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Still looking for base64url decoding tools, both on-line and for MacOS homebrew

Posted by jpluimers on 2021/02/11

Remember the screenshot on the right from yesterdays post Kristian Köhntopp explaining theories?

In the end, I:

  1. saved that tweet
  2. in archive.is
  3. then took the screenshot image from there (see below)
  4. cut out the grey parts, then posted the middle part

I much more would have used the screenshot functionality of Google as described here by Terence Eden:

[WayBack] twitter – How to convert a tweet to image – Stack Overflow

Google has a secret screenshot API

For example, you can use it to get a screenshot of a tweet like this

https://www.googleapis.com/pagespeedonline/v1/runPagespeed?screenshot=true&strategy=mobile&url=https%3a%2f%2ftwitter.com%2fedent%2fstatus%2f661570680253755392

At the bottom of that JSON response, you’ll see

"screenshot": {
     "data": "_9j_4AAQSkZJRgAB.....=",
     "height": 569,
     "mime_type": "image/jpeg",
     "width": 320
 }

You will need to Base64 decode it using the URL and Filename safe alphabet.

That will give you a JPG screenshot of the Tweet.

I was hoping for an on-line way, so I followed [WayBack] Google’s Secret Screenshot API – Terence Eden’s Blog.

The blog post pointed me to a Python based script ([WayBack] Python-Twitter-Hacks/websiteScreenshot.py at master · edent/Python-Twitter-Hacks · GitHub) but had no online way.

So I tried out a few on-line things myself that failed:

Then I found out the script was just a proof of concept with hard coded URL and filename.

So I forked the repository, and fixed the script basing it on Python 3.

More on that next week.

Related:

  • [WayBack] RFC 4648 – The Base16, Base32, and Base64 Data Encodings: Base 64 Encoding with URL and Filename Safe Alphabet:
       The Base 64 encoding with an URL and filename safe alphabet has been
       used in [12].
    ...
       An alternative alphabet has been suggested that would use "~" as the
       63rd character.  Since the "~" character has special meaning in some
       file system environments, the encoding described in this section is
       recommended instead.
    ...
       This encoding may be referred to as "base64url".  This encoding
       should not be regarded as the same as the "base64" encoding and
       should not be referred to as only "base64". 
    ...
       This encoding is technically identical to the previous one, except
       for the 62:nd and 63:rd alphabet character, as indicated in Table 2.
    ...
             Table 2: The "URL and Filename safe" Base 64 Alphabet
    
         Value Encoding  Value Encoding  Value Encoding  Value Encoding
             0 A            17 R            34 i            51 z
             1 B            18 S            35 j            52 0
             2 C            19 T            36 k            53 1
             3 D            20 U            37 l            54 2
             4 E            21 V            38 m            55 3
             5 F            22 W            39 n            56 4
             6 G            23 X            40 o            57 5
             7 H            24 Y            41 p            58 6
             8 I            25 Z            42 q            59 7
             9 J            26 a            43 r            60 8
            10 K            27 b            44 s            61 9
            11 L            28 c            45 t            62 - (minus)
            12 M            29 d            46 u            63 _
            13 N            30 e            47 v           (underline)
            14 O            31 f            48 w
            15 P            32 g            49 x
            16 Q            33 h            50 y         (pad) =
    

–jeroen

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