Base64、DES、MD5加密解密原理

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一、编码规则 Base编码的思想是是采用个基本的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。按顺序排列位数据，再把这位数据分成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BASE编码：

1、首先取ABC对应的ASCII码&#;。A（）B（）C（）；2、再取二进制&#;A（）B（）C（）； 3、然后把这三个字节的二进制码接起来（）；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&#;，（）（）（）（），其中蓝色部分为真实数据； 5、再把这四个字节数据转化成进制数得（）（）（9）（3）； 6、最后根据BASE给出的个基本字符表，查出对应的ASCII码字符（Q）（U）（J）（D），这里的&#;实际就是数据在字符表中的索引。

注：BASE字符表：ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz&#;/

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> /// Base编码类。 /// 将byte[]类型转换成Base编码的string类型。 /// </summary> public class BaseEncoder { byte[] source; int length, length2; int blockCount; int paddingCount; public static BaseEncoder Encoder = new BaseEncoder(); public BaseEncoder() { } 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 & ) >> 2); temp = (byte)((b1 & 3) << 4); temp2 = (byte)((b2 & ) >> 4); temp2 &#;= temp; temp = (byte)((b2 & ) << 2); temp3 = (byte)((b3 & ) >> 6); temp3 &#;= temp; temp4 = (byte)(b3 & ); 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[]{ '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 <= )) { return lookupTable[(int)b]; } else { return ' '; } } }

解码类：

/// <summary> /// Base解码类 /// 将Base编码的string类型转换成byte[]类型 /// </summary> public class BaseDecoder { char[] source; int length, length2, length3; int blockCount; int paddingCount; public static BaseDecoder Decoder = new BaseDecoder(); public BaseDecoder() { } 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 & ) >> 4); b1 &#;= b; b = (byte)((temp2 & ) << 4); b2 = (byte)((temp3 & ) >> 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[]{ '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 < ; 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base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.ToBaseString(bytes);解码：byte[] outputb = Convert.FromBaseString("ztKwrsTj");string orgStr= Encoding.Default.GetString(outputb);四.C#中DES和MD5的加密解密处理

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