public static byte[] decode(byte[] source, int off, int len, byte[] decodabet)
      throws Base64DecoderException {
    int len34 = len * 3 / 4;
    byte[] outBuff = new byte[2 + len34]; // Upper limit on size of output
    int outBuffPosn = 0;

    byte[] b4 = new byte[4];
    int b4Posn = 0;
    int i = 0;
    byte sbiCrop = 0;
    byte sbiDecode = 0;
    for (i = 0; i < len; i++) {
      sbiCrop = (byte) (source[i + off] & 0x7f); // Only the low seven bits
      sbiDecode = decodabet[sbiCrop];

      if (sbiDecode >= WHITE_SPACE_ENC) { // White space Equals sign or better
        if (sbiDecode >= EQUALS_SIGN_ENC) {
          // An equals sign (for padding) must not occur at position 0 or 1
          // and must be the last byte[s] in the encoded value
          if (sbiCrop == EQUALS_SIGN) {
            int bytesLeft = len - i;
            byte lastByte = (byte) (source[len - 1 + off] & 0x7f);
            if (b4Posn == 0 || b4Posn == 1) {
              throw new Base64DecoderException(
                  "invalid padding byte '=' at byte offset " + i);
            } else if ((b4Posn == 3 && bytesLeft > 2)
                || (b4Posn == 4 && bytesLeft > 1)) {
              throw new Base64DecoderException(
                  "padding byte '=' falsely signals end of encoded value "
                      + "at offset " + i);
            } else if (lastByte != EQUALS_SIGN && lastByte != NEW_LINE) {
              throw new Base64DecoderException(
                  "encoded value has invalid trailing byte");
            }
            break;
          }

          b4[b4Posn++] = sbiCrop;
          if (b4Posn == 4) {
            outBuffPosn += decode4to3(b4, 0, outBuff, outBuffPosn, decodabet);
            b4Posn = 0;
          }
        }
      } else {
        throw new Base64DecoderException("Bad Base64 input character at " + i
            + ": " + source[i + off] + "(decimal)");
      }
    }

    // Because web safe encoding allows non padding base64 encodes, we
    // need to pad the rest of the b4 buffer with equal signs when
    // b4Posn != 0.  There can be at most 2 equal signs at the end of
    // four characters, so the b4 buffer must have two or three
    // characters.  This also catches the case where the input is
    // padded with EQUALS_SIGN
    if (b4Posn != 0) {
      if (b4Posn == 1) {
        throw new Base64DecoderException("single trailing character at offset "
            + (len - 1));
      }
      b4[b4Posn++] = EQUALS_SIGN;
      outBuffPosn += decode4to3(b4, 0, outBuff, outBuffPosn, decodabet);
    }

    byte[] out = new byte[outBuffPosn];
    System.arraycopy(outBuff, 0, out, 0, outBuffPosn);
    return out;
  }
Decodes Base64 content using the supplied decodabet and returns
the decoded byte array.

@param source    The Base64 encoded data
@param off       The offset of where to begin decoding
@param len       The length of characters to decode
@param decodabet the decodabet for decoding Base64 content
@return decoded data
android/android_framework/base/src/main/java/com/google/android/vending/licensing/util/Base64.java