Base64是網路上最常見的傳送8Bit位元組代碼的編碼方式之一，大家可以查看RFC2045～RFC2049，上面有MIME的詳細規格。 Base64要求把每三個8Bit的位元組轉換為四個6Bit的位元組（3*8 = 4*6 = 24），然後把6Bit再添兩位高位0，組成四個8Bit的位元組，也就是說，轉換後的字符串理論上將要比原來的長1/3
php 的函數：base64_encode() 和base64_decode()
base64的編,解碼原理
Base64 編碼其實是將3個8位字節轉換為4個6位元組,( 3*8 = 4*6 = 24 ) 這4個六位元組其實仍然是8位元,只不過高兩位被設定為0. 當一個位元組只有6位元有效時,它的取值空間為0 到2的6次方減1 即63,也就是說被轉換的Base64編碼的每一個編碼的取值空間為(0~63) 。
事實上，0~63之間的ASCII碼有許多不可見字符，所以應該再做一個映射，映射表為
'A' ~ 'Z' ? ASCII（0 ~ 25）
'a' ~ 'z' ? ASCII（26 ~ 51）
'0' ~ '9' ? ASCII（52 ~ 61）
' ' ? ASCII（62）
'/' ? ASCII (63)可以將3個8位元組，轉換為4個可見字元。
特定的字節分割方法為：(圖（畫得不好，領會精神:-))
aaaaaabb ccccdddd eeffffff //abcdef其實就是1或0，為了看的清楚就用abcdefbiddddd
~ ~~~~~~~ ~~~~~~~~ ~~~~~~~~
字節1 位元組2 位元組3
||
/
ff00cc bboffof 07070000cc 0707870787007870787007878787878787878 -87x bbffoff;上面的三個位元組位元原文，下面四個位元組為Base64編碼，其前兩位均為0。
這樣分割的時候，原文的位元組數量應該是3的倍數，當這個條件不能滿足時，用全零字節
補足，轉換時Base64編碼用=號代替，這就是為什麼有些Base64編碼以一個或兩個等號結
束的原因，但等號最多有兩個，因為：若F(origin)代表原文的位元組數，F(remain)代
表餘數，則
F (remain) = F(origin) MOD 3 成立。
所以F(remain)的可能取值為0,1,2.
如果設n = [F(origin) – F(remain)] / 3
當F(remain) = 0 時，恰好轉換為4*n個位元組的Base64編碼。
當F(remain) = 1 時，由於一個原文字節可以分割為屬於兩個Base64編碼的位元組，為了
讓Base64編碼是4的倍數，所以應該要補2個等號。
當F(remain) = 2 時，由於兩個原文字節可以分割為屬於3個Base64編碼的位元組，同理，
應該補上一個等號。
base64 編碼後的字串末尾會有0到2個等號，這些等號在解碼是並不必要，所以可以刪除。
在網路GET 和POST參數列表的時候，'+'不能正常傳輸，可以把它替換成'|'
這樣經過base64編碼後的字符串就只有'|'和'/'，所以經過這樣處理base64編碼的字串可以作為參數列表的以個參數值來傳送
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以下是老外寫的一個實作：
package com.meterware.httpunit;
/******************************************************************************************************************** <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]));
}
}
