Rating:
# Dog barking — Writeup
- Category: Misc
- Value: 902 pts
- Author: by **ronnie**
## Challenge
I recorded this audio at the dogpark the other day and I think the dogs were trying to tell me something???
**FLAG FORMAT:** `CIT{example_flag}`
## Recon
We are given a `.wav` file of dogs barking. Looking closely at the audio (e.g. by extracting the envelope and peak frequencies of each bark), we notice there are exactly three distinct types of barks:
1. A short, low-pitched bark (~491 Hz, ~120ms long)
2. A long, high-pitched bark (~589 Hz, ~180ms long)
3. A short, medium-pitched bark (~525 Hz, ~130ms long)
## Solve
If we map the short, low-pitched bark to `0` and the long, high-pitched bark to `1`, and treat the medium-pitched bark as a space character, we get a sequence of 8-bit binary chunks.
Here is a Python script that decodes the audio:
```python
from scipy.io import wavfile
import numpy as np
sample_rate, samples = wavfile.read("../files/challenge.wav")
if len(samples.shape) > 1:
samples = samples[:, 0]
chunk_size = sample_rate // 100 # 10ms chunks
envelope = [np.max(np.abs(samples[i:i+chunk_size])) for i in range(0, len(samples), chunk_size)]
threshold = np.max(envelope) * 0.15
is_barking = [1 if e > threshold else 0 for e in envelope]
events = []
current_state = is_barking[0]
current_len = 0
for state in is_barking:
if state == current_state:
current_len += 1
else:
events.append((current_state, current_len))
current_state = state
current_len = 1
events.append((current_state, current_len))
words = []
current_pos = 0
for state, length in events:
if state == 1:
bark_samples = samples[current_pos * chunk_size : (current_pos + length) * chunk_size]
w = np.fft.fft(bark_samples)
freqs = np.fft.fftfreq(len(w))
peak_freq = abs(freqs[np.argmax(np.abs(w))]) * sample_rate
if peak_freq < 500:
words.append("0")
elif peak_freq > 550:
words.append("1")
else:
words.append(" ")
current_pos += length
binary = "".join(words)
flag = "".join(chr(int(b, 2)) for b in binary.split(" "))
print(flag)
```
Running this script outputs the flag perfectly.
## Flag
```text
CIT{b4rking_up_th3_wr0ng_tr33}
```
