+-------------------------------------------------------------------------------
| Binary Golf Grand Prix '23
|
| by hed0x 
|   ~ hed0x@linuxmail.org
|
| Jul 29, 2023.
+-------------------------------------------------------------------------------

; ------------------------------------------------------------------------------
; Introduction
; ------------------------------------------------------------------------------

This is my first writeup ever. I never coded in Assembly, although I always
wanted to learn it. I believe that doing something towards a challenge, motivates
me much more that just writing code.

Here is what I've done for the Binary Golf Grand Prix '23


; ------------------------------------------------------------------------------
; BINARY GOLF GRAND PRIX (BGGP) 4/2023: The challenge
; ------------------------------------------------------------------------------

The next lines were extracted from the BGGP4 web page [1]:

Goal: Create the smallest self-replicating file.

Requirements:
    - Produce exactly 1 copy of itself
    - Name the copy "4"
    - Not execute the copied file 
    - Print, return, or display the number 4 




; ------------------------------------------------------------------------------
; My solution
; ------------------------------------------------------------------------------

The very first piece of code set the memory offset where the code will be loaded.
For more information on .COM files, please check [5][6].

- - -
    org 100h
- - -

The next code is responsible to copy data (1024 bytes) starting from the label 
'start' to the memory segment 0x7C.

- - -
    mov ax, cs
    mov ds, ax
    mov es, ax
    mov si, start
    mov di, 7C0h
    mov cx, 1024
    cld
    rep movsb
- - -

We start it by printing the character '4' on the screen, using BIOS 
interrupt 0x10 [3]. This interrupt allows us to write the character, stored in 
register al, to the teletype (tty):

- - -
    mov ah, 0x0E
    mov al, '4'
    int 0x10
- - -

To create a file we use the interrupt 21 Service 60 (code 0x3C). The only parameter
is the file name (fn) that should be stores in the register dx.

- - -
    mov dx, fn
    mov ah, 0x3C
    int 0x21
- - -

The output of the previous operation is the file handler stored in register ax.
We move it to the register bx just because we will need it there for when writing
the data to the disk.

- - -
    mov bx, ax
- - -

Now we write the data into a file. The number of bytes should be in the cx 
register, the buffer address should be in dx and the code of the operation to be 
performed should be stored in ah register. According to [2] the code is 0x40.
Last, interrupt 0x21 is called to perform the operation.

- - -
    mov cx, size
    mov dx, start
    mov ah, 0x40 ; Write to file function
    int 0x21
- - -

The next part closes the file handle. The code 0x3E refers to this operation [2].

- - -
    mov ah, 0x3E
    int 0x21
- - -

The data used in previous commands are the following:

- - -
fn db '4.COM', 0
size equ $ - start
```


; ------------------------------------------------------------------------------
; Assembler
; ------------------------------------------------------------------------------

The whole source code can be found at [7]. To build it I used the NASM assembler
with the following command [4]:

> nasm -f bin bggp4-hed0x.asm -o bggp4-hed0x.com



; ------------------------------------------------------------------------------
; Checksum
; ------------------------------------------------------------------------------

The SHA256 checksum of both original file and copy are exactly the same:

> sha256sum bggp4-hed0x.com
    99622d24b81ec6ec663669a00679e8fd02cba2400d2ce1893420122622ec336d *bggp4-hed0x.com

> sha256sum 4.COM
    99622d24b81ec6ec663669a00679e8fd02cba2400d2ce1893420122622ec336d *4.COM



; ------------------------------------------------------------------------------
; Base64 encoding
; ------------------------------------------------------------------------------

> base64 bggp4-hed0x.com
    jMiO2I7AvgABv8AHuQAE/POktA6wNM0QujABtDzNIYnDuTYAugABtEDNIbQ+zSHDNC5DT00A




; ------------------------------------------------------------------------------
; The final submission
; ------------------------------------------------------------------------------

---BEGIN BGGP4 ENTRY---
Name or handle: hed0x
Contact Info: hed0x@linuxmail.org
Website, twitter, other online presence: http://hed0x.github.io
Target File Type: COM
SHA256 Hash: 99622d24b81ec6ec663669a00679e8fd02cba2400d2ce1893420122622ec336d
Target Environment (How do we run the file?): DOSBox
Any additional info?: No.
Link to PoC video, screenshot, or console output, if any: N/A.
Link to writeup, if any: http://hed0x.github.io/txt/bggp4/binary-golf-2023.txt
File contents (base64 encoded please): jMiO2I7AvgABv8AHuQAE/POktA6wNM0QujABtDzNIYnDuTYAugABtEDNIbQ+zSHDNC5DT00A
---END BGGP4 ENTRY---
    


; ------------------------------------------------------------------------------
; References
; ------------------------------------------------------------------------------

[1] https://binary.golf/
[2] https://grandidierite.github.io/dos-interrupts/
[3] https://grandidierite.github.io/bios-interrupts/
[4] https://www.nasm.us
[5] https://www.nasm.us/xdoc/2.16.01/html/nasmdoc9.html#section-9.2
[6] COM Walkthrough: https://imgur.com/cuyliWz
[7] https://github.com/hed0x/hed0x.github.io/tree/main/txt/bggp4