JAVA implementation of base64_encode and base64_decode

Base64 is one of the most common encoding methods for transmitting 8-bit byte codes on the Internet. You can check RFC2045~RFC2049, which has detailed specifications for MIME. Base64 requires converting every three 8Bit bytes into four 6Bit bytes (3*8 = 4*6 = 24), and then adding two high-bit 0s to the 6Bit to form four 8Bit bytes, that is Said, the converted string will theoretically be 1/3 longer than the original

php functions: base64_encode() and base64_decode()

base64 encoding and decoding principle

Base64 encoding actually converts 3 8-bit bytes into 4 6-bit bytes, (3*8 = 4*6 = 24) These 4 six-bit bytes are actually still 8 bits, but they are two higher bit is set to 0. When only 6 bits of a byte are valid, its value space is from 0 to 2 to the 6th power minus 1, which is 63, that is to say, the value space of each encoding of the converted Base64 encoding is (0~63).

In fact, there are many invisible characters in the ASCII code between 0 and 63, so another mapping should be done. The mapping table is

'A' ~ 'Z' ? ASCII (0 ~ 25)

'a' ~ 'z' ? ASCII (26 ~ 51)

'0' ~ '9' ? ASCII (52 ~ 61)

' ' ? ASCII (62)

'/' ? ASCII (63)

In this way, 3 8-bit bytes can be converted into 4 visible characters.

The specific byte splitting method is: (picture (the drawing is not good, understand the spirit:-))

aaaaaabb ccccdddd eeffffff //abcdef is actually 1 or 0, for the sake of viewing If it is clear, use abcdef instead

~~~~~~~~ ~~~~~~~~ ~~~~~~~~

Byte 1 Byte 2 Byte 3

　|| The first two digits are both 0.

When split like this, the number of bytes of the original text should be a multiple of 3. When this condition cannot be met, use all zero bytes

to make up for it. Use the = sign for Base64 encoding during conversion. Instead, this is why some Base64 encodings end with one or two equal signs

, but there are at most two equal signs, because: if F(origin) represents the number of bytes of the original text, F( remain) represents the remainder, then

F(remain) = F(origin) MOD 3 is established.

So the possible values ​​of F(remain) are 0,1,2.

If n = [F(origin) – F(remain)] / 3

When F(remain) = 0, it is converted to Base64 encoding of 4*n bytes.

When F(remain) = 1, since one original text byte can be split into two Base64-encoded bytes, in order to

make the Base64 encoding a multiple of 4, it should To add 2 equal signs.

When F(remain) = 2, since the two original text bytes can be split into three Base64-encoded bytes,

should be added with an equal sign.

base64 There will be 0 to 2 equal signs at the end of the encoded string. These equal signs are not necessary for decoding, so they can be deleted.
In the network GET and POST parameter lists, '+' cannot be transmitted normally, you can replace it with '|'
In this way, the base64-encoded string will only have '|' and '/', so The base64-encoded string processed in this way can be transmitted as a parameter value in the parameter list

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The following is an implementation written by a foreigner:
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] = &#39;= &#39;; 
  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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