# Low-Cost Full Correlated-Power-Noise Generator to Counteract Side-Channel Attacks
## 5 Security Evaluation and Experimental Results
### CPA Attack  
The CPA attack has been applied in the final round. The quantification of security for each attack is determined by assessing the acquisition of SubBytes, in addition to the determination of Measurements to Disclose (MTD) the secret key, which, in turn, defines the minimum number of required input patterns for the retrieval of the secret key. The attacks have been conducted using three different keys to calculate average values, while the input data were randomly generated. The first keys for both implementations, _key\({}_{1}\)_, are a public NIST key test while the rest are pseudo-random. Table 3 provides a summary of the results of the attack, employing a dataset comprising 20,000 ciphertexts. The average of revealed SubBytes is 6 for the unprotected AES and 3.33 for the proposed countermeasure. Therefore, obtaining the SubBytes in the attack is made more challenging, resulting in a 44.5% reduction in their number employing a set of 20,000 traces. In support of these results, the Figure 10 is presented. For each key, a graphical representation illustrates the revealed SubBytes relative to the number of power consumption traces employed. We can observe that the number of revealed SubBytes for the protected AES (in blue) is lower for nearly all data set sizes used in the attack. Even for key 1, where 6 SubBytes are revealed for both implementations with the complete dataset, a greater number of traces are required to uncover the initial bytes. Thus, while 2 SubBytes are already known for the unprotected AES with 4000 traces, none have been obtained for the protected AES. Comparing the results obtained with other countermeasures proposed in the literature, we can see that we have to evaluate the trade-off between security levels achieved and degradation in performance. In relation to hiding techniques based on DPL logic styles, we find SABL or WDDL countermeasures as the most widespread. However, the cost in the design phases is high due to the need for a full-custom or semi-custom design process. On the other hand, the degradation in area in the case of SABL countermeasure amounts to a factor of \(\times 2\), while in WDDL it can reach increments of around \(\times 7\)[4]. In the case of similar power noise generators, as the one presented in [7], the authors present an area overhead of \(\times 1.44\), while maintaining the same operating frequency. However, authors claim only first-order security levels in the first operation round of the AES cipher. Our proposal although protecting all encryption rounds, it achieves the same operating frequency as the unprotected implementation, with a minimal area overhead of \(\times 1.29\). Based on these data, the following conclusions can be given. Paying attention to the revealed SubBytes for each implementation, it can be observed that the unprotected AES presents a greater threat than the proposed AES. The protected CPNG AES includes a second _SubByte_ operation that causes a significant noise correlated with the original key, the input data or state cipher for intermediate rounds, and, thus, an interference key which is changes in each round. The overall power consumption is influenced, and its correlation with the original key is also impacted, rendering the CPA attack more challenging. It is \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Implementation**} & \multicolumn{4}{c}{**Unprotected AES**} & \multicolumn{4}{c}{**Protected CPNG AES**} \\ \cline{2-7}  & **Key 1** & **Key 2** & **Key 3** & **Key 1** & **Key 2** & **Key 3** \\ \hline Revealed SubBytes & 6 & 4 & 8 & 6 & 2 & 2 \\ \hline MTD Key Mean & 16.63 & 17.68 & 17.38 & 17.75 & 18.88 & 19.38 \\ \hline Revealed SubBytes Mean & & 6 & & & 3.33 & \\ \hline MTD Mean & & 17.13 & & & 18.66 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: DPA attack results with set of 20,000 traces for unprotected and protected CPNG AES. important to note that there are different types of attacks that exploit hardware implementations beyond power attacks, such as fault-injection attacks.