## 3 Benchmark Results
### Symmetric Cryptography
This means that AES is not only the best algorithm for ciphering large documents, but also for small ones. For example, in Hx2790, to cipher 1 KB of information takes around 270 ns using DES, 745 ns using 3 DES, 152 ns using AES-128, and 204 ns using AES-256. Regarding 3 DES, we evaluated the 3 DES-EDE variant, which involves two DES encryptions and a DES decryption using three DES keys (168 bits in total). The results give evidence that 3 DES is three times slower than DES. In the same way that temporal costs, Table 4 summarizes the energy costs of the selected block cipher algorithms. We have seen in Section 3.1 that the required power to execute cryptographic functions in Hx2790 is almost twice as large as in iPAQ3970. However, since Hx2790 runs block cipher algorithms four times faster that the other, the total consumed energy is the half. Thus, we can conclude that Hx2790 is more efficient. One issue of AES is that decryption does not perform exactly the same steps than encryption, and so, the code is partially different. In [7], [PERSON] _et al._ model the computational operations involved in the AES encryption scheme and make a theoretical evaluation of its expected consumed energy. They estimate the number of computational operations for deciphering an AES message is three times greater than the operations for ciphering. Since the energy consumption is also related to the number of processor cycles which are used in the computation of basic operations, they calculate the decryption process spends nearly three times more energy than encryption. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Energy (mJ)**} \\ \cline{2-4}  & **iPAQ3970** & **Hx2790** & **Ratio** \\ \hline DES (64) & 0.35 + 0.64x & 0.01 + 0.32x & 2.01 \\ \hline
3 DES (192) & 0.56 + 1.66x & 0.03 + 0.85x & 1.95 \\ \hline AES (128) & 0.07 + 0.46x & 0.19x & 2.38 \\ \hline AES (256) & 0.07 + 0.64.x & 0.27x & 2.35 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Energy costs of symmetric algorithms in a PDA (mJ). Actually, depending on the implementation, AES encryption and decryption differences can be more significant in one sense or another. In OpenSSL, AES is accelerated via a 10 KB lookup table implementation. The OpenSSL compilation for Linux Familiar takes full profit of this design and the effect is that a decryption process in the iPAQ3970 is even faster than encryption (20% faster). In the same way, the consumed energy is in general lower, except when using small input files (up to 100 KB). Yet, in the Windows mobile platform the results are not quite the same. The throughput of the encryption and decryption operations in the Hx2790 is more or less the same for both cases. As for the energy, the encryption better results are very significant, but the differences between ciphering and deciphering keep coming down as the size of the input text grows. This means that deciphering has some complex operations that do not depend on the size of the input text and that become dissolved with the rest of computations as the size of the ciphertext grows. The differences in AES encryption and decryption processes lead to slightly different results in throughput and energy consumption. Nevertheless, data presented in Tables 3 and 4, and the following Figure 2, is the average of both operations. Figure 2 makes a comparison of the quantity of data that can be ciphered in a second of time and with a Joule of energy. Temporal results can be compared with that obtained by other implementations and tests:  
\(\bullet\) In [3], authors launched some benchmarking tests in two PDAs of similar characteristics than ours: an iPAQ4150 with a PXA255 XScale CPU at 400 MHz, and a Dell Axim x30 with a PXA270 XScale CPU at 640 MHz. The throughput for AES-256 working in ECB cipher mode is 98 KB/s in iPAQ4150, and 149.5 KB/s in Axim x30. \(\bullet\) Tests in [2] were executed in an iPAQ4700 with a PXA270 XScale CPU at 624 MHz. They evaluated the throughput for AES-256 working in ECB cipher mode, and the result is 1382.7 KB/s.