private function readRecordData($data, $pos, $len)
    {
        $data = substr($data, $pos, $len);

        // File not encrypted, or record before encryption start point
        if ($this->encryption == self::MS_BIFF_CRYPTO_NONE || $pos < $this->encryptionStartPos) {
            return $data;
        }

        $recordData = '';
        if ($this->encryption == self::MS_BIFF_CRYPTO_RC4) {
            $oldBlock = floor($this->rc4Pos / self::REKEY_BLOCK);
            $block = floor($pos / self::REKEY_BLOCK);
            $endBlock = floor(($pos + $len) / self::REKEY_BLOCK);

            // Spin an RC4 decryptor to the right spot. If we have a decryptor sitting
            // at a point earlier in the current block, re-use it as we can save some time.
            if ($block != $oldBlock || $pos < $this->rc4Pos || !$this->rc4Key) {
                $this->rc4Key = $this->makeKey($block, $this->md5Ctxt);
                $step = $pos % self::REKEY_BLOCK;
            } else {
                $step = $pos - $this->rc4Pos;
            }
            $this->rc4Key->RC4(str_repeat("\0", $step));

            // Decrypt record data (re-keying at the end of every block)
            while ($block != $endBlock) {
                $step = self::REKEY_BLOCK - ($pos % self::REKEY_BLOCK);
                $recordData .= $this->rc4Key->RC4(substr($data, 0, $step));
                $data = substr($data, $step);
                $pos += $step;
                $len -= $step;
                ++$block;
                $this->rc4Key = $this->makeKey($block, $this->md5Ctxt);
            }
            $recordData .= $this->rc4Key->RC4(substr($data, 0, $len));

            // Keep track of the position of this decryptor.
            // We'll try and re-use it later if we can to speed things up
            $this->rc4Pos = $pos + $len;
        } elseif ($this->encryption == self::MS_BIFF_CRYPTO_XOR) {
            throw new Exception('XOr encryption not supported');
        }

        return $recordData;
    }