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 ])); } } |