基于Java实现的Base64加密、解密原理代码

一、概述

1.base64是什么：

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编码可用于在http环境下传递较长的标识信息。例如，在java persistence系统hibernate中，就采用了base64来将一个较长的唯一标识符（一般为128-bit的uuid）编码为一个字符串，用作http表单和http get url中的参数。在其他应用程序中，也常常需要把二进制数据编码为适合放在url（包括隐藏表单域）中的形式。此时，采用base64编码不仅比较简短，同时也具有不可读性，即所编码的数据不会被人用肉眼所直接看到

2.简介：

标准的base64并不适合直接放在url里传输，因为url编码器会把标准base64中的“/”和“+”字符变为形如“%xx”的形式，而这些“%”号在存入数据库时还需要再进行转换，因为ansi sql中已将“%”号用作通配符。

为解决此问题，可采用一种用于url的改进base64编码，它不在末尾填充'='号，并将标准base64中的“+”和“/”分别改成了“*”和“-”，这样就免去了在url编解码和数据库存储时所要作的转换，避免了编码信息长度在此过程中的增加，并统一了数据库、表单等处对象标识符的格式。

另有一种用于正则表达式的改进base64变种，它将“+”和“/”改成了“!”和“-”，因为“+”,“*”以及前面在ircu中用到的“[”和“]”在正则表达式中都可能具有特殊含义。

此外还有一些变种，它们将“+/”改为“_-”或“._”（用作编程语言中的标识符名称）或“.-”（用于xml中的nmtoken）甚至“_:”（用于xml中的name）。

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。

3.规则：

关于这个编码的规则：

①.把3个字符变成4个字符..
②.每76个字符加一个换行符..
③.最后的结束符也要处理..
这样说会不会太抽象了？不用着急，我们来看一个例子：
转换前： aaaaaabb ccccdddd eeffffff
转换后： 00aaaaaa 00bbcccc 00ddddee 00ffffff
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）

二、java实现代码示例：

public final class base64 {

 static private final int baselength = 255;
 static private final int lookuplength = 64;
 static private final int twentyfourbitgroup = 24;
 static private final int eightbit = 8;
 static private final int sixteenbit = 16;
 static private final int sixbit = 6;
 static private final int fourbyte = 4;
 static private final int sign = -128;
 static private final char pad = '=';
 static private final boolean fdebug = false;
 static final private byte[] base64alphabet = new byte[baselength];
 static final private char[] lookupbase64alphabet = new char[lookuplength];

 static {

 for (int i = 0; i < baselength; i++) {
  base64alphabet[i] = -1;
 }
 for (int i = 'z'; i >= 'a'; i--) {
  base64alphabet[i] = (byte) (i - 'a');
 }
 for (int i = 'z'; i >= 'a'; i--) {
  base64alphabet[i] = (byte) (i - 'a' + 26);
 }

 for (int i = '9'; i >= '0'; i--) {
  base64alphabet[i] = (byte) (i - '0' + 52);
 }

 base64alphabet['+'] = 62;
 base64alphabet['/'] = 63;

 for (int i = 0; i <= 25; i++)
  lookupbase64alphabet[i] = (char) ('a' + i);

 for (int i = 26, j = 0; i <= 51; i++, j++)
  lookupbase64alphabet[i] = (char) ('a' + j);

 for (int i = 52, j = 0; i <= 61; i++, j++)
  lookupbase64alphabet[i] = (char) ('0' + j);
 lookupbase64alphabet[62] = (char) '+';
 lookupbase64alphabet[63] = (char) '/';

 }

 protected static boolean iswhitespace(char octect) {
 return (octect == 0x20 || octect == 0xd || octect == 0xa || octect == 0x9);
 }

 protected static boolean ispad(char octect) {
 return (octect == pad);
 }

 protected static boolean isdata(char octect) {
 return (base64alphabet[octect] != -1);
 }

 protected static boolean isbase64(char octect) {
 return (iswhitespace(octect) || ispad(octect) || isdata(octect));
 }

 /**
 * encodes hex octects into base64
 * 
 * @param binarydata
 *      array containing binarydata
 * @return encoded base64 array
 */
 public static string encode(byte[] binarydata) {

 if (binarydata == null)
  return null;

 int lengthdatabits = binarydata.length * eightbit;
 if (lengthdatabits == 0) {
  return "";
 }

 int fewerthan24bits = lengthdatabits % twentyfourbitgroup;
 int numbertriplets = lengthdatabits / twentyfourbitgroup;
 int numberquartet = fewerthan24bits != 0 ? numbertriplets + 1
  : numbertriplets;
 int numberlines = (numberquartet - 1) / 19 + 1;
 char encodeddata[] = null;

 encodeddata = new char[numberquartet * 4 + numberlines];

 byte k = 0, l = 0, b1 = 0, b2 = 0, b3 = 0;

 int encodedindex = 0;
 int dataindex = 0;
 int i = 0;
 if (fdebug) {
  system.out.println("number of triplets = " + numbertriplets);
 }

 for (int line = 0; line < numberlines - 1; line++) {
  for (int quartet = 0; quartet < 19; quartet++) {
  b1 = binarydata[dataindex++];
  b2 = binarydata[dataindex++];
  b3 = binarydata[dataindex++];

  if (fdebug) {
   system.out.println("b1= " + b1 + ", b2= " + b2 + ", b3= "
    + b3);
  }

  l = (byte) (b2 & 0x0f);
  k = (byte) (b1 & 0x03);

  byte val1 = ((b1 & sign) == 0) ? (byte) (b1 >> 2)
   : (byte) ((b1) >> 2 ^ 0xc0);

  byte val2 = ((b2 & sign) == 0) ? (byte) (b2 >> 4)
   : (byte) ((b2) >> 4 ^ 0xf0);
  byte val3 = ((b3 & sign) == 0) ? (byte) (b3 >> 6)
   : (byte) ((b3) >> 6 ^ 0xfc);

  if (fdebug) {
   system.out.println("val2 = " + val2);
   system.out.println("k4 = " + (k << 4));
   system.out.println("vak = " + (val2 | (k << 4)));
  }

  encodeddata[encodedindex++] = lookupbase64alphabet[val1];
  encodeddata[encodedindex++] = lookupbase64alphabet[val2
   | (k << 4)];
  encodeddata[encodedindex++] = lookupbase64alphabet[(l << 2)
   | val3];
  encodeddata[encodedindex++] = lookupbase64alphabet[b3 & 0x3f];

  i++;
  }
  encodeddata[encodedindex++] = 0xa;
 }

 for (; i < numbertriplets; i++) {
  b1 = binarydata[dataindex++];
  b2 = binarydata[dataindex++];
  b3 = binarydata[dataindex++];

  if (fdebug) {
  system.out.println("b1= " + b1 + ", b2= " + b2 + ", b3= " + b3);
  }

  l = (byte) (b2 & 0x0f);
  k = (byte) (b1 & 0x03);

  byte val1 = ((b1 & sign) == 0) ? (byte) (b1 >> 2)
   : (byte) ((b1) >> 2 ^ 0xc0);

  byte val2 = ((b2 & sign) == 0) ? (byte) (b2 >> 4)
   : (byte) ((b2) >> 4 ^ 0xf0);
  byte val3 = ((b3 & sign) == 0) ? (byte) (b3 >> 6)
   : (byte) ((b3) >> 6 ^ 0xfc);

  if (fdebug) {
  system.out.println("val2 = " + val2);
  system.out.println("k4 = " + (k << 4));
  system.out.println("vak = " + (val2 | (k << 4)));
  }

  encodeddata[encodedindex++] = lookupbase64alphabet[val1];
  encodeddata[encodedindex++] = lookupbase64alphabet[val2 | (k << 4)];
  encodeddata[encodedindex++] = lookupbase64alphabet[(l << 2) | val3];
  encodeddata[encodedindex++] = lookupbase64alphabet[b3 & 0x3f];
 }

 // form integral number of 6-bit groups
 if (fewerthan24bits == eightbit) {
  b1 = binarydata[dataindex];
  k = (byte) (b1 & 0x03);
  if (fdebug) {
  system.out.println("b1=" + b1);
  system.out.println("b1<<2 = " + (b1 >> 2));
  }
  byte val1 = ((b1 & sign) == 0) ? (byte) (b1 >> 2)
   : (byte) ((b1) >> 2 ^ 0xc0);
  encodeddata[encodedindex++] = lookupbase64alphabet[val1];
  encodeddata[encodedindex++] = lookupbase64alphabet[k << 4];
  encodeddata[encodedindex++] = pad;
  encodeddata[encodedindex++] = pad;
 } else if (fewerthan24bits == sixteenbit) {
  b1 = binarydata[dataindex];
  b2 = binarydata[dataindex + 1];
  l = (byte) (b2 & 0x0f);
  k = (byte) (b1 & 0x03);

  byte val1 = ((b1 & sign) == 0) ? (byte) (b1 >> 2)
   : (byte) ((b1) >> 2 ^ 0xc0);
  byte val2 = ((b2 & sign) == 0) ? (byte) (b2 >> 4)
   : (byte) ((b2) >> 4 ^ 0xf0);

  encodeddata[encodedindex++] = lookupbase64alphabet[val1];
  encodeddata[encodedindex++] = lookupbase64alphabet[val2 | (k << 4)];
  encodeddata[encodedindex++] = lookupbase64alphabet[l << 2];
  encodeddata[encodedindex++] = pad;
 }

 encodeddata[encodedindex] = 0xa;

 return new string(encodeddata);
 }

 /**
 * decodes base64 data into octects
 * 
 * @param binarydata
 *      byte array containing base64 data
 * @return array containind decoded data.
 */
 public static byte[] decode(string encoded) {

 if (encoded == null)
  return null;

 char[] base64data = encoded.tochararray();
 // remove white spaces
 int len = removewhitespace(base64data);

 if (len % fourbyte != 0) {
  return null;// should be divisible by four
 }

 int numberquadruple = (len / fourbyte);

 if (numberquadruple == 0)
  return new byte[0];

 byte decodeddata[] = null;
 byte b1 = 0, b2 = 0, b3 = 0, b4 = 0, marker0 = 0, marker1 = 0;
 char d1 = 0, d2 = 0, d3 = 0, d4 = 0;

 int i = 0;
 int encodedindex = 0;
 int dataindex = 0;
 decodeddata = new byte[(numberquadruple) * 3];

 for (; i < numberquadruple - 1; i++) {

  if (!isdata((d1 = base64data[dataindex++]))
   || !isdata((d2 = base64data[dataindex++]))
   || !isdata((d3 = base64data[dataindex++]))
   || !isdata((d4 = base64data[dataindex++])))
  return null;// if found "no data" just return null

  b1 = base64alphabet[d1];
  b2 = base64alphabet[d2];
  b3 = base64alphabet[d3];
  b4 = base64alphabet[d4];

  decodeddata[encodedindex++] = (byte) (b1 << 2 | b2 >> 4);
  decodeddata[encodedindex++] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf));
  decodeddata[encodedindex++] = (byte) (b3 << 6 | b4);
 }

 if (!isdata((d1 = base64data[dataindex++]))
  || !isdata((d2 = base64data[dataindex++]))) {
  return null;// if found "no data" just return null
 }

 b1 = base64alphabet[d1];
 b2 = base64alphabet[d2];

 d3 = base64data[dataindex++];
 d4 = base64data[dataindex++];
 if (!isdata((d3)) || !isdata((d4))) {// check if they are pad characters
  if (ispad(d3) && ispad(d4)) { // two pad e.g. 3c[pad][pad]
  if ((b2 & 0xf) != 0)// last 4 bits should be zero
   return null;
  byte[] tmp = new byte[i * 3 + 1];
  system.arraycopy(decodeddata, 0, tmp, 0, i * 3);
  tmp[encodedindex] = (byte) (b1 << 2 | b2 >> 4);
  return tmp;
  } else if (!ispad(d3) && ispad(d4)) { // one pad e.g. 3cq[pad]
  b3 = base64alphabet[d3];
  if ((b3 & 0x3) != 0)// last 2 bits should be zero
   return null;
  byte[] tmp = new byte[i * 3 + 2];
  system.arraycopy(decodeddata, 0, tmp, 0, i * 3);
  tmp[encodedindex++] = (byte) (b1 << 2 | b2 >> 4);
  tmp[encodedindex] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf));
  return tmp;
  } else {
  return null;// an error like "3c[pad]r", "3cdx", "3cxd", "3cxx"
    // where x is non data
  }
 } else { // no pad e.g 3cql
  b3 = base64alphabet[d3];
  b4 = base64alphabet[d4];
  decodeddata[encodedindex++] = (byte) (b1 << 2 | b2 >> 4);
  decodeddata[encodedindex++] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf));
  decodeddata[encodedindex++] = (byte) (b3 << 6 | b4);

 }

 return decodeddata;
 }

 /**
 * remove whitespace from mime containing encoded base64 data.
 * 
 * @param data
 *      the byte array of base64 data (with ws)
 * @return the new length
 */
 protected static int removewhitespace(char[] data) {
 if (data == null)
  return 0;

 // count characters that's not whitespace
 int newsize = 0;
 int len = data.length;
 for (int i = 0; i < len; i++) {
  if (!iswhitespace(data[i]))
  data[newsize++] = data[i];
 }
 return newsize;
 }
 public static void main(string[] args) {
 system.out.println(encode("*".getbytes()));
 }
}