矛与盾的较量（4）——奇妙的Base64编码

柔情只为你懂 2022-08-24 05:12 129阅读 0赞

各位看官应该都是资深的网虫了，小弟斗胆在此问问大家，平时上网时，除了泡MM、到论坛灌水、扔版砖……之外，进行的最多的是什么活动？对了，你一定会说：是收发电子邮件！（谁敢说自己没收/发过电子邮件的？拉出去枪毙了！！）  
  
收/发E-mail的时候有一个安全性的问题——假想一下，你花了一整天时间给系花写的情书，在发送的过程中被隔壁宿舍张三那小子截获了（难道他是黑客？？），更糟的是他是你的情敌啊……天，后果不堪设想！！因此，我们必须有一种比较可靠的加密方法，能够对电子邮件的明文进行转换，至少要得出一个无法被别人一眼就看出内容来的东西，而且编码/解码的速度还要足够快。（这时你可以再假想一下啦，张三那家伙截获了你的肉麻情书，可是他一看：“咦？怎么乱七八糟的？垃圾邮件！！”——这样一来你不就逃过大难了？！）  
  
Base64就是在这种背景下产生的加密方法。它的特点是：1、速度非常快。2、能够将字符串A转换成字符串B，而且如果你光看字符串B，是绝对猜不出字符串A的内容来的。不信吗？让我们来看看下面这串东西：  
  
xOO6w6Osu7bTrbniwdnAz8LetcTnzbfXzOy12KOh  
  
呵呵，是什么啊？猜出来了吗？其实它就是下面这段文字经过Base64编码产生的东东：  
  
你好，欢迎光临老罗的缤纷天地！  
  
介绍说完啦，让我们开始探讨实质性的东西。  
  
Base64是网络上最常见的用于传输8Bit字节代码的编码方式之一，大家可以查看RFC2045～RFC2049，上面有MIME的详细规范。  
  
Base64要求把每三个8Bit的字节转换为四个6Bit的字节（3\*8 = 4\*6 = 24），然后把6Bit再添两位高位0，组成四个8Bit的字节，也就是说，转换后的字符串理论上将要比原来的长1/3。  
  
这样说会不会太抽象了？不怕，我们来看一个例子：

<table> 
 <tbody> 
 <tr> 
 <td>转换前</td> 
 <td>aaaaaabb</td> 
 <td>ccccdddd</td> 
 <td>eeffffff</td> 
 <td></td> 
 </tr> 
 <tr> 
 <td>转换后</td> 
 <td>00aaaaaa</td> 
 <td>00bbcccc</td> 
 <td>00ddddee</td> 
 <td>00ffffff</td> 
 </tr> 
 </tbody> 
</table>

应该很清楚了吧？上面的三个字节是原文，下面的四个字节是转换后的Base64编码，其前两位均为0。  
  
转换后，我们用一个码表来得到我们想要的字符串（也就是最终的Base64编码），这个表是这样的：（摘自RFC2045）  
  
  
                            Table 1: The Base64 Alphabet  
  
      Value Encoding  Value Encoding  Value Encoding  Value Encoding  
           0 A            17 R            34 i            51 z  
           1 B            18 S            35 j            52 0  
           2 C            19 T            36 k            53 1  
           3 D            20 U            37 l            54 2  
           4 E            21 V            38 m            55 3  
           5 F            22 W            39 n            56 4  
           6 G            23 X            40 o            57 5  
           7 H            24 Y            41 p            58 6  
           8 I            25 Z            42 q            59 7  
           9 J            26 a            43 r            60 8  
          10 K            27 b            44 s            61 9  
          11 L            28 c            45 t            62 +  
          12 M            29 d            46 u            63 /  
          13 N            30 e            47 v  
          14 O            31 f            48 w         (pad) =  
          15 P            32 g            49 x  
          16 Q            33 h            50 y  
  
  
让我们再来看一个实际的例子，加深印象！

<table> 
 <tbody> 
 <tr> 
 <td>转换前</td> 
 <td>10101101</td> 
 <td>10111010</td> 
 <td>01110110</td> 
 <td></td> 
 </tr> 
 <tr> 
 <td>转换后</td> 
 <td>00101011</td> 
 <td>00011011</td> 
 <td>00101001</td> 
 <td>00110110</td> 
 </tr> 
 <tr> 
 <td>十进制</td> 
 <td>43</td> 
 <td>27</td> 
 <td>41</td> 
 <td>54</td> 
 </tr> 
 <tr> 
 <td>对应码表中的值</td> 
 <td>r</td> 
 <td>b</td> 
 <td>p</td> 
 <td>2</td> 
 </tr> 
 </tbody> 
</table>

所以上面的24位编码，编码后的Base64值为 rbp2  
解码同理，把 rbq2 的二进制位连接上再重组得到三个8位值，得出原码。  
（解码只是编码的逆过程，在此我就不多说了，另外有关MIME的RFC还是有很多的，如果需要详细情况请自行查找。）  
  
用更接近于编程的思维来说，编码的过程是这样的：  
  
第一个字符通过右移2位获得第一个目标字符的Base64表位置，根据这个数值取到表上相应的字符，就是第一个目标字符。  
然后将第一个字符左移4位加上第二个字符右移4位，即获得第二个目标字符。  
再将第二个字符左移2位加上第三个字符右移6位，获得第三个目标字符。  
最后取第三个字符的右6位即获得第四个目标字符。  
  
在以上的每一个步骤之后，再把结果与 0x3F 进行 AND 位操作，就可以得到编码后的字符了。  
  
（感谢 Athena 指出以上描述中原有的一些错误！^\_^）  
  
So easy! That’s all!!!  
  
可是等等……聪明的你可能会问到，原文的字节数量应该是3的倍数啊，如果这个条件不能满足的话，那该怎么办呢？  
  
我们的解决办法是这样的：原文的字节不够的地方可以用全0来补足，转换时Base64编码用=号来代替。这就是为什么有些Base64编码会以一个或两个等号结束的原因，但等号最多只有两个。因为：  
  
余数 = 原文字节数 MOD 3  
  
所以余数任何情况下都只可能是0，1，2这三个数中的一个。如果余数是0的话，就表示原文字节数正好是3的倍数（最理想的情况啦）。如果是1的话，为了让Base64编码是4的倍数，就要补2个等号；同理，如果是2的话，就要补1个等号。  
  
讲到这里，大伙儿应该全明白了吧？如果还有不清楚的话就返回去再仔细看看，其实不难理解的。  
  
下面我给出一个演示Base64编码/解码的程序，希望能对您有用。同时也希望您帮我完善它，利用它做出更多的用途，到时别忘了通知我一声啊！（我现在太忙了）  
  
  
**DLL的源代码：Base64Dll.asm**

<table> 
 <tbody> 
 <tr> 
 <td><a rel="nofollow">;*********************************************** </a><a rel="nofollow">;程序名称：演示Base64编码/解码原理 </a><a rel="nofollow">;作者：罗聪 </a><a rel="nofollow">;日期：2002-9-14 </a><a rel="nofollow">;出处：http://laoluoc.yeah.net（老罗的缤纷天地） </a><a rel="nofollow">;注意事项：如欲转载，请保持本程序的完整，并注明： </a><a rel="nofollow">;转载自“老罗的缤纷天地”（http://laoluoc.yeah.net） </a><a rel="nofollow">;*********************************************** </a><a rel="nofollow"> </a><a rel="nofollow">.386 </a><a rel="nofollow">.model flat, stdcall </a><a rel="nofollow">option casemap:none </a><a rel="nofollow"> </a><a rel="nofollow">include /masm32/include/windows.inc </a><a rel="nofollow">include /masm32/include/kernel32.inc </a><a rel="nofollow">include /masm32/include/user32.inc </a><a rel="nofollow">includelib /masm32/lib/kernel32.lib </a><a rel="nofollow">includelib /masm32/lib/user32.lib </a><a rel="nofollow"> </a><a rel="nofollow">DllEntry&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;proto :HINSTANCE, :DWORD, :DWORD </a><a rel="nofollow">Base64Encode&nbsp;&nbsp;&nbsp;&nbsp;proto :DWORD, :DWORD </a><a rel="nofollow">Base64Decode&nbsp;&nbsp;&nbsp;&nbsp;proto :DWORD, :DWORD </a><a rel="nofollow"> </a><a rel="nofollow">.data </a><a rel="nofollow">;Base64 -&gt; ASCII mapping table </a><a rel="nofollow">base64_alphabet &nbsp;&nbsp;&nbsp;&nbsp;db&nbsp;&nbsp;&nbsp;&nbsp;"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=" </a><a rel="nofollow"> </a><a rel="nofollow">;ASCII -&gt; Base64 mapping table </a><a rel="nofollow">base64table&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; db 43 dup (255) </a><a rel="nofollow">&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;db 62,255,255,255,63,52,53,54,55,56,57,58,59,60,61,255 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;db 255,255,0,255,255,255,0,1,2,3,4,5,6,7,8,9,10,11,12,13 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;db 14,15,16,17,18,19,20,21,22,23,24,25,255,255,255,255 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;db 255,255,26,27,28,29,30,31,32,33,34,35,36,37,38 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;db 39,40,41,42,43,44,45,46,47,48,49,50,51 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;db 132 dup (255) </a><a rel="nofollow"> </a><a rel="nofollow">.code </a><a rel="nofollow">DllEntry&nbsp;&nbsp;&nbsp;&nbsp;proc&nbsp;&nbsp;&nbsp;&nbsp;hInst: HINSTANCE, reason: DWORD, reserved1: DWORD </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov eax, TRUE </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;ret </a><a rel="nofollow">DllEntry&nbsp;&nbsp;&nbsp;&nbsp;endp </a><a rel="nofollow"> </a><a rel="nofollow"> </a><a rel="nofollow">;********************************************************** </a><a rel="nofollow">;函数功能：进行Base64编码 </a><a rel="nofollow">;参数： </a><a rel="nofollow">;&nbsp;&nbsp;&nbsp;&nbsp;source&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;&nbsp;&nbsp;传入的字符串 </a><a rel="nofollow">;&nbsp;&nbsp;&nbsp;&nbsp;destination&nbsp;&nbsp;&nbsp;&nbsp; =&nbsp;&nbsp;&nbsp;&nbsp;返回的编码 </a><a rel="nofollow">;********************************************************** </a><a rel="nofollow">Base64Encode&nbsp;&nbsp;&nbsp;&nbsp;proc&nbsp;&nbsp;&nbsp;&nbsp;uses ebx edi esi source:DWORD, destination:DWORD </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;LOCAL&nbsp;&nbsp;&nbsp;&nbsp;sourcelen:DWORD </a><a rel="nofollow"> </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;invoke lstrlen, source </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov sourcelen, eax </a><a rel="nofollow"> </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;esi, source </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;edi, destination </a><a rel="nofollow">@@base64loop: </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;xor eax, eax </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;.if sourcelen == 1 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;lodsb&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;source ptr + 1 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;mov ecx, 2&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;bytes to output = 2 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;mov edx, 03D3Dh&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;padding = 2 byte </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dec sourcelen&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;length - 1 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;.elseif sourcelen == 2 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;lodsw&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;source ptr + 2 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;mov ecx, 3&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;bytes to output = 3 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;mov edx, 03Dh&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;padding = 1 byte </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sub sourcelen, 2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;length - 2 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;.else </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;lodsd </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;mov ecx, 4&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;bytes to output = 4 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;xor edx, edx&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;padding = 0 byte </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dec esi&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;source ptr + 3 (+4-1) </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sub sourcelen, 3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;length - 3 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;.endif </a><a rel="nofollow"> </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;xchg al,ah&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;flip eax completely </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;rol&nbsp;&nbsp;eax, 16&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;can this be done faster </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;xchg al,ah </a><a rel="nofollow"> </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;@@: </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;push&nbsp;&nbsp;eax </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;and&nbsp;&nbsp; eax, 0FC000000h&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;get the last 6 high bits </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;rol&nbsp;&nbsp; eax, 6&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;rotate them into al </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp; al,&nbsp;&nbsp;byte ptr [offset base64_alphabet + eax]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;get encode character </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;stosb&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;write to destination </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;pop&nbsp;&nbsp; eax </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;shl&nbsp;&nbsp; eax, 6&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;shift left 6 bits </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;dec&nbsp;&nbsp; ecx </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;jnz&nbsp;&nbsp; @B&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;loop </a><a rel="nofollow"> </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;cmp&nbsp;&nbsp; sourcelen, 0 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;jnz&nbsp;&nbsp; @@base64loop&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;main loop </a><a rel="nofollow"> </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp; eax, edx&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;add padding and null terminate </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;stosd </a><a rel="nofollow"> </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;ret </a><a rel="nofollow">Base64Encode&nbsp;&nbsp;&nbsp;&nbsp;endp </a><a rel="nofollow"> </a><a rel="nofollow"> </a><a rel="nofollow">;********************************************************** </a><a rel="nofollow">;函数功能：进行Base64解码 </a><a rel="nofollow">;参数： </a><a rel="nofollow">;&nbsp;&nbsp;&nbsp;&nbsp;source&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;&nbsp;&nbsp;传入的编码 </a><a rel="nofollow">;&nbsp;&nbsp;&nbsp;&nbsp;destination&nbsp;&nbsp;&nbsp;&nbsp; =&nbsp;&nbsp;&nbsp;&nbsp;返回的字符串 </a><a rel="nofollow">;********************************************************** </a><a rel="nofollow">Base64Decode&nbsp;&nbsp;&nbsp;&nbsp;proc&nbsp;&nbsp;&nbsp;&nbsp;uses ebx edi esi source:DWORD, destination:DWORD </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;LOCAL&nbsp;&nbsp;&nbsp;&nbsp;sourcelen:DWORD </a><a rel="nofollow"> </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;invoke lstrlen, source </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov sourcelen, eax </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp; </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;&nbsp;&nbsp;esi, source&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ;esi &lt;- source </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;&nbsp;&nbsp;edi, destination&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;edi &lt;- destination </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;&nbsp;&nbsp;ecx, sourcelen </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;shr&nbsp;&nbsp;&nbsp;&nbsp;ecx, 2 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;cld </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp; </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;;-------------[decoding part]--------------- </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp; </a><a rel="nofollow">@@outer_loop: </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;push&nbsp;&nbsp; ecx </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;&nbsp;&nbsp;ecx, 4 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;xor&nbsp;&nbsp;&nbsp;&nbsp;ebx, ebx </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;lodsd </a><a rel="nofollow">@@inner_loop: </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;push&nbsp;&nbsp; eax </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;and&nbsp;&nbsp;&nbsp;&nbsp;eax, 0ffh </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;&nbsp;&nbsp;al, byte ptr [offset base64table + eax] </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;cmp&nbsp;&nbsp;&nbsp;&nbsp;al, 255 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;je&nbsp;&nbsp;&nbsp;&nbsp; @@invalid_char </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;shl&nbsp;&nbsp;&nbsp;&nbsp;ebx, 6 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;or&nbsp;&nbsp;&nbsp;&nbsp; bl, al </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;pop&nbsp;&nbsp;&nbsp;&nbsp;eax </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;shr&nbsp;&nbsp;&nbsp;&nbsp;eax, 8 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;dec&nbsp;&nbsp;&nbsp;&nbsp;ecx </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;jnz&nbsp;&nbsp;&nbsp;&nbsp;@@inner_loop </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;&nbsp;&nbsp;eax, ebx </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;shl&nbsp;&nbsp;&nbsp;&nbsp;eax, 8 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;xchg&nbsp;&nbsp; ah, al </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;ror&nbsp;&nbsp;&nbsp;&nbsp;eax, 16 </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;xchg&nbsp;&nbsp; ah, al </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;stosd </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;dec&nbsp;&nbsp;&nbsp;&nbsp;edi </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;pop&nbsp;&nbsp;&nbsp;&nbsp;ecx </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;dec&nbsp;&nbsp;&nbsp;&nbsp;ecx </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;jnz&nbsp;&nbsp;&nbsp;&nbsp;@@outer_loop </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;xor&nbsp;&nbsp;&nbsp;&nbsp;eax, eax </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;jmp&nbsp;&nbsp;&nbsp;&nbsp;@@decode_done </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp; </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;;------------------------------------------- </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp; </a><a rel="nofollow">@@invalid_char: </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;mov&nbsp;&nbsp;&nbsp;&nbsp;eax, -1 </a><a rel="nofollow">@@decode_done: </a><a rel="nofollow">&nbsp;&nbsp;&nbsp;&nbsp;ret </a><a rel="nofollow">Base64Decode ENDP </a><a rel="nofollow"> </a><a rel="nofollow"> </a><a rel="nofollow">end DllEntry </a><a rel="nofollow">;********************&nbsp;&nbsp;&nbsp;&nbsp;over&nbsp;&nbsp;&nbsp;&nbsp;******************** </a><a rel="nofollow">;by LC</a></td> 
 </tr> 
 </tbody> 
</table>

--------------------

**测试程序：base64.asm**

--------------------

**测试程序的资源文件：base64.rc**

<table> 
 <tbody> 
 <tr> 
 <td>#include "resource.h" #define IDC_BUTTON_ENCODE&nbsp;&nbsp;&nbsp;&nbsp;3000 #define IDC_BUTTON_DECODE&nbsp;&nbsp;&nbsp;&nbsp;3001 #define IDC_EDIT_INPUT&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 3002 #define IDC_STATIC&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -1 LC_DIALOG DIALOGEX 10, 10, 195, 60 STYLE DS_SETFONT | DS_CENTER | WS_MINIMIZEBOX | WS_VISIBLE | WS_CAPTION | &nbsp;&nbsp;&nbsp;&nbsp;WS_SYSMENU CAPTION "Base64 demo by LC" FONT 9, "宋体", 0, 0, 0x0 BEGIN &nbsp;&nbsp;&nbsp;&nbsp;LTEXT&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; "请输入字符串：", IDC_STATIC, 11, 7, 130, 10 &nbsp;&nbsp;&nbsp;&nbsp;EDITTEXT&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;IDC_EDIT_INPUT, 11, 20, 173, 12, ES_AUTOHSCROLL &nbsp;&nbsp;&nbsp;&nbsp;DEFPUSHBUTTON&nbsp;&nbsp; "编码(&amp;E)", IDC_BUTTON_ENCODE, 38, 39, 52, 15 &nbsp;&nbsp;&nbsp;&nbsp;PUSHBUTTON&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;"解码(&amp;D)", IDC_BUTTON_DECODE, 104, 39, 52, 15 END</td> 
 </tr> 
 </tbody> 
</table>

**如果你发现了有bug，一定要告诉我啊，并请来信讨论！[lcother@163.net][lcother_163.net]**  
  
最后给大家留下一个小小的习题，你知道下面这串Base64编码的原文是什么吗？  ：）  
0LvQu8T6xM3XxdDU19O/tM3qztK1xEJhc2U2NL3Ms8yjoSCjuqOp

转自：[http://www.luocong.com/articles/show\_article.asp?Article\_ID=17][http_www.luocong.com_articles_show_article.asp_Article_ID_17]

[lcother_163.net]: mailto:lcother@163.net?subject=老罗，有关Base64的问题想跟你讨论！
[http_www.luocong.com_articles_show_article.asp_Article_ID_17]: http://www.luocong.com/articles/show_article.asp?Article_ID=17