x25, 0xC6, 0x44, 0x24, 0x31, 0x00, 0xFF, 0xD0, 0x48, 0x83, 0xC4, 0x40, 0x5B, 0x5E, 0x5F, 0xC3, 0x31, 0xC0, 0xEB, 0xCF, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90

; 32 bit shellcode - FatalAppExitA(0, "32bit Hello!")
b32_shellcode: 
db 0xE9, 0x1E, 0x01, 0x00, 0x00, 0x90, 0x66, 0x83, 0x39, 0x00, 0x74, 0x24, 0x53, 0x31, 0xC0, 0x8D, 0x59, 0x02, 0x0F, 0xB7, 0x0B, 0x83, 0xC3, 0x02, 0x89, 0xCA, 0x83, 0xCA, 0x20, 0x0F, 0xB7, 0xD2, 0x01, 0xD0, 0xC1, 0xC8, 0x08, 0x66, 0x85, 0xC9, 0x75, 0xE8, 0x5B, 0xC3, 0x8D, 0x74, 0x26, 0x00, 0x31, 0xC0, 0xC3, 0x80, 0x39, 0x00, 0x74, 0x28, 0x53, 0x31, 0xC0, 0x8D, 0x59, 0x01, 0x66, 0x90, 0x0F, 0xB6, 0x0B, 0x83, 0xC3, 0x01, 0x89, 0xCA, 0x83, 0xCA, 0x20, 0x0F, 0xBE, 0xD2, 0x01, 0xD0, 0xC1, 0xC8, 0x08, 0x84, 0xC9, 0x75, 0xE9, 0x5B, 0xC3, 0x8D, 0xB4, 0x26, 0x00, 0x00, 0x00, 0x00, 0x31, 0xC0, 0xC3, 0x57, 0x56, 0x53, 0x64, 0xA1, 0x30, 0x00, 0x00, 0x00, 0x8B, 0x40, 0x0C, 0x8B, 0x58, 0x14, 0x8D, 0x70, 0x14, 0x39, 0xF3, 0x74, 0x27, 0x89, 0xCF, 0xEB, 0x09, 0x8D, 0x76, 0x00, 0x8B, 0x1B, 0x39, 0xF3, 0x74, 0x1A, 0x8B, 0x4B, 0x28, 0xE8, 0x78, 0xFF, 0xFF, 0xFF, 0x39, 0xF8, 0x75, 0xEE, 0x8B, 0x43, 0x10, 0x5B, 0x5E, 0x5F, 0xC3, 0x8D, 0xB4, 0x26, 0x00, 0x00, 0x00, 0x00, 0x5B, 0x31, 0xC0, 0x5E, 0x5F, 0xC3, 0x8B, 0x41, 0x3C, 0x8B, 0x44, 0x01, 0x78, 0x85, 0xC0, 0x74, 0x6F, 0x55, 0x01, 0xC8, 0x57, 0x56, 0x53, 0x83, 0xEC, 0x08, 0x8B, 0x78, 0x18, 0x89, 0x44, 0x24, 0x04, 0x85, 0xFF, 0x74, 0x28, 0x8B, 0x58, 0x20, 0x89, 0x14, 0x24, 0x89, 0xCE, 0x31, 0xED, 0x01, 0xCB, 0x85, 0xDB, 0x74, 0x0E, 0x8B, 0x0B, 0x01, 0xF1, 0xE8, 0x55, 0xFF, 0xFF, 0xFF, 0x3B, 0x04, 0x24, 0x74, 0x1D, 0x83, 0xC5, 0x01, 0x83, 0xC3, 0x04, 0x39, 0xEF, 0x75, 0xE4, 0x83, 0xC4, 0x08, 0x31, 0xC0, 0x5B, 0x5E, 0x5F, 0x5D, 0xC3, 0x89, 0xF6, 0x8D, 0xBC, 0x27, 0x00, 0x00, 0x00, 0x00, 0x8B, 0x7C, 0x24, 0x04, 0x8D, 0x04, 0x6E, 0x03, 0x47, 0x24, 0x0F, 0xB7, 0x00, 0x8D, 0x04, 0x86, 0x03, 0x47, 0x1C, 0x03, 0x30, 0x83, 0xC4, 0x08, 0x5B, 0x89, 0xF0, 0x5E, 0x5F, 0x5D, 0xC3, 0x90, 0x31, 0xC0, 0xC3, 0x57, 0xB8, 0x41, 0x00, 0x00, 0x00, 0x56, 0x53, 0x83, 0xEC, 0x30, 0x8D, 0x4C, 0x24, 0x22, 0xC7, 0x44, 0x24, 0x22, 0x46, 0x61, 0x74, 0x61, 0xC7, 0x44, 0x24, 0x26, 0x6C, 0x41, 0x70, 0x70, 0xC7, 0x44, 0x24, 0x2A, 0x45, 0x78, 0x69, 0x74, 0x66, 0x89, 0x44, 0x24, 0x2E, 0xE8, 0xDF, 0xFE, 0xFF, 0xFF, 0x89, 0xC7, 0x64, 0xA1, 0x30, 0x00, 0x00, 0x00, 0x8B, 0x40, 0x0C, 0x8B, 0x58, 0x14, 0x8D, 0x70, 0x14, 0x39, 0xDE, 0x75, 0x0D, 0xEB, 0x55, 0x90, 0x8D, 0x74, 0x26, 0x00, 0x8B, 0x1B, 0x39, 0xDE, 0x74, 0x0E, 0x8B, 0x4B, 0x10, 0x89, 0xFA, 0xE8, 0x26, 0xFF, 0xFF, 0xFF, 0x85, 0xC0, 0x74, 0xEC, 0x8D, 0x54, 0x24, 0x15, 0xC7, 0x44, 0x24, 0x15, 0x33, 0x32, 0x62, 0x69, 0xC7, 0x44, 0x24, 0x19, 0x74, 0x20, 0x48, 0x65, 0xC7, 0x44, 0x24, 0x1D, 0x6C, 0x6C, 0x6F, 0x21, 0xC6, 0x44, 0x24, 0x21, 0x00, 0x89, 0x54, 0x24, 0x04, 0xC7, 0x04, 0x24, 0x00, 0x00, 0x00, 0x00, 0xFF, 0xD0, 0x83, 0xEC, 0x08, 0x83, 0xC4, 0x30, 0x5B, 0x5E, 0x5F, 0xC3, 0x8D, 0x74, 0x26, 0x00, 0x31, 0xC0, 0xEB, 0xC0, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90, 0x90

You can just use

xor rax, rax
test rax, rax
je @b64_shellcode

on x32 same bytes will become

dec eax
xor eax, eax
dec eax
test eax, eax
je @b64_shellcode

so, merging 2 shellcodes can be achived without assembler at all, just via simple byte concatation

\x48\x31\xC0\x48\x85\xC0\x0F\x84 + 4 byte (length x32 shell code) + x32 shellcode + x64 shellcode.