# Multi-Line Prefetch Covert Channel with Huge Pages
## 5 Results  
### Throughput and Accuracy  
To evaluate the efficiency of our multi-line encoding covert channel, we measured both the throughput and accuracy across different page sizes and encoding strategies. As shown in Table 2, the read-only multi-line encoding achieved a throughput of approximately 4623 KB/s with 4 KB pages and up to 4940 KB/s with 2 MB huge pages, with an accuracy of up to 81.23%. The write-access encoding, which leveraged PREPETCHW to induce transitions from the I to M state rather than S to M, demonstrated a slightly higher precision at 83.34%, although with a slightly lower throughput--4345 KB/s on 4 KB pages and 4828 KB/s with 2 MB huge pages. Compared to the original single-line encoding approach from prior work [1], which achieves a throughput of only 822 KB/s and transmits just a single bit per iteration, our multi-line encoding achieves up to a 4940 KB/s throughput and transmits 9 bits per iteration--representing a 6\(\times\) increase in the bandwidth capacity. The original design cannot convey meaningful data efficiently due to its limited capacity. In contrast, our approach can encode and transmit 9 bits per iteration by accessing multiple cache lines, allowing for the efficient transmission of complex messages. Moreover, if greater decoding accuracy is desired, a bucket-based method can be employed: for example, transmitting message \"1\" by accessing 10 lines in one iteration, message \"2\" by accessing 20 lines, and so on. This technique trades throughput for enhanced resilience to noise and improved decoding reliability, offering flexibility between performance and accuracy. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Encoding Strategy** & **Page Size** & **Throughput (KB/s)** & **Accuracy (\%)** \\ \hline Single-Line Encoding [1] & 4 KB & 822 & – \\ \hline Multi-Line Read-Only & 4 KB & 4623 & 80.55 \\ Multi-Line Read-Only & 2 MB & 4940 & 81.23 \\ \hline Multi-Line Write-Access & 4 KB & 4345 & 82.16 \\ Multi-Line Write-Access & 2 MB & 4828 & 83.34 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Throughput and accuracy comparison of encoding strategies. While the reference paper [1] achieved a reported accuracy of 96.2% using the single-line encoding scheme, our local reproduction under varying experimental conditions revealed a broader accuracy range of 60-80%. This divergence in the results suggests potential sensitivity to environmental factors not fully replicated in our setup. To uphold transparency and avoid overstating the outcomes, we have opted to omit the accuracy metrics for [1]'s results from the table, as they may not reliably reflect the scheme's performance in generalized scenarios. We further evaluated the influence of the bucket size on the decoding accuracy under the read-only 2 MB huge page setting. Our experiments show that as the bucket size increases, the accuracy improves up to a point and then plateaus. Specifically, with a bucket size of 1 or 5, the accuracy remains at 66.67%; increasing the bucket size to 10 improves the accuracy to 71.43%; and at a bucket size of 25, we achieve the peak accuracy of 81.23%. Further increases in the bucket size beyond 25 yield no significant improvements, with the accuracy remaining stable at 81.23%. A bucket size of \(b\) introduces redundancy into the encoding. The timing of a coherence event is above or below a statistical threshold at the receiver to indicate an encoded 1 or 0. Many system-level aspects determine this statistical model--how many other processes are active and what else may engender an aliased coherence event, timer accuracy, or jitter, among many others. Once a certain level of redundancy resulting in a robust statistical threshold is reached, an additional bucket size \(b\) does not overcome any additional noise, resulting in a saturated accuracy. This suggests that a moderate bucket size provides a good balance between throughput and accuracy. Figure 2 illustrates the relationship between bucket size and accuracy.