C＃实现Base64编码与解码

一、编码规则
      Base64编码的思想是是采用64个基本的ASCII码字符对数据进行重新编码。它将需要编码的数据拆分成字节数组。以3个字节为一组。按顺序排列24 位数据，再把这24位数据分成4组，即每组6位。再在每组的的最高位前补两个0凑足一个字节。这样就把一个3字节为一组的数据重新编码成了4个字节。当所要编码的数据的字节数不是3的整倍数，也就是说在分组时最后一组不够3个字节。这时在最后一组填充1到2个0字节。并在最后编码完成后在结尾添加1到2个 “=”。

例：将对ABC进行BASE64编码：

1、首先取ABC对应的ASCII码值。A（65）B（66）C（67）；
2、再取二进制值A（01000001）B（01000010）C（01000011）；
3、然后把这三个字节的二进制码接起来（010000010100001001000011）；
4、 再以6位为单位分成4个数据块,并在最高位填充两个0后形成4个字节的编码后的值，（00010000）（00010100）（00001001）（00000011），其中蓝色部分为真实数据；
5、再把这四个字节数据转化成10进制数得（16）（20）（9）（3）；
6、最后根据BASE64给出的64个基本字符表，查出对应的ASCII码字符（Q）（U）（J）（D），这里的值实际就是数据在字符表中的索引。

注：BASE64字符表：ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/

二、解码规则
      解码过程就是把4个字节再还原成3个字节再根据不同的数据形式把字节数组重新整理成数据。

三、C#中的实现

编码类：

/// <summary>
    /// Base64编码类。
    /// 将byte[]类型转换成Base64编码的string类型。
    /// </summary>
    publicclass Base64Encoder
    {
        byte[] source;
        int length, length2;
        int blockCount;
        int paddingCount;
        publicstatic Base64Encoder Encoder= new Base64Encoder();

        public Base64Encoder()
        {
        }

        privatevoid init(byte[] input)
        {
            source = input;
            length = input.Length;
            if ((length% 3)== 0)
            {
                paddingCount =0;
                blockCount = length/ 3;
            }
            else
            {
                paddingCount =3 - (length% 3);
                blockCount = (length+ paddingCount)/ 3;
            }
            length2 = length+ paddingCount;
        }

        publicstring GetEncoded(byte[] input)
        {
            //初始化
            init(input);

            byte[] source2;
            source2 =new byte[length2];

            for (int x= 0; x< length2; x++)
            {
                if (x< length)
                {
                    source2[x] = source[x];
                }
                else
                {
                    source2[x] =0;
                }
            }

            byte b1, b2, b3;
            byte temp, temp1, temp2, temp3, temp4;
            byte[] buffer= newbyte[blockCount* 4];
            char[] result= newchar[blockCount* 4];
            for (int x= 0; x< blockCount; x++)
            {
                b1 = source2[x* 3];
                b2 = source2[x* 3+ 1];
                b3 = source2[x* 3+ 2];

                temp1 = (byte)((b1& 252)>> 2);

                temp = (byte)((b1& 3)<< 4);
                temp2 = (byte)((b2& 240)>> 4);
                temp2 += temp;

                temp = (byte)((b2& 15)<< 2);
                temp3 = (byte)((b3& 192)>> 6);
                temp3 += temp;

                temp4 = (byte)(b3& 63);

                buffer[x *4] = temp1;
                buffer[x *4 +1] = temp2;
                buffer[x *4 +2] = temp3;
                buffer[x *4 +3] = temp4;

            }

            for (int x= 0; x< blockCount* 4; x++)
            {
                result[x] = sixbit2char(buffer[x]);
            }


            switch (paddingCount)
            {
                case0: break;
                case1: result[blockCount* 4- 1]= '=';break;
                case2: result[blockCount* 4- 1]= '=';
                    result[blockCount *4 -2] ='=';
                    break;
                default:break;
            }
            returnnew string(result);
        }
        privatechar sixbit2char(byte b)
        {
            char[] lookupTable= newchar[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 ((b>= 0)&& (b <= 63))
            {
                return lookupTable[(int)b];
            }
            else
            {

                return' ';
            }
        }

    }

解码类：

/// <summary>
    /// Base64解码类
    /// 将Base64编码的string类型转换成byte[]类型
    /// </summary>
    publicclass Base64Decoder
    {
        char[] source;
        int length, length2, length3;
        int blockCount;
        int paddingCount;
        publicstatic Base64Decoder Decoder= new Base64Decoder();

        public Base64Decoder()
        {
        }

        privatevoid init(char[] input)
        {
            int temp= 0;
            source = input;
            length = input.Length;

            for (int x= 0; x< 2; x++)
            {
                if (input[length- x - 1]== '=')
                    temp++;
            }
            paddingCount = temp;

            blockCount = length/ 4;
            length2 = blockCount* 3;
        }

        publicbyte[] GetDecoded(string strInput)
        {
            //初始化
            init(strInput.ToCharArray());

            byte[] buffer= newbyte[length];
            byte[] buffer2= newbyte[length2];

            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;
            }

            length3 = length2- paddingCount;
            byte[] result= newbyte[length3];

            for (int x= 0; x< length3; x++)
            {
                result[x] = buffer2[x];
            }

            return result;
        }

        privatebyte char2sixbit(char c)
        {
            char[] lookupTable= newchar[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== '=')
                return0;
            else
            {
                for (int x= 0; x< 64; x++)
                {
                    if (lookupTable[x]== c)
                        return (byte)x;
                }

                return0;
            }

        }
    }
    //解码类结束

提示：

上面的代码只是说明base64编码的原理，以便用更多语言重写。但.net里面可以使用更简单的方法：
编码：

byte[] bytes=Encoding.Default.GetBytes("要转换的字符串");
Convert.ToBase64String(bytes);

解码：

//"ztKwrsTj"是“我爱你”的base64编码
byte[] outputb= Convert.FromBase64String("ztKwrsTj");
string orgStr= Encoding.Default.GetString(outputb);