static String base64encode(byte[] a) {
    int aLen = a.length;
    int numFullGroups = aLen/3;
    int numBytesInPartialGroup = aLen - 3*numFullGroups;
    int resultLen = 4*((aLen + 2)/3);
    StringBuffer result = new StringBuffer(resultLen);
    char[] intToAlpha = intToBase64;

    // Translate all full groups from byte array elements to Base64
    int inCursor = 0;
    for (int i=0; i<numFullGroups; i++) {
      int byte0 = a[inCursor++] & 0xff;
      int byte1 = a[inCursor++] & 0xff;
      int byte2 = a[inCursor++] & 0xff;
      result.append(intToAlpha[byte0 >> 2]);
      result.append(intToAlpha[(byte0 << 4)&0x3f | (byte1 >> 4)]);
      result.append(intToAlpha[(byte1 << 2)&0x3f | (byte2 >> 6)]);
      result.append(intToAlpha[byte2 & 0x3f]);
    }

    // Translate partial group if present
    if (numBytesInPartialGroup != 0) {
      int byte0 = a[inCursor++] & 0xff;
      result.append(intToAlpha[byte0 >> 2]);
      if (numBytesInPartialGroup == 1) {
        result.append(intToAlpha[(byte0 << 4) & 0x3f]);
        result.append("==");
      } else {
        // assert numBytesInPartialGroup == 2;
        int byte1 = a[inCursor++] & 0xff;
        result.append(intToAlpha[(byte0 << 4)&0x3f | (byte1 >> 4)]);
        result.append(intToAlpha[(byte1 << 2)&0x3f]);
        result.append('=');
      }
    }
    // assert inCursor == a.length;
    // assert result.length() == resultLen;
    return result.toString();
  }

  /**
   * This array is a lookup table that translates 6-bit positive integer
   * index values into their "Base64 Alphabet" equivalents as specified
   * in Table 1 of RFC 2045.
   */
  static final char intToBase64[] = {
    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
    'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
    'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
    'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/'
  };