.Net Base64Decode

출처: http://www.codeproject.com/KB/cs/base64encdec.aspx 

[ 사용법]

    string a = "가나다라마바사아";

    Console.WriteLine(GetDecoded(a.ToCharArray()));

[소스]

        public string GetDecoded(char[] input)

        {

            char[] source;

            int length, length2, length3;

            int blockCount;

            int paddingCount;

            int temp = 0;

            source = input;

            length = input.Length;

            //find how many padding are there

            for (int x = 0; x < 2; x++)

            {

                if (input[length - x - 1] == '=')

                    temp++;

            }

            paddingCount = temp;

            //calculate the blockCount;

            //assuming all whitespace and carriage returns/newline were removed.

            blockCount = length / 4;

            length2 = blockCount * 3;

            byte[] buffer = new byte[length];//first conversion result

            byte[] buffer2 = new byte[length2];//decoded array with padding

            for (int x = 0; x < length; x++)

            {

                buffer[x] = char2sixbit(source[x]);

            }

            byte b, b1, b2, b3;

            byte temp1, temp2, temp3, temp4;

            for (int x = 0; x < blockCount; x++)

            {

                temp1 = buffer[x * 4];

                temp2 = buffer[x * 4 + 1];

                temp3 = buffer[x * 4 + 2];

                temp4 = buffer[x * 4 + 3];

                b = (byte)(temp1 << 2);

                b1 = (byte)((temp2 & 48) >> 4);

                b1 += b;

                b = (byte)((temp2 & 15) << 4);

                b2 = (byte)((temp3 & 60) >> 2);

                b2 += b;

                b = (byte)((temp3 & 3) << 6);

                b3 = temp4;

                b3 += b;

                buffer2[x * 3] = b1;

                buffer2[x * 3 + 1] = b2;

                buffer2[x * 3 + 2] = b3;

            }

            //remove paddings

            length3 = length2 - paddingCount;

            byte[] result = new byte[length3];

            for (int x = 0; x < length3; x++)

            {

                result[x] = buffer2[x];

            }

            StringBuilder sb = new StringBuilder();

            //byte[] temp=myDecoder.GetDecoded();

            sb.Append(System.Text.UTF8Encoding.UTF8.GetChars(result));

            return sb.ToString();

        }

        private byte char2sixbit(char c)

        {

            char[] lookupTable = new char[64]

                    { 'A','B','C','D','E','F','G','H','I','J','K','L','M','N',

                      'O','P','Q','R','S','T','U','V','W','X','Y','Z',

                      'a','b','c','d','e','f','g','h','i','j','k','l','m','n',
                      'o','p','q','r','s','t','u','v','w','x','y','z',
                      '0','1','2','3','4','5','6','7','8','9','+','/'};

            if (c == '=')

                return 0;

            else

            {

                for (int x = 0; x < 64; x++)

                {

                    if (lookupTable[x] == c)

                        return (byte)x;

                }

                //should not reach here

                return 0;

            }

        }