Rating:
# Car Crash — Writeup
- **Category:** Steganography
- **Points:** 980
- **Flag:** `CIT{7E3qU4wE}`
## Observations
- `exiftool` on [car_crash.png](files/car_crash.png) reports six `tEXt` chunks with
plausible-but-nonsense keys — `render-group-bsDFp`, `palette-trace-A8sBeb`,
`map-stamp`, `digest_path`, `manifestLayer-9aG4A`, `palette_refs-BB6oT7C6` —
each carrying a base64-looking blob. These turn out to be **decoys**
(one blob has length 97, which is not a valid base64 length — a hint they
are not meant to be decoded).
- The alpha channel has **only two distinct values** (254 and 255): a strong
steganography signal that its bit-0 (LSB) plane carries data.
- Viewing any single-channel LSB plane as a 1-bit image shows noisy text-like
patterns but nothing readable. Likewise for concatenations and for per-chunk
decodings.
- The trick: the RGBA LSB planes *each* carry rendered text plus a common
carrier noise. **XOR-ing two planes cancels the carrier and reveals one
clean text layer.**
## Solution
Render the bit-0 plane of `G XOR A` as a black-and-white image:
```python
from PIL import Image
img = Image.open("car_crash.png")
w, h = img.size
px = list(img.getdata())
out = Image.new("L", (w, h))
out.putdata([((p[1] ^ p[3]) & 1) * 255 for p in px])
out.save("ga_xor.png")
```
Inside the rendered noise, a centered base64 string is clearly visible:
```text
Q0lUezdFM3FVNHdFfQ==
```
Decoding it gives the flag:
```python
>>> base64.b64decode("Q0lUezdFM3FVNHdFfQ==")
b'CIT{7E3qU4wE}'
```
## Why G⊕A specifically
The author drew a single text layer into two channel LSB planes, XOR-ed with
a shared noise pattern. XOR-ing those two planes back together cancels the
noise and leaves the text. Any other channel pair still carries a different,
uncancelled noise — that's why only the G⊕A view is legible.
## Reproduce
```sh
python3 scripts/solve.py
# writes files/extractions/ga_xor.png and prints the flag
```
## Red herrings
- The six `tEXt` chunks of base64-looking blobs.
- `zsteg`'s wbStego hit on `b1,bgr,lsb,xy` — a false positive from the
3-bit periodicity of a per-pixel RGB LSB stream.
