How to deal with ISO-2022-JP ( and other character sets ) in a Twitter update?

Answer 1

Your understanding of how the text is encoded seems correct. In python

'GyRCJEtFajlGJEckLSRrGyhC'.decode('base64').decode('ISO-2022-JP')

returns the correct unicode string. Note that you need to decode base64 first in order to get the ISO-2022-JP-encoded text.

Answer 2

This produced the output you are looking for:

using System;
using System.Text;

class Program {
  static void Main(string[] args) {
    string input = "に投稿できる";
    Console.WriteLine(EncodeTwit(input));
    Console.ReadLine();
  }
  public static string EncodeTwit(string txt) {
    var enc = Encoding.GetEncoding("iso-2022-jp");
    byte[] bytes = enc.GetBytes(txt);
    char[] chars = new char[(bytes.Length * 3 + 1) / 2];
    int len = Convert.ToBase64CharArray(bytes, 0, bytes.Length, chars, 0);
    return "=?ISO-2022-JP?B?" + new string(chars, 0, len) + "?=";
  }
}

Standards are great, there are so many to choose from. ISO never disappoints, there are no less than 3 ISO-2022-JP encodings. If you have trouble then also try encodings 50221 and 50222.