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import argparse
import sys
from typing import List, Optional, Tuple
import av
import numpy as np
def pack_bits(bits01: np.ndarray, bitorder: str = "big") -> bytes:
bits01 = bits01.astype(np.uint8)
n = (bits01.size // 8) * 8
bits01 = bits01[:n]
return np.packbits(bits01, bitorder=bitorder).tobytes()
def _plane_to_1d(plane: av.video.plane.VideoPlane, width: int, height: int) -> np.ndarray:
buf = np.frombuffer(plane, dtype=np.uint8)
if plane.line_size == width:
return buf[: width * height]
rows = buf.reshape(height, plane.line_size)
return rows[:, :width].reshape(-1)
def extract_first_part_pixels(path: str) -> bytes:
container = av.open(path)
try:
frame = next(container.decode(video=0))
frame = frame.reformat(format="yuv444p")
w, h = frame.width, frame.height
n = w * h
y = _plane_to_1d(frame.planes[0], w, h)[:n]
u = _plane_to_1d(frame.planes[1], w, h)[:n]
v = _plane_to_1d(frame.planes[2], w, h)[:n]
bits = np.empty(n * 3, dtype=np.uint8)
bits[0::3] = v & 1
bits[1::3] = y & 1
bits[2::3] = u & 1
data = pack_bits(bits, bitorder="big")
if len(data) < 4:
raise ValueError("LSB stream too short")
L = int.from_bytes(data[0:4], "big")
payload = data[4 : 4 + L]
return payload
finally:
container.close()
def _durations_from_packets(path: str) -> List[int]:
container = av.open(path)
try:
vstream = next(s for s in container.streams if s.type == "video")
durs: List[Optional[int]] = []
pts_list: List[Optional[int]] = []
for pkt in container.demux(vstream):
if pkt.dts is None and pkt.pts is None:
continue
pts_list.append(pkt.pts)
durs.append(pkt.duration)
if all(d is not None for d in durs) and len(durs) > 0:
vals = [int(d) for d in durs] # type: ignore
if len(set(vals)) >= 2:
return vals
pts_clean = [p for p in pts_list if p is not None]
if len(pts_clean) < 2:
return []
inferred = []
for i in range(len(pts_clean) - 1):
inferred.append(int(pts_clean[i + 1] - pts_clean[i]))
inferred.append(inferred[-1])
return inferred
finally:
container.close()
def _iter_boxes(data: bytes, start: int, end: int) -> List[Tuple[int, int, int, bytes]]:
boxes = []
off = start
while off + 8 <= end:
size = int.from_bytes(data[off : off + 4], "big")
btype = data[off + 4 : off + 8]
header = 8
if size == 1:
if off + 16 > end:
break
size = int.from_bytes(data[off + 8 : off + 16], "big")
header = 16
elif size == 0:
size = end - off
if size < header or off + size > end:
break
boxes.append((off, size, header, btype))
off += size
return boxes
def _find_trak_boxes(data: bytes) -> List[Tuple[int, int, int]]:
traks: List[Tuple[int, int, int]] = []
top = _iter_boxes(data, 0, len(data))
for off, size, header, btype in top:
if btype == b"moov":
for off2, size2, header2, btype2 in _iter_boxes(data, off + header, off + size):
if btype2 == b"trak":
traks.append((off2, size2, header2))
return traks
def _get_handler_type(data: bytes, trak_off: int, trak_size: int) -> Optional[bytes]:
for off, size, header, btype in _iter_boxes(data, trak_off + 8, trak_off + trak_size):
if btype != b"mdia":
continue
for off2, size2, header2, btype2 in _iter_boxes(data, off + header, off + size):
if btype2 != b"hdlr":
continue
base = off2 + header2
if base + 12 > off2 + size2:
return None
return data[base + 8 : base + 12]
return None
def _get_stts_entries(data: bytes, trak_off: int, trak_size: int) -> Optional[List[int]]:
for off, size, header, btype in _iter_boxes(data, trak_off + 8, trak_off + trak_size):
if btype != b"mdia":
continue
for off2, size2, header2, btype2 in _iter_boxes(data, off + header, off + size):
if btype2 != b"minf":
continue
for off3, size3, header3, btype3 in _iter_boxes(data, off2 + header2, off2 + size2):
if btype3 != b"stbl":
continue
for off4, size4, header4, btype4 in _iter_boxes(data, off3 + header3, off3 + size3):
if btype4 != b"stts":
continue
base = off4 + header4
if base + 8 > off4 + size4:
return None
entry_count = int.from_bytes(data[base + 4 : base + 8], "big")
pos = base + 8
durations: List[int] = []
for _ in range(entry_count):
if pos + 8 > off4 + size4:
break
count = int.from_bytes(data[pos : pos + 4], "big")
delta = int.from_bytes(data[pos + 4 : pos + 8], "big")
if count > 0:
durations.extend([delta] * count)
pos += 8
return durations if durations else None
return None
def _durations_from_stts(path: str) -> List[int]:
data = open(path, "rb").read()
for trak_off, trak_size, _ in _find_trak_boxes(data):
htype = _get_handler_type(data, trak_off, trak_size)
if htype != b"vide":
continue
durs = _get_stts_entries(data, trak_off, trak_size)
if durs:
return durs
for trak_off, trak_size, _ in _find_trak_boxes(data):
durs = _get_stts_entries(data, trak_off, trak_size)
if durs:
return durs
raise ValueError("No stts durations found")
def extract_last_part_timing(path: str) -> bytes:
durs = _durations_from_packets(path)
if len(set(durs)) < 2:
durs = _durations_from_stts(path)
uniq = sorted(set(durs))
if len(uniq) < 2:
raise ValueError(f"Need 2 distinct duration values, got {uniq}")
if len(uniq) > 2:
uniq = uniq[:2]
small, large = uniq[0], uniq[1]
bits = np.array([1 if d == large else 0 for d in durs], dtype=np.uint8)
data = pack_bits(bits, bitorder="big")
return data.rstrip(b"\x00")
def combine_flag(first_payload: bytes, last_payload: bytes) -> str:
fp = first_payload.decode("utf-8", errors="replace")
lp = last_payload.decode("utf-8", errors="replace")
if "hkcert25{" in fp:
fp = fp[fp.index("hkcert25{") :]
fp = fp.replace("first part:", "")
if "last part:" in lp:
lp = lp.split("last part:", 1)[1].lstrip()
return fp + lp
def main():
ap = argparse.ArgumentParser()
ap.add_argument("input", help="Path to .AVIF")
args = ap.parse_args()
first = extract_first_part_pixels(args.input)
last = extract_last_part_timing(args.input)
print("[+] first payload :", first)
print("[+] last payload :", last)
flag = combine_flag(first, last)
print("\n[+] FLAG =", flag)
if __name__ == "__main__":
main()
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