原理:
Base64 使用US-ASCII子集的64个字符,即大小写的26个英文字母,0-9,+,/。
编码总是基于3个字符,每个字符用8位二进制表示,因此一共24位,再分为4四组,每组6位,表示一个Base64的值。如下:
"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", "+", "/"
Base64值为0就是A,为27的就是b。这样,每3个字符产生4位的Base64字符。如果被加密的字符串每3个一组,还剩1或2个字符,使用特殊字符"="补齐Base64的4字。
如, 编码只有2个字符“me”,m的ascii是109,e的是101,用二进制表示分别是01101101、01100101,连接起来就是 0110110101100101,再按6位分为一组:011011、010110、010100(不足6位补0),分别ascii分别是27、22、 20,即Base64值为bWU,Base64不足4字,用=补齐,因此bWU=就me的Base64值。
用java的按位逻辑和移位运算就可以实现该算法。但实际上,并不用我们自己去编程实现。现有实现该加密解密算法的程序很多,如javamail的MimeUtility。
实践:javamail的MimeUtility实现对字符串的加密解密
第一步,在eclipse新建一个java项目,并引进javamail.jar
第二步,在com.mascot.encrypt包下创建测试单元,注意引进javax.mail.internet.MimeUtility
package com.mascot.encrypt; import java.io.BufferedReader; import java.io.ByteArrayInputStream; import java.io.ByteArrayOutputStream; import java.io.InputStreamReader; import javax.mail.internet.MimeUtility; public class Base64 { public static BufferedReader decode(String b64string) throws Exception { return new BufferedReader( new InputStreamReader( MimeUtility.decode( new ByteArrayInputStream( b64string.getBytes()), "base64"))); } public static void main(String args[]) throws Exception{ System.out.println(encodeAsString("hello")); System.out.println(decodeAsString("aGVsbG8=")); System.out.println(decodeAsString("aGVsbG8A")); } public static String decodeAsString(String b64string) throws Exception { if (b64string == null) { return b64string; } String returnString = decode(b64string).readLine(); if (returnString == null) { return returnString; } return returnString.trim(); } public static ByteArrayOutputStream encode(String plaintext) throws Exception { ByteArrayOutputStream out = new ByteArrayOutputStream(); byte[] in = plaintext.getBytes(); ByteArrayOutputStream inStream = new ByteArrayOutputStream(); inStream.write(in, 0, in.length); //补0 if ((in.length % 3 ) == 1){ inStream.write(0); inStream.write(0); } else if((in.length % 3 ) == 2){ inStream.write(0); } inStream.writeTo( MimeUtility.encode(out, "base64") ); return out; } public static String encodeAsString(String plaintext) throws Exception { return encode(plaintext).toString(); } }
第三步,运行程序,结果:
aGVsbG8A
hello
hello
注意到hello的加密结果并不是aGVsbG8=,而是aGVsbG8A,这是因为程序补齐都是补0,而0对应的Base64值就是A,这是我们程序实现与上述理论不同造成的。
以上原文地址为:http://www.blogjava.net/topquan/archive/2006/10/14/75132.html