幹壞事是進步最大的原動力
Google 宣佈從八月開始將不再更新 Android 4.4 (KitKat，KK) 的 Google Play 服務：「Google Play services discontinuing updates for KitKat (API levels 19 & 20) starting August 2023」。
Therefore, we are no longer supporting KK in future releases of Google Play services. KK devices will not receive versions of the Play Services APK beyond 23.30.99.
可以看到講的是 client 端的 apk 停止更新，服務應該還是可以用一陣子，直到 Google Play 有改動，讓舊版的 client 無法用。
Android 4.4 也是個經典的版本，當初在用 LG G2 的時候是 4.2，用了一陣子有 4.4，最後一版是 5.0，再新的版本就得刷機用第三方了。
忘記在哪邊看到的，分析 Android APK 檔的軟體：「Droidefense: Advance Android Malware Analysis Framework」。
Droidefense (originally named atom: analysis through observation machine)* is the codename for android apps/malware analysis/reversing tool. It was built focused on security issues and tricks that malware researcher have on they every day work. For those situations on where the malware has anti-analysis routines, Droidefense attemps to bypass them in order to get to the code and 'bad boy' routine. Sometimes those techniques can be virtual machine detection, emulator detection, self certificate checking, pipes detection. tracer pid check, and so on.
Droidefense uses an innovative idea in where the code is not decompiled rather than viewed. This allow us to get the global view of the execution workflow of the code with a 100% accuracy on gathered information. With this situation, Droidefense generates a fancy html report with the results for an easy understanding.
看起來是輔助用的工具... 先記錄下來 XD
在「Intern Impact: Brotli compression for Play Store app downloads」這邊介紹了 Google Play Store 引入 Brotli 的情況。
選擇 Brotli 除了是 Google 自家研發出來的東西以外，另外是考量到 Brotli 的壓縮與解壓縮速 (尤其是後者) 不會增加太多，卻可以多出不少壓縮率。維基百科這邊說明的是文字的部份：
Replacing deflate with brotli typically gives an increase of 20% in compression density for text files, while compression and decompression speeds are roughly unchanged.
不過實際在 Google Play Store 上測試 binary 的效果也不錯：
當然，如同之前提到的「Google 再次改善 Android 的 APK 更新，讓下載的量更小」，在去年 12 月時 Google 對於背景更新的下載 File-by-File 的更新來降低流量 (但在手機上會需要大量的 CPU 資源計算，不過因為是背景 idle 時跑而不會影響使用者，所以被採用)，透過這兩個改善互相搭配繼續壓低流量。
在接下來的幾個禮拜會生效：
Brotli compression for app downloads is rolling out now, and users should start to enjoy the benefits over the coming weeks.
Google 的人再次更新了演算法，將下載的量再次減少，從本來的 47% 降到 65%：「Saving Data: Reducing the size of App Updates by 65%」。
今年七月的時候，更新演算法導入了 bsdiff，使得本來要抓整包 APK 的量，變成抓 diff 的部份，這使得下載的流量降了 47%：「Improvements for smaller app downloads on Google Play」。
Using bsdiff, we were able to reduce the size of app updates on average by 47% compared to the full APK size.
現在則改成不直接對 APK 做 diff，而是對未壓縮的檔案做，再把差異包起來，則可以降到 65%：
Today, we're excited to share a new approach that goes further — File-by-File patching. App Updates using File-by-File patching are, on average, 65% smaller than the full app, and in some cases more than 90% smaller.
主要的原因在於 APK 的壓縮使用的 DEFLATE 演算法對於變更非常敏感，改變一個字元就會讓後續整串都改變，導致差異很大而跑 diff algorithm 的效果不好：
用 File-by-File 的好處主要來自於是對未壓縮的檔案比較差異，這代表沒有變動的檔案完全不會進來攪和，而對 binary 檔案的效果也比較好 (大部份的程式碼還是一樣)。不過這對於已經有壓縮的圖片的效果就比較差了，這也是 APK 一般肥大常見的原因。
有兩件事情值得注意的，一個是 Google 的人為了使用者體驗，只有在 auto update 時才會走 File-by-File 的更新，主要原因是 File-by-File 的解開速度慢不少：
For now, we are limiting the use of this new patching technology to auto-updates only, i.e. the updates that take place in the background, usually at night when your phone is plugged into power and you're not likely to be using it. This ensures that users won't have to wait any longer than usual for an update to finish when manually updating an app.
另外一個是，這個新方式讓 Google 每天省下 6PB 的流量，如果流量都是平均打散的話，大約是 600Gbps：
The savings, compared to our previous approach, add up to 6 petabytes of user data saved per day!
這種規模改善起來很有感覺 XDDD
維基百科對 APT (Advanced Persistent Threat) 的定義是：
Advanced Persistent Threat (APT) APT is a set of stealthy and continuous computer hacking processes, often orchestrated by human(s) targeting a specific entity.
針對特定個人或團體進行攻擊，這邊的 entity 通常是指有權限存取系統，或是手上握有機敏資料的人，這些人的帳號密碼，或是系統權限是有價值的。
這幾年因為行動裝置普及，再加上行動裝置上驗證起來會比較麻煩，成為 APT 攻擊的首選。
下面就原文照登：
