InstagramBase64.cs

public string Base64Encode(string plainText)
        {
            Dictionary<char, int> Base64Table = new Dictionary<char, int>()
            #region Table
            {
            {'+',62},
            {'/',63},
            {'0',52},
            {'1',53},
            {'2',54},
            {'3',55},
            {'4',56},
            {'5',57},
            {'6',58},
            {'7',59},
            {'8',60},
            {'9',61},
            {'A',0},
            {'B',1},
            {'C',2},
            {'D',3},
            {'E',4},
            {'F',5},
            {'G',6},
            {'H',7},
            {'I',8},
            {'J',9},
            {'K',10},
            {'L',11},
            {'M',12},
            {'N',13},
            {'O',14},
            {'P',15},
            {'Q',16},
            {'R',17},
            {'S',18},
            {'T',19},
            {'U',20},
            {'V',21},
            {'W',22},
            {'X',23},
            {'Y',24},
            {'Z',25},
            {'a',26},
            {'b',27},
            {'c',28},
            {'d',29},
            {'e',30},
            {'f',31},
            {'g',32},
            {'h',33},
            {'i',34},
            {'j',35},
            {'k',36},
            {'l',37},
            {'m',38},
            {'n',39},
            {'o',40},
            {'p',41},
            {'q',42},
            {'r',43},
            {'s',44},
            {'t',45},
            {'u',46},
            {'v',47},
            {'w',48},
            {'x',49},
            {'y',50},
            {'z',51}
            };
            #endregion
            long Decimal = long.Parse(plainText);
            Stack s = new Stack();
            do
            {
                s.Push(Decimal % 64);
                Decimal /= 64;
            } while (Decimal != 0);
            string Output = "";
            foreach (var Char in s.ToArray())
            {
                Output += Base64Table.FirstOrDefault(x => x.Value == int.Parse(Char.ToString())).Key;
            }
            return Output;
        }