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：


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>

public class Base64Encoder

{

byte[] source;

int length, length2;

int blockCount;

int paddingCount;

public static Base64Encoder Encoder = new Base64Encoder();

public Base64Encoder()

{

}

private void 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;

}

public string 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 = new byte[blockCount * 4];

char[] result = new char[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)

{

case 0: break;

case 1: result[blockCount * 4 - 1] = '='; break;

case 2: result[blockCount * 4 - 1] = '=';

result[blockCount * 4 - 2] = '=';

break;

default: break;

}

return new string(result);

}

private char sixbit2char(byte b)

{

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 ((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>

public class Base64Decoder

{

char[] source;

int length, length2, length3;

int blockCount;

int paddingCount;

public static Base64Decoder Decoder = new Base64Decoder();

public Base64Decoder()

{

}

private void 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;

}

public byte[] GetDecoded(string strInput)

{

//初始化

init(strInput.ToCharArray());

byte[] buffer = new byte[length];

byte[] buffer2 = new byte[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 = new byte[length3];

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

{

result[x] = buffer2[x];

}

return result;

}

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;

}

return 0;

}

}

}

//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);