## 2 Related Work  
A WSN is a collection of nodes organized into a cooperative network [13]. Applications of WSNs are growing rapidly, which range from indoor deployment scenarios in offices to outdoor deployments in adversarial territory. However, due to their deployment environments and resource limitations, these networks are vulnerable to many security threats that can deteriorate the performance [14].  
These attacks are often classified as external attacks and internal attacks [15]. In an external attack, the attacker node is not an authorized participant of the sensor network. Adversaries can severely limit the usefulness of a WSN through denial-of-service (DoS) attacks [16; 17]. An attacker may simply disrupt the network operation by broadcasting a high-energy signal. If the transmission is powerful enough, the entire system communication could be jammed. The standard defense against jamminginvolves various forms of spread-spectrum or frequency hopping communication [18]. However, cryptographically-secure spread-spectrum radios are not commercially available. In addition, this defense is not sufficiently secure against node capture attacks and cryptographic key extracting [14].  
Node compromise is the major problem in sensor networks that leads to internal attacks. In contrast to disabled nodes, compromised nodes actively seek to disrupt or paralyze the network [19]. Routing and data forwarding are essential services for enabling communication in sensor networks. Unfortunately, many routing protocols suffer from security vulnerabilities [20]. The simplest attacks involve injecting malicious routing information into the network, resulting in routing inconsistencies.  
Another area that has received a great deal of attention in WSN security is key management. A taxonomy of key management protocols in WSNs is described in [21].  
Non-constant execution time can be caused by conditional branching and various optimization techniques. If such operations involve secret parameters, these timing variations can leak some information, and a careful statistical analysis could even lead to the total recovery of these secret keys. Because timing attacks gain secret information from the response times and rather than brute force attacks or theoretical weaknesses in the algorithms, they are a real threat to WSNs as an internal attack. However, this threat is not covered by traditional notions of cryptographic security [8]. It was commonly believed that timing attacks can be directed only towards smart cards or affect the inter-process locally, but more recent research reveals that remote timing attacks are also possible and should be taken into consideration [10; 11].  
The interposition of random delays in the cryptographic algorithm execution flow is a simple, but rather effective countermeasure against side-channel and fault attacks by mitigating the information leakage. Random delays are easily deployed even if the source code of the application is not at hand. They are widely used for the protection of cryptographic implementations in embedded devices [22]. To date, based on random delay insertion, a processor architecture resistant to side-channel attacks is proposed in [23] using a combination of randomized scheduling, randomized instruction insertion and randomized pipeline-delay. The researchers in [24] present a design and hardware implementation of asynchronous AES with random noise injection for improved side-channel attack resistance.