## 2 Previous Work
### Appropriate Authentication Techniques for WSNs  
In this subsection, we use some findings from our previous paper [23] specifying the best cipher suitable for the severely constrained sensor nodes in each of the two main categories of the different authentication techniques. In the first main category, namely the private key cryptography, we deduced that RC5 is the most feasible to be used for WSN scenarios since it strikes the best balance by consuming less energy than most other algorithms while providing better security than algorithms with less energy consumption. The main drawback of the use of symmetric keys is that both the sender and receiver share the same secret key to perform encryption and decryption. This drawback gets worse in the case where all the nodes in the network share the same key to send their readings periodically to the base station (BS). This approach provides some authentication that the sender is a member of the network, assuming the case of a weak attacker that cannot completely take over a node. Also using this scheme, there cannot be \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline
**CENTERA** & Flat & No & No & Very Limited & No & No & No & Low & Good & Possible & No & C \\
**SPIN** & Flat & Possible & No & Limited & Yes & Yes & No & Low & Limited & Yes & Yes & D \\
**Directed Diffusion** & Flat & Limited & No & Limited & Yes & Yes & Yes & Low & Limited & Yes & Yes & D \\  & & Very & & & & & & & & & & \\
**Rumor Routing** & Flat & Limited & No & N/A & No & Yes & No & Low & Good & No & Yes & D \\
**GBR** & Flat & Limited & No & N/A & No & Yes & No & Low & Limited & No & Yes & D \\
**MCFA** & Flat & No & No & N/A & No & No & No & Low & Good & No & No & D \\
**CADR** & Flat & No & No & Limited & No & Yes & No & Low & Limited & No & No & D \\
**COUGAR** & Flat & Limited & No & N/A & No & Yes & No & Low & Limited & No & Yes & D \\
**ACQUIRE** & Flat & Limited & No & N/A & No & No & No & Low & Limited & No & Yes & D \\
**EAR** & Flat & Limited & No & N/A & No & No & No & Low & Limited & No & Yes & D \\
**LEACH** & Hierarchical & Fixed BS & No & Maximum & No & Yes & Yes & High & Good & No & No & D \\
**TEEN \& APTEEN** & Hierarchical & Fixed BS & No & Maximum & No & Yes & Yes & High & Good & No & No & D \\
**PEAGSIS** & Hierarchical & Fixed BS & No & Maximum & No & No & Yes & Low & Good & No & No & D \\
**MECN \& SMECN** & Hierarchical & No & No & Maximum & No & No & No & Low & Low & No & No & D \\
**SOP** & Hierarchical & No & No & N/A & No & No & No & Low & Low & No & No & D \\
**HPAR** & Hierarchical & No & No & N/A & No & No & No & Low & Good & No & No & D \\
**VGA** & Hierarchical & No & No & N/A & Yes & Yes & Yes & High & Good & Yes & No & D \\
**Sensor aggregate** & Hierarchical & Limited & No & N/A & No & Yes & No & Low & Good & No & Possible & D \\
**TTDD** & Hierarchical & Yes & Yes & Limited & No & No & No & Moderate & Low & Possible & Possible & D \\
**GAF** & Location & Limited & No & Limited & No & No & No & Low & Good & No & No & D \\
**GEAR** & Location & Limited & No & Limited & No & No & No & Low & Limited & No & No & D \\
**SPAN** & Location & Limited & No & N/A & Yes & No & No & Low & Limited & No & No & D \\
**MFR** & Location & No & No & N/A & No & No & No & Low & Limited & No & No & D \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison among Most Popular WSN Routing Techniques. accountability of the exact node that sent false or malicious data into the network. This highly affects trust and reputation schemes and routing protocols, since one malfunctioning node can jeopardize the whole network with no way to point out, punish, or ban such a node. Another drawback of symmetric key cryptography is that such schemes do not scale well with large numbers of sensor nodes [24]. Thus, in some scenarios, symmetric key cryptography may just not be enough to guarantee the normal and correct functioning of the wireless sensor network; and so the need for other types of encryption is required.