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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 }