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
php 的函数：base64_encode() 和 base64_decode()
base64的编,解码原理
Base64 编码其实是将3个8位字节转换为4个6位字节,( 3*8 = 4*6 = 24 ) 这4个六位字节 其实仍然是8位,只不过高两位被设置为0. 当一个字节只有6位有效时,它的取值空间为0 到 2的6次方减1 即63,也就是说被转换的Base64编码的每一个编码的取值空间为(0~63) 。
事实上，0~63之间的ASCII码有许多不可见字符，所以应该再做一个映射，映射表为
'A' ~ 'Z' ? ASCII（0 ~ 25）
'a' ~ 'z' ? ASCII（26 ~ 51）
'0' ~ '9' ? ASCII（52 ~ 61）
' ' ? ASCII（62）
'/' ? ASCII（63）
这样就可以将3个8位字节，转换为4个可见字符。
具体的字节拆分方法为：(图（画得不好，领会精神 :-))
aaaaaabb ccccdddd eeffffff //abcdef其实就是1或0，为了看的清楚就用abcdef代替
~~~~~~~~ ~~~~~~~~ ~~~~~~~~
字节 1 字节 2 字节 3
||
\/
00aaaaaa 00bbcccc 00ddddee 00ffffff
注：上面的三个字节位原文，下面四个字节为Base64编码，其前两位均为0。
这样拆分的时候，原文的字节数量应该是3的倍数，当这个条件不能满足时，用全零字节
补足，转化时Base64编码用=号代替，这就是为什么有些Base64编码以一个或两个等号结
束的原因，但等号最多有两个，因为：如果F(origin)代表原文的字节数，F(remain)代
表余数，则
F(remain) = F(origin) MOD 3 成立。
所以F(remain)的可能取值为0,1,2.
如果设 n = [F(origin) – F(remain)] / 3
当F(remain) = 0 时，恰好转换为4*n个字节的Base64编码。
当F(remain) = 1 时，由于一个原文字节可以拆分为属于两个Base64编码的字节，为了
让Base64编码是4的倍数，所以应该为补2个等号。
当F(remain) = 2 时，由于两个原文字节可以拆分为属于3个Base64编码的字节，同理，
应该补上一个等号。
base64 编码后的字符串末尾会有0到2个等号，这些等号在解码是并不必要，所以可以删除。
在网络GET 和 POST参数列表的时候，‘+'不能正常传输，可以把它替换成‘|'
这样经过base64编码后的字符串就只有‘|'和‘/‘，所以经过这样处理base64编码的字符串可以作为参数列表的以个参数值来传输
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以下是老外写的一个实现：
package com.meterware.httpunit;
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/******************************************************************************************************************** <br>* $Id: Base64.java,v 1.4 2002/12/24 15:17:17 russgold Exp $ <br>* <br>* Copyright (c) 2000-2002 by Russell Gold <br>* <br>* Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated <br>* documentation files (the "Software "), to deal in the Software without restriction, including without limitation <br>* the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and <br>* to permit persons to whom the Software is furnished to do so, subject to the following conditions: <br>* <br>* The above copyright notice and this permission notice shall be included in all copies or substantial portions <br>* of the Software. <br>* <br>* THE SOFTWARE IS PROVIDED "AS IS ", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO <br>* THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE <br>* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF <br>* CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER <br>* DEALINGS IN THE SOFTWARE. <br>* <br>*******************************************************************************************************************/ <br><br>/** <br>* A utility class to convert to and from base 64 encoding. <br>* <br>* @author <a href= "mailto:russgold@httpunit.org "> Russell Gold </a> <br>**/ <br>
public class Base64 {
final static String encodingChar = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/ "; /**
* Returns the base 64 encoded equivalent of a supplied string.
* @param source the string to encode
*/
public static String encode( String source ) {
char[] sourceBytes = getPaddedBytes( source );
int numGroups = (sourceBytes.length + 2) / 3;
char[] targetBytes = new char[4];
char[] target = new char[ 4 * numGroups ];
for (int group = 0; group < numGroups; group++) {
convert3To4( sourceBytes, group*3, targetBytes );
for (int i = 0; i < targetBytes.length; i++) {
target[ i + 4*group ] = encodingChar.charAt( targetBytes[i] );
}
}
int numPadBytes = sourceBytes.length - source.length();
for (int i = target.length-numPadBytes; i < target.length; i++) target[i] = '= ';
return new String( target );
}
private static char[] getPaddedBytes( String source ) {
char[] converted = source.toCharArray();
int requiredLength = 3 * ((converted.length+2) /3);
char[] result = new char[ requiredLength ];
System.arraycopy( converted, 0, result, 0, converted.length );
return result;
}
private static void convert3To4( char[] source, int sourceIndex, char[] target ) {
target[0] = (char) ( source[ sourceIndex ] > > > 2);
target[1] = (char) (((source[ sourceIndex ] & 0x03) < < 4) | (source[ sourceIndex+1 ] > > > 4));
target[2] = (char) (((source[ sourceIndex+1 ] & 0x0f) < < 2) | (source[ sourceIndex+2 ] > > > 6));
target[3] = (char) ( source[ sourceIndex+2 ] & 0x3f);
}
/**
* Returns the plaintext equivalent of a base 64-encoded string.
* @param source a base 64 string (which must have a multiple of 4 characters)
*/
public static String decode( String source ) {
if (source.length()%4 != 0) throw new RuntimeException( "valid Base64 codes have a multiple of 4 characters " );
int numGroups = source.length() / 4;
int numExtraBytes = source.endsWith( "== " ) ? 2 : (source.endsWith( "= " ) ? 1 : 0);
byte[] targetBytes = new byte[ 3*numGroups ];
byte[] sourceBytes = new byte[4];
for (int group = 0; group < numGroups; group++) {
for (int i = 0; i < sourceBytes.length; i++) {
sourceBytes[i] = (byte) Math.max( 0, encodingChar.indexOf( source.charAt( 4*group+i ) ) );
}
convert4To3( sourceBytes, targetBytes, group*3 );
}
return new String( targetBytes, 0, targetBytes.length - numExtraBytes );
}
private static void convert4To3( byte[] source, byte[] target, int targetIndex ) {
target[ targetIndex ] = (byte) (( source[0] < < 2) | (source[1] > > > 4));
target[ targetIndex+1 ] = (byte) (((source[1] & 0x0f) < < 4) | (source[2] > > > 2));
target[ targetIndex+2 ] = (byte) (((source[2] & 0x03) < < 6) | (source[3]));
}
}
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