Code:
xor eax, eax
xor ebx, ebx
mov esi, *start adress of your code to crypt*
mov edi, esi

start:
lodsb
add bl, 25   ; changeable!
add bh, 33  ;  changeable!!
add ah, 23  ;  changeable!!
add al, ah
xor al, bl
sub al, bh
stosb
cmp esi, *end adress of your code to crypt*
jle start
jmp OEP