format PE GUI
include 'win32ax.inc'

; MSDN Random Example:
; https://msdn.microsoft.com/en-us/library/windows/desktop/aa379942(v=vs.85).aspx

.data
      LENGTH               equ 10         ; number of random BYTES
      buffer               rb  255        ; random hex string
      hcrypt               dd  ?
      ascii                db  '0123456789ABCDEF'

      RESERVED             equ NULL
      PROV_RSA_FULL        equ 1           ; values from wincrypt.h:
      CRYPT_VERIFYCONTEXT  equ 0xF0000000  ; http://sourceforge.net/u/earnie/winapi/winapi/ci/Initial/tree/include/wincrypt.h

.code
      start:
                    ; wincrypto funcitons are from Advapi32.dll
            invoke  CryptAcquireContext, hcrypt, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT
            invoke  CryptGenRandom, [hcrypt], LENGTH, buffer
            invoke  CryptReleaseContext, [hcrypt], RESERVED

                    ; now buffer contains LENGTH random BYTES
                    ; we need to transform it to HEX string
                    ; gonna fill buffer with hex characters backwards
                    mov edi, LENGTH * 2    ; edi is index of hex string
                    mov esi, LENGTH        ; esi is index of random generated byte
                    mov [buffer + edi], 0  ; terminate our hex string
                    mov eax, 0             ; eax is index of ascii character
      bin2hex:
                    mov al, [buffer + esi - 1]  ; copy random byte to AL
                    shr al, 4                   ; get last 4 bits of the byte
                    mov cl, [ascii + eax]       ; copy corresponding ascii character to its place
                    mov [buffer + edi - 1], cl  ;
                    dec edi
                    mov al, [buffer + esi - 1]
                    shl al, 4                   ; get first 4 bits of the byte
                    shr al, 4                   ;
                    mov cl, [ascii + eax]
                    mov [buffer + edi - 1], cl  ; copy ascii character to its place
                    dec edi
                    dec esi
                    jnz bin2hex                 ; repeat until index of random generated byte is ZERO

            invoke  MessageBox, 0, buffer, 'Random Hex', MB_OK
      finish:
            invoke  ExitProcess, 0


.end  start