gistfile1.asm

; Quick hack to print 16 bytes from /dev/urandom with minimal overhead.
; nasm -f elf64 -o random.o random.asm && ld -s -o random random.o
; klg, Jun 2014

read  equ 00h
write equ 01h
open  equ 02h
exit  equ 3Ch

LEN   equ 16

  global _start

_start:

  sub rsp, 2*LEN+1

  mov rdi, path
  mov rsi, 0
  mov rdx, 0
  mov rax, open
  syscall

  cmp rax, 0
  jl done

  mov rdi, rax
  mov rsi, rsp
  mov rdx, LEN
  mov rax, read
  syscall

  cmp rax, LEN
  jne done

  mov rcx, rax
binhex:
  mov rbx, 0
  mov bl, [rcx+rsp-1]
  mov rdx, rbx
  shr rbx, 4
  and rdx, 0xf
  mov al, [hexa+rdx]
  mov [2*rcx+rsp-1], al
  mov al, [hexa+rbx]
  mov [2*rcx+rsp-2], al
  loop binhex
  mov [rsp+LEN*2], byte 0x0a

  mov rsi, rsp
  mov rdx, 2*LEN+1
  mov rdi, 1
  mov rax, write
  syscall

  cmp rax, 2*LEN+1
  jne done

  mov rax, 0

done:
  mov rdi, rax
  mov rax, exit
  syscall

path  db "/dev/urandom", 0
hexa  db "0123456789abcdef"

; vim: set ft=nasm ai :