HackThisSite - Steganography Mission 2

Challenge

did I hear that correctly?

我这个听对了吗？

题目提供一个 WAV 音频文件，要求找出隐藏的密码。

Solution

先确认 WAV 的基本参数：

$ soxi 2.wav
Input File     : '2.wav'
Channels       : 2
Sample Rate    : 22050
Precision      : 16-bit
Duration       : 00:00:10.00 = 220500 samples ~ 750 CDDA sectors
Bit Rate       : 706k
Sample Encoding: 16-bit Signed Integer PCM

这是 22050 Hz、16-bit、双声道、10 秒的 PCM 音频。直接播放只能听到噪声；反向播放、交换声道也没有可懂内容。频谱分析显示，异常的时间变化集中在约 1.2–1.6 kHz，说明载荷不在普通听感或 WAV 元数据里，而是画在时频平面上。

命令行直接渲染频谱：

$ sox 2.wav -n spectrogram -o spec.png -z 90

为了让字符更清楚，限制频率范围并提高时频分辨率。下面的脚本使用 NFFT=1024、HOP=64，将 0–2000 Hz 的频谱保存为灰度图：

#!/usr/bin/env python3
"""Render the spectrogram band that carries the password.

Usage:
  uv run --with numpy --with pillow python render_spec.py 2.wav spec.png
"""
import sys
import wave

import numpy as np
from PIL import Image

NFFT, HOP = 1024, 64
FMAX = 2000
DYNAMIC_RANGE = 70.0


def load_mono(path):
    with wave.open(path, "rb") as w:
        sample_rate = w.getframerate()
30 collapsed lines
        channels = w.getnchannels()
        frames = w.readframes(w.getnframes())
    pcm = np.frombuffer(frames, dtype=np.int16).astype(np.float64)
    return sample_rate, pcm.reshape(-1, channels).mean(axis=1)


def render(path):
    sample_rate, samples = load_mono(path)
    window = np.hanning(NFFT)
    freqs = np.fft.rfftfreq(NFFT, 1.0 / sample_rate)
    keep = int(np.searchsorted(freqs, FMAX))
    frame_count = 1 + (len(samples) - NFFT) // HOP
    magnitude = np.empty((keep, frame_count))

    for index in range(frame_count):
        start = index * HOP
        frame = samples[start:start + NFFT] * window
        magnitude[:, index] = np.abs(np.fft.rfft(frame))[:keep]

    db = 20.0 * np.log10(magnitude + 1e-9)
    db -= db.max()
    image = np.clip((db + DYNAMIC_RANGE) / DYNAMIC_RANGE, 0.0, 1.0)
    image = (image[::-1] * 255).astype(np.uint8)
    return image


def main():
    source = sys.argv[1] if len(sys.argv) > 1 else "2.wav"
    target = sys.argv[2] if len(sys.argv) > 2 else "spec.png"
    image = Image.fromarray(render(source), "L")
    image = image.resize((image.width, image.height * 4), Image.NEAREST)
    image.save(target)
    print("wrote %s (%d x %d, 0-%d Hz)" %
          (target, image.width, image.height, FMAX))


if __name__ == "__main__":
    main()

$ uv run --with numpy --with pillow python render_spec.py 2.wav spec.png
wrote spec.png (3430 x 372, 0-2000 Hz)

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