Update YT decrypter and add document

decrypter/0_latest.txt

@@ -1,26 +1,24 @@
-# 19417
-s 17
+# 19660
+s 67
 c 1
-R
-s 65
+s 31
 >
-r -800848978
-R
-e 330905968
-r -1801395041
-r -45203544
-r 969944705
-s -1266939812
-s 1071587255
-r 2070384319
-t hwadna7 Aa0-_
-s 61910453
-R
-t UDjUn 0aA-_
-r -1700764381
-R
-R
-r 859083917
-r -2140614363
-r -307172264
+t split Aa0-_
+e 349754986
+s 1399367760
+e 1563194841
+r 1680918897
+s -1227273718
+r 945509789
+s -386866156
+r -2141966360
+r -505298648
+e 349713772
+r 1006958380
+e 191776033
+r 1956089104
+t finally Aa0-_
+s -1467723118
+s -703780294
+s 1903244263
 R

decrypter/README.md

@@ -0,0 +1,55 @@
+## YouTube "decrypter" file
+YouTube employs some measures for anti-third-party client. Something called "signature cipher" or "n parameter" is one of them.  
+We can fetch stream URLs from `ytInitialPlayerResponse` in the video page or `/player` endpoint, but the URLs are unusable or rate-limited without some parameter conversion.  
+The conversion is done in `base.js` on the official web client, and the process seems to be always a combination of a certain simple operations.  
+We could download `base.js` and execute the conversion code on ThirdTube, but `base.js` is a large(1~2 MB) file and it slows down video loading,
+so we make a text file(2-300 B) describing the operations in the conversion process and let the app load it.  
+This document describes the format of the file.  
+
+## How ThirdTube loads the file
+1. embedded in romfs of the app file (updated with the ThirdTube releases)
+2. local cache (updated along with 3.)
+3. `decrypter/0_latest.txt` in this repository (latest)
+
+On startup, the app loads the decrypter in the following order, and uses the last one that is succeeded to load.
+
+## How to find conversion function in base.js
+
+### sig
+1. Search for `a=a.split("")`. It should be at the start of a function, followed by \[VAR\].\[FUNC\](a,[x]). The function is the conversion function.
+2. Search for `var [VAR]=` to see the content of `FUNC`s.
+
+### n
+1. Search for `("n")`. The unique match should be followed by `&&(b=[ARR][0](b))`.
+2. Go for the defintion of `[ARR]` by searching `var [ARR]` and get the conversion function name
+
+## Format
+```
+# [STS]
+[one or more lines for "sig" conversion]
+>
+[one or more lines for "n" conversion]
+```
+`STS` is like a version of the decrypter and seems to be the day count from unix epoch to the deployment of the decrypter.  
+A line for "sig"/"n" conversion is one of the following:  
+`s [x]` : **s**wap `a[0]` and `a[x mod a.size()]`  
+`e [x]` : **e**rase `a[x mod a.size()]` from `a`  
+`c [x]` : (**c**ut) erase the first `x` items of `a`  
+`r [x]` : **r**otate `a` right for `x mod a.size()` items  
+`R` : reverse `a`  
+`t [key] [charset]` : **t**ransform `a` with the following procedure:
+
+ 1. Replace `A` in `charset` with `ABC...Z` (string containing all latin uppercase letters in order)
+ 2. Replace `a` in `charset` with `abc...z` (string containing all latin lowercase letters in order)
+ 3. Replace `0` in `charset` with `012...9` (string containing all digit letters in order)  
+    (These three steps are done in order to be able to compress the "decrypter" file)
+ 4. Let f(c) be the index in `charset` at which the character `c` appears
+ 5. For $i = 0, 1, 2, ..., \mathrm{a.size()-1}$ do the following :  
+     1. Assign `charset[(f(a[i]) - f(key[i])) mod 64]` to `a[i]` (64 is always equal to `charset.size()`)  
+     2. Push `a[i]` to the back of `key`  
+
+Here, `a` denotes the original string of "sig"/"n" parameter and `x mod y` denotes an integer between $0$ and $y - 1$ that is congruent to $x$ mod $y$.  
+When the url of the streams are fetched, ThirdTube converts the `sig` parameter by executing the lines for `sig` conversion in order and `n` in the same way.  
+While YouTube updates the decrypter regularly (around every three days?), we can use old decrypter by requesting the stream with old `STS`.  
+However, they seem to invalidate decrypters older than 2-3 weeks, so we have to update the decrypter regularly.  
+

decrypter/archive/0/19660.txt

@@ -0,0 +1,24 @@
+# 19660
+s 67
+c 1
+s 31
+>
+t split Aa0-_
+e 349754986
+s 1399367760
+e 1563194841
+r 1680918897
+s -1227273718
+r 945509789
+s -386866156
+r -2141966360
+r -505298648
+e 349713772
+r 1006958380
+e 191776033
+r 1956089104
+t finally Aa0-_
+s -1467723118
+s -703780294
+s 1903244263
+R