Base32String.java

/*
 * Copyright 2009 Google Inc. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.android.apps.authenticator;

import java.util.HashMap;
import java.util.Locale;

/**
 * Encodes arbitrary byte arrays as case-insensitive base-32 strings.
 * <p>
 * The implementation is slightly different than in RFC 4648. During encoding,
 * padding is not added, and during decoding the last incomplete chunk is not
 * taken into account. The result is that multiple strings decode to the same
 * byte array, for example, string of sixteen 7s ("7...7") and seventeen 7s both
 * decode to the same byte array.
 * TODO(sarvar): Revisit this encoding and whether this ambiguity needs fixing.
 *
 * @author sweis@google.com (Steve Weis)
 * @author Neal Gafter
 */
public class Base32String {
  // singleton

  private static final Base32String INSTANCE =
    new Base32String("ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"); // RFC 4648/3548

  static Base32String getInstance() {
    return INSTANCE;
  }

  // 32 alpha-numeric characters.
  private String ALPHABET;
  private char[] DIGITS;
  private int MASK;
  private int SHIFT;
  private HashMap<Character, Integer> CHAR_MAP;

  static final String SEPARATOR = "-";

  protected Base32String(String alphabet) {
    this.ALPHABET = alphabet;
    DIGITS = ALPHABET.toCharArray();
    MASK = DIGITS.length - 1;
    SHIFT = Integer.numberOfTrailingZeros(DIGITS.length);
    CHAR_MAP = new HashMap<Character, Integer>();
    for (int i = 0; i < DIGITS.length; i++) {
      CHAR_MAP.put(DIGITS[i], i);
    }
  }

  public static byte[] decode(String encoded) throws DecodingException {
    return getInstance().decodeInternal(encoded);
  }

  protected byte[] decodeInternal(String encoded) throws DecodingException {
    // Remove whitespace and separators
    encoded = encoded.trim().replaceAll(SEPARATOR, "").replaceAll(" ", "");

    // Remove padding. Note: the padding is used as hint to determine how many
    // bits to decode from the last incomplete chunk (which is commented out
    // below, so this may have been wrong to start with).
    encoded = encoded.replaceFirst("[=]*$", "");

    // Canonicalize to all upper case
    encoded = encoded.toUpperCase(Locale.US);
    if (encoded.length() == 0) {
      return new byte[0];
    }
    int encodedLength = encoded.length();
    int outLength = encodedLength * SHIFT / 8;
    byte[] result = new byte[outLength];
    int buffer = 0;
    int next = 0;
    int bitsLeft = 0;
    for (char c : encoded.toCharArray()) {
      if (!CHAR_MAP.containsKey(c)) {
        throw new DecodingException("Illegal character: " + c);
      }
      buffer <<= SHIFT;
      buffer |= CHAR_MAP.get(c) & MASK;
      bitsLeft += SHIFT;
      if (bitsLeft >= 8) {
        result[next++] = (byte) (buffer >> (bitsLeft - 8));
        bitsLeft -= 8;
      }
    }
    // We'll ignore leftover bits for now.
    //
    // if (next != outLength || bitsLeft >= SHIFT) {
    //  throw new DecodingException("Bits left: " + bitsLeft);
    // }
    return result;
  }

  public static String encode(byte[] data) {
    return getInstance().encodeInternal(data);
  }

  protected String encodeInternal(byte[] data) {
    if (data.length == 0) {
      return "";
    }

    // SHIFT is the number of bits per output character, so the length of the
    // output is the length of the input multiplied by 8/SHIFT, rounded up.
    if (data.length >= (1 << 28)) {
      // The computation below will fail, so don't do it.
      throw new IllegalArgumentException();
    }

    int outputLength = (data.length * 8 + SHIFT - 1) / SHIFT;
    StringBuilder result = new StringBuilder(outputLength);

    int buffer = data[0];
    int next = 1;
    int bitsLeft = 8;
    while (bitsLeft > 0 || next < data.length) {
      if (bitsLeft < SHIFT) {
        if (next < data.length) {
          buffer <<= 8;
          buffer |= (data[next++] & 0xff);
          bitsLeft += 8;
        } else {
          int pad = SHIFT - bitsLeft;
          buffer <<= pad;
          bitsLeft += pad;
        }
      }
      int index = MASK & (buffer >> (bitsLeft - SHIFT));
      bitsLeft -= SHIFT;
      result.append(DIGITS[index]);
    }
    return result.toString();
  }

  @Override
  // enforce that this class is a singleton
  public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
  }

  public static class DecodingException extends Exception {
    public DecodingException(String message) {
      super(message);
    }
  }
}