Slow Motion

A looping loading animation left running on a harbour crane control panel — a GIF that never finishes. The hidden message is carried in the least-significant bits of the red channel, sampled row-wise across every frame.

Files

• slow_motion.gif — the animation
• challenge04_slow_motion_README.md — brief
• hotwa-solve/solve.py

Analysis

GIF frames are palettised; after converting each frame to RGB, the red channel's LSB looks like noise to the eye but survives the palette quantisation applied by the challenge author. The trick is where to sample: only the first row of each frame (y = 0), iterating x across the width, frame after frame.

from PIL import Image, ImageSequence

bits = []
im = Image.open("slow_motion.gif")
for frame in ImageSequence.Iterator(im):
    f = frame.convert("RGB")
    for x in range(f.width):
        r, _, _ = f.getpixel((x, 0))
        bits.append(str(r & 1))

Reassembly

The bitstream needs alignment: try each of the 8 bit offsets, and both MSB-first and LSB-first grouping, decoding bytes until an ASCII flag pattern {...} appears.

def bits_to_bytes(bits, msb_first=True):
    out = bytearray()
    for i in range(len(bits) // 8):
        chunk = bits[i*8:(i+1)*8]
        if not msb_first:
            chunk = chunk[::-1]
        out.append(int("".join(chunk), 2))
    return bytes(out)

for offset in range(8):
    for msb in (True, False):
        blob = bits_to_bytes(bits[offset:], msb_first=msb)
        m = re.search(rb"\{[A-Za-z0-9_\-]+\}", blob)
        if m:
            print(m.group(0).decode())

The offset/MSB search only has to cover 16 possibilities, and one of them yields the flag inside readable ASCII.

Takeaway

Animation formats are stego-friendly because per-frame LSB writing survives palette quantisation if you keep the payload shallow. Iterating frames in order and sampling a fixed row keeps the channel contiguous despite inter-frame compression.

Flag

Flag: not recovered in the writeup.