Merge branch 'master' into copyedit-chs

README.md

@@ -19,6 +19,12 @@ cd digital_video_introduction
 
 All the **hands-on should be performed from the folder you cloned** this repository. For the **jupyter examples** you must start the server `./s/start_jupyter.sh` and copy the URL and use it in your browser.
 
+# Changelog
+
+* added DRM system
+* released version 1.0.0
+* added simplified Chinese translation
+
 # Index
 
 - [Intro](#intro)
@@ -123,9 +129,9 @@ Finally, we can define a **video** as a **succession of *n* frames** in **time**
 
 ![video](/i/video.png "video")
 
-The number of bits per second needed to show a video is its **bit rate**. 
+The number of bits per second needed to show a video is its **bit rate**.
 
-> bit rate = width * height * * bit depth * frames per second 
+> bit rate = width * height * * bit depth * frames per second
 
 For example, a video with 30 frames per second, 24 bits per pixel, resolution of 480x240 will need **82,944,000 bits per second** or 82.944 Mbps (30x480x240x24) if we don't employ any kind of compression.
 
@@ -233,7 +239,7 @@ These schemas are known as subsampling systems and are expressed as a 3 part rat
 
 > An exception to this exists with 4:1:0, which provides a single chroma sample within each `4 x 4` block of luma resolution.
 
-Common schemes used in modern codecs are: **4:4:4** *(no subsampling)***, **4:2:2, 4:1:1, 4:2:0, 4:1:0 and 3:1:1**. 
+Common schemes used in modern codecs are: **4:4:4** *(no subsampling)***, **4:2:2, 4:1:1, 4:2:0, 4:1:0 and 3:1:1**.
 
 > **YCbCr 4:2:0 merge**
 >
@@ -731,11 +737,60 @@ HEVC has bigger and more **partitions** (and **sub-partitions**) options than AV
 
 ## Content protection
 
+We can use **a simple token system** to protect the content. The user without a token tries to request a video and the CDN forbids her or him while a user with a valid token can play the content, it works pretty similarly to most of the web authentication systems.
+
 ![token protection](/i/token_protection.png "token_protection")
 
+The sole use of this token system still allows a user to download a video and distribute it. Then the **DRM (digital rights management)** systems can be used to try to avoid this.
+
 ![drm](/i/drm.png "drm")
 
-[TODO]
+In real life production systems, people often use both techniques to provide authorization and authentication.
+
+### DRM
+#### Main systems
+
+* FPS - [**FairPlay Streaming**](https://developer.apple.com/streaming/fps/)
+* PR - [**PlayReady**](https://www.microsoft.com/playready/)
+* WV - [**Widevine**](http://www.widevine.com/)
+
+
+#### What?
+
+DRM means Digital rights management, it's a way **to provide copyright protection for digital media**, for instance, digital video and audio. Although it's used in many places [it's not universally accepted](https://en.wikipedia.org/wiki/Digital_rights_management#DRM-free_works).
+
+#### Why?
+
+Content creator (mostly studios) want to protect its intelectual property against copy to prevent unauthorized redistribution of digital media.
+
+#### How?
+
+We're going to describe an abstract and generic form of DRM in a very simplified way.
+
+Given a **content C1** (i.e. an hls or dash video streaming), with a **player P1** (i.e. shaka-clappr, exo-player or ios) in a **device D1** (i.e. a smartphone, TV, tablet or desktop/notebook) using a **DRM system DRM1** (widevine, playready or FairPlay).
+
+The content C1 is encrypted with a **symmetric-key K1** from the system DRM1, generating the **encrypted content C'1**.
+
+![drm general flow](/i/drm_general_flow.jpeg "drm general flow")
+
+The player P1, of a device D1, has two keys (asymmetric), a **private key PRK1** (this key is protected<sup>1</sup> and only known by **D1**) and a **public key PUK1**.
+
+> **<sup>1</sup>protected**: this protection can be **via hardware**, for instance, this key can be stored inside a special (read-only) chip that works like [a black-box](https://en.wikipedia.org/wiki/Black_box) to provide decryption, or **by software** (less safe), the DRM system provides means to know which type of protection a given device has.
+
+
+When the **player P1 wants to play** the **content C'1**, it needs to deal with the **DRM system DRM1**, giving its public key **PUK1**. The DRM system DRM1 returns the **key K1 encrypted** with the client''s public key **PUK1**. In theory, this response is something that **only D1 is capable of decrypting**.
+
+`K1P1D1 = enc(K1, PUK1)`
+
+**P1** uses its DRM local system (it could be a [SOC](https://en.wikipedia.org/wiki/System_on_a_chip), a specialized hardware or software), this system is **able to decrypt** the content using its private key PRK1, it can decrypt **the symmetric-key K1 from the K1P1D1** and **play C'1**. At best case, the keys are not exposed through RAM.
+
+ ```
+ K1 = dec(K1P1D1, PRK1)
+
+ P1.play(dec(C'1, K1))
+ ```
+
+![drm decoder flow](/i/drm_decoder_flow.jpeg "drm decoder flow")
 
 # How to use jupyter
 
@@ -790,7 +845,7 @@ Software:
 
 Non-ITU Codecs:
 
-* https://aomedia.googlesource.com/ 
+* https://aomedia.googlesource.com/
 * https://github.com/webmproject/libvpx/tree/master/vp9
 * https://people.xiph.org/~xiphmont/demo/daala/demo1.shtml
 * https://people.xiph.org/~jm/daala/revisiting/
@@ -868,4 +923,3 @@ Miscellaneous:
 * https://www.youtube.com/watch?v=LFXN9PiOGtY
 * https://www.youtube.com/watch?v=Lto-ajuqW3w&list=PLzH6n4zXuckpKAj1_88VS-8Z6yn9zX_P6
 * https://www.youtube.com/watch?v=LWxu4rkZBLw
-

simplified-chinese/README-cn.md

@@ -24,6 +24,12 @@ cd digital_video_introduction
 * 发行版本 1.0.0
 * 新增中文翻译
 
+# 更新日志
+
+* 添加 DRM 系统
+* 发布版本 1.0.0
+* 添加简体中文翻译
+
 # 目录
 
 - [介绍](#介绍)
@@ -95,8 +101,10 @@ cd digital_video_introduction
 >
 > ![NES palette](/i/nes-color-palette.png "NES palette")
 
+
 例如以下几张图片。第一张包含所有颜色平面。剩下的分别是红、绿、蓝色平面（显示为灰调）（译注：颜色强度高的地方显示为亮色，强度低为暗色）。
 
+
 ![RGB channels intensity](/i/rgb_channels_intensity.png "RGB channels intensity")
 
 我们可以看到，对于最终的成像，红色平面对强度的贡献更多（三个平面最亮的是红色平面），蓝色平面（最后一张图片）的贡献大多只在马里奥的眼睛和他衣服的一部分。所有颜色平面对马里奥的胡子（最暗的部分）均贡献较少。
@@ -747,7 +755,6 @@ HEVC 比 AVC 有更大和更多的**分区**（和**子分区**）选项，更
 * PR - [**PlayReady**](https://www.microsoft.com/playready/)
 * WV - [**Widevine**](http://www.widevine.com/)
 
-
 #### 是什么
 
 DRM 指的是数字版权管理，是一种**为数字媒体提供版权保护**的方法，例如数字视频和音频。尽管用在了很多场合，但它并[没有被普遍接受](https://en.wikipedia.org/wiki/Digital_rights_management#DRM-free_works).
@@ -776,11 +783,11 @@ DRM 指的是数字版权管理，是一种**为数字媒体提供版权保护**
 
 **P1** 使用它的本地 DRM 系统（这可以使用 [SoC](https://zh.wikipedia.org/wiki/系统芯片) ，一个专门的硬件和软件，这个系统可以使用它的私钥 PRK1 用来**解密**内容，它可以解密被加密过的**K1P1D1 的对称密钥 K1**。理想情况下，密钥不会被导出到内存以外的地方。
 
- ```
+```
  K1 = dec(K1P1D1, PRK1)
 
  P1.play(dec(C'1, K1))
- ```
+```
 
 ![DRM 解码流程](/i/drm_decoder_flow.jpeg "DRM 解码流程")