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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.apache.commons.fileupload.util.mime;
18  
19  import java.io.IOException;
20  import java.io.OutputStream;
21  
22  /**
23   * @since 1.3
24   */
25  final class Base64Decoder {
26  
27      /**
28       * Decoding table value for invalid bytes.
29       */
30      private static final int INVALID_BYTE = -1; // must be outside range 0-63
31  
32      /**
33       * Decoding table value for padding bytes, so can detect PAD afer conversion.
34       */
35      private static final int PAD_BYTE = -2; // must be outside range 0-63
36  
37      /**
38       * Mask to treat byte as unsigned integer.
39       */
40      private static final int MASK_BYTE_UNSIGNED = 0xFF;
41  
42      /**
43       * Number of bytes per encoded chunk - 4 6bit bytes produce 3 8bit bytes on output.
44       */
45      private static final int INPUT_BYTES_PER_CHUNK = 4;
46  
47      /**
48       * Set up the encoding table.
49       */
50      private static final byte[] ENCODING_TABLE = {
51          (byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D', (byte) 'E', (byte) 'F', (byte) 'G',
52          (byte) 'H', (byte) 'I', (byte) 'J', (byte) 'K', (byte) 'L', (byte) 'M', (byte) 'N',
53          (byte) 'O', (byte) 'P', (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T', (byte) 'U',
54          (byte) 'V', (byte) 'W', (byte) 'X', (byte) 'Y', (byte) 'Z',
55          (byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd', (byte) 'e', (byte) 'f', (byte) 'g',
56          (byte) 'h', (byte) 'i', (byte) 'j', (byte) 'k', (byte) 'l', (byte) 'm', (byte) 'n',
57          (byte) 'o', (byte) 'p', (byte) 'q', (byte) 'r', (byte) 's', (byte) 't', (byte) 'u',
58          (byte) 'v', (byte) 'w', (byte) 'x', (byte) 'y', (byte) 'z',
59          (byte) '0', (byte) '1', (byte) '2', (byte) '3', (byte) '4', (byte) '5', (byte) '6',
60          (byte) '7', (byte) '8', (byte) '9',
61          (byte) '+', (byte) '/'
62      };
63  
64      /**
65       * The padding byte.
66       */
67      private static final byte PADDING = (byte) '=';
68  
69      /**
70       * Set up the decoding table; this is indexed by a byte converted to an unsigned int,
71       * so must be at least as large as the number of different byte values,
72       * positive and negative and zero.
73       */
74      private static final byte[] DECODING_TABLE = new byte[Byte.MAX_VALUE - Byte.MIN_VALUE + 1];
75  
76      static {
77          // Initialise as all invalid characters
78          for (int i = 0; i < DECODING_TABLE.length; i++) {
79              DECODING_TABLE[i] = INVALID_BYTE;
80          }
81          // set up valid characters
82          for (int i = 0; i < ENCODING_TABLE.length; i++) {
83              DECODING_TABLE[ENCODING_TABLE[i]] = (byte) i;
84          }
85          // Allow pad byte to be easily detected after conversion
86          DECODING_TABLE[PADDING] = PAD_BYTE;
87      }
88  
89      /**
90       * Hidden constructor, this class must not be instantiated.
91       */
92      private Base64Decoder() {
93          // do nothing
94      }
95  
96      /**
97       * Decode the base 64 encoded byte data writing it to the given output stream,
98       * whitespace characters will be ignored.
99       *
100      * @param data the buffer containing the Base64-encoded data
101      * @param out the output stream to hold the decoded bytes
102      *
103      * @return the number of bytes produced.
104      * @throws IOException thrown when the padding is incorrect or the input is truncated.
105      */
106     public static int decode(byte[] data, OutputStream out) throws IOException {
107         int outLen = 0;
108         byte[] cache = new byte[INPUT_BYTES_PER_CHUNK];
109         int cachedBytes = 0;
110 
111         for (byte b : data) {
112             final byte d = DECODING_TABLE[MASK_BYTE_UNSIGNED & b];
113             if (d == INVALID_BYTE) {
114                 continue; // Ignore invalid bytes
115             }
116             cache[cachedBytes++] = d;
117             if (cachedBytes == INPUT_BYTES_PER_CHUNK) {
118                 // CHECKSTYLE IGNORE MagicNumber FOR NEXT 4 LINES
119                 final byte b1 = cache[0];
120                 final byte b2 = cache[1];
121                 final byte b3 = cache[2];
122                 final byte b4 = cache[3];
123                 if (b1 == PAD_BYTE || b2 == PAD_BYTE) {
124                     throw new IOException("Invalid Base64 input: incorrect padding, first two bytes cannot be padding");
125                 }
126                 // Convert 4 6-bit bytes to 3 8-bit bytes
127                 // CHECKSTYLE IGNORE MagicNumber FOR NEXT 1 LINE
128                 out.write((b1 << 2) | (b2 >> 4)); // 6 bits of b1 plus 2 bits of b2
129                 outLen++;
130                 if (b3 != PAD_BYTE) {
131                     // CHECKSTYLE IGNORE MagicNumber FOR NEXT 1 LINE
132                     out.write((b2 << 4) | (b3 >> 2)); // 4 bits of b2 plus 4 bits of b3
133                     outLen++;
134                     if (b4 != PAD_BYTE) {
135                         // CHECKSTYLE IGNORE MagicNumber FOR NEXT 1 LINE
136                         out.write((b3 << 6) | b4);        // 2 bits of b3 plus 6 bits of b4
137                         outLen++;
138                     }
139                 } else if (b4 != PAD_BYTE) { // if byte 3 is pad, byte 4 must be pad too
140                     throw new // line wrap to avoid 120 char limit
141                     IOException("Invalid Base64 input: incorrect padding, 4th byte must be padding if 3rd byte is");
142                 }
143                 cachedBytes = 0;
144             }
145         }
146         // Check for anything left over
147         if (cachedBytes != 0) {
148             throw new IOException("Invalid Base64 input: truncated");
149         }
150         return outLen;
151     }
152 }