---
title: Lab Meeting Minutes 2022/4/1
tags: lab_meeting
---
> Outline
> [TOC]
---
# PERAL Lab Meeting
- 時間：111 年 4 月 1 日 10:00
- 地點：TC 321
- 出席者：吳坤熹老師、謝萬霖、周以恆、吳騰然、劉怡君、田蕙瑜、洪胤勛、丘世宇、莊才賢
- 會議主題： [Frequency Hopping in Steganography & Chi-square Test](https://l.facebook.com/l.php?u=https%3A%2F%2Fms15.voip.edu.tw%2F~phoebe%2Ffiles%2Fppt%2FSpread%2520Spectrum%2520%26%2520Chi-square%2520Test.pptx%3Ffbclid%3DIwAR10fcw2yGjDWtN7p21yYl4hVX0B0pCLTBhSvojv_WsNkTjz8Ff1ppnXzpA&h=AT1QjZuq58bRv4tT_S5qMSKqygp8sGRE7CG-B5JaKF0faKr0StiD6nr3MtpFqJtefN1mKEFGWRfdJIH8YhaqvHrX_Kp6ULNU9uST0ZeIT5BIS0TkBZKp_uuQ-Rfz1rriB2FVo0HjDv5-DEJth-Sx&__tn__=H-R&c[0]=AT3n3l9zPQG51yr5XoxFXeXJfGzbu_iePOLKvBk3YXgLUf3EkJv1uLgVE7AmTI1BpqqXf2Aph9sLdh1TrwbS4g4tf3AvdlERub54T9O1c8XYBpw647n6pEe3dB_5xRYTOMqFpbhaYWNWpLzZXqAgIH7Ev_AgvK8h95OkjiHp3RTOgAdvW-mpClx6lj2nSU7Tzxm7UYJN08rw28wsBUw)
- 主講者: 劉怡君
- 主記: 田蕙瑜

## 會議內容
### Outline
- Spread Spectrum
- Direct-sequence Spread Spectrum(DSSS)
- Frequency-hopping Spread Spectrum(FHSS)
- Spread Spectrum in Steganography
- Chi-square Test
- Chi-square Test Examples
- Crack Caesar Cipher with Chi-squared Algorithm

### Spread Spectrum
- Direct-sequence Spread Spectrum(DSSS)
- DSSS encoded datd signal with a higher bit rate sequence
- Use redundancy to get recovery.
- The smaller this duration, the larger the bandwidth of the resulting DSSS signals.

- Frequency-hopping Spread Spectrum(FHSS)
- Sends data on one frequency at a time, and uses different frequencies over time.
- The hopping pattern is known only between receiver and transmitter.
- By pseudo random but not in realistic
- Why not in realistic?
- If only one pseudo random, it's too easy to catch the data or collision.
- Solution: Use multiple pseudo random to send data at the same time.(?)
Design a algorithm to send multiple data by diff. pseudo random frequencys at once.
- p.4 不一樣的是載波(carrier wave)的頻率，而 data wave 固定。

- Spread Spectrum in Steganography
- p.5 將資訊藏在音訊浮水印
1. Transmit audio by a transmit formula(e.g. Fourier transform, discrete cosine transform and etc.)
- 傅立葉轉換將 Time domain 轉成 Frequency domain
2. Then substitute some data by Steganography
3. Finally, use a inverse function transmit to audio

- Watermark vs Steganography
- Watermark 是為能敵擋住攻擊
- Steganography 是為不被察覺

- Chi-square Test
- 檢測隱寫術夠不夠安全
- Compare Critical value and Obtained Value.
- If Obtained Value is more upper, then the null hypothesis will be rejected.
-

### 建議&問題
1. PPT 字體大小要比 Word 大[name=Solomon]
2. p.3 DSSS Redundancy 利用比率會不會太低落? [name=Edgar]
3. p.3 頻寬會變大? [name=Choffin]
4. p.3 bandwidth definition? [name=Edgar]
5. 頻率的切換如何協議? [name=Angela]
- Pseudo-Random
6. 頻率越快傳輸速度越快，那怎麼保持傳輸資料的同步 [name=Jerry]
- data rate 速度都一樣
- Like 收音機，同一時間轉到不同頻率去接收訊息
7. 目前 Chi-Square Algorithm 只有針對a-z, 那 exceted value or case(p.5) 針對封包或 audio file 如何準備/看哪裡? [name=Edgar]
8. p.7 前面大於表不相關? [name=Choffin]
- The null hypothesis states that a quantity (of interest) is larger or equal to zero and smaller or equal to zero.(wiki)
If null hypothesis is rejected, the Alternative Hypotheses (Our assumption will effect).
9. 單純的 wav file 如何做 watermark? [name=Choffin]
- 把原本的 audio file 抽掉幾個 bits，把祕密插在這些 bits 上
10. p.4 Why FHSS 抗干擾較高? [name=Jerry]
- 使用多個頻率，可避部分頻率訊號不好時，仍能保證通訊品質。
11. p.4 可干擾 FHSS 嗎? [name=Branko]
- 可
12. p.3 Transmitted Signal 要怎麼回復成原本的 0 或 1 [name=Branko]
- Exclusive Or
13. In demo program, Frequency meaning? [name=Toby]
- A-Z 可能出現的概率 by counting。
14. p.5 Watermark ...@Henry [name=Henry]
- @[逮到你了！歌詞網站Genius控Google盜取內容 - 數位時代](https://www.bnext.com.tw/article/53697/lyrics-site-genius-accuses-google-of-lifting-its-content)
15. p.7 When explaining statistic example, it's better to define hypothesis first. [name=Angela]
16. 若今天知道加密演算法(e.g. 凱薩加密)，而不知道 key 時，Chi-Square 可利用統計去找出可能的 key。而雖未驗證，但有論文指出若觀測 audio file 的 2-bit ，會有 4 cases，則未被加入資料的 audio 經 chi-test 後應皆 0.25。[name=Solomon]

## 待追蹤事項
1. 目前 Chi-Square Algorithm 只有針對a-z, 那 exceted value or case(p.5) 針對封包或 audio file 如何準備/看哪裡
2. Chi-square test 透過已知統計數據去推論結果，延伸到隱寫術，若是結合 FHSS 技術將資料在 packet header 跟 payload 間 hopping 藏匿，就無法透過特定的統計資料(僅針對header統計，或是僅針對payload統計)去判定封包是否異常。

## 臨時動議
1. 以後負責聯絡學長姐的同學，要記得了解學長姐居住地、產業近況等等(值星記下)[name=Solomon]
2. Henry 交接事項 => 全部交給見如，見如再去分配給其他學弟
3. 4/2 聚餐取消，請 Henry 去連絡學長姐聚餐取消，並詢問是否併入新竹廠。
4. 下星期因應期中考暫停一次 Meeting，下一次預計 4/15 日 10:30 A.M[name=Toby]
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散會結束時間： 12:24
下次 Meeting 時間： 2022/04/15 10:30