# Challenges, Threats, Security Issues and New Trends of Underwater Wireless Sensor Networks
## 4 Security Issues of UWSNs  
### Security Requirements  
As discussed in Section 3, UWSNs are vulnerable to various attacks. Hence, UWSNs should meet some security requirements in actual applications. As a branch of WSNs, the security requirements of UWSNs are similar to terrestrial WSNs [7]. But due to the particularities and constraints of UWSNs discussed in Section 2, there are also some special security requirements.  
(1) Confidentiality  
This concerns preventing unauthorized nodes from understanding the contents of the sensitive data (e.g., security credentials and secret keys). Confidentiality is not restricted only to the survivability of a user's information (e.g., strategic or tactical military information), but also to the survivability of the MAC, routing information, etc. These sensitive data should be prevented from reading or tampering by a malicious attacker. Confidentiality can be achieved by applying a low power-efficient encryption technique which is suitable for UWSNs. The cipher text stealing technique [57] is a lightweight typical encryption technique used in UWSNs.  
(2) Authentication  
As discussed above, the acoustic channel is open and, moreover, the malicious attacker can easily capture and modify packets if there is no encryption technique. Hence, receiving node needs to identify the source of the data in order to filter malicious attacks. Only authorized nodes have the right to access and share channel, services, applications and data on that network. Intrusion detection and a trust management mechanism can be utilized to identify abnormal activities to remove malicious nodes from the network. These mechanisms ensure that only the authorized nodes have permission to perform in the network.  
(3) Integrity  
Data integrity is to ensure that the received data is not modified, removed, or corrupted in transition by unauthorized nodes either by radio failure or malicious attack. This is most essential in circumstances such as military operations and equipment controls where such changes could cause serious damage. The message authentication code [58] for data authentication has been widely applied in WSNs and UWSNs, which has good scalability, low latency, reliability, adaptability and ease of implementation.  
(4) Freshness  
Freshness is to ensure that the received data is fresh and it is not the retransmission of legacy data. Routing updates should be delivered in real time. The delay of the update messages might reflect the wrong state of the network and lead to a large loss in information.  
(5) Availability  
Availability is to ensure that the network must be robust enough. Even if some nodes fail or the system is attacked, it will still be able to provide services. Proper redundancy tactics and self-adaptive tactics can supply availability for UWSNs.  
(6) Isolation  
Isolation is to ensure that nodes should be able to identify abnormal activities and isolate malicious nodes. Moreover, MAC protocols and routing protocols should be immune to malicious attacks. Proper trust management and lightweight cryptography algorithms can be used to isolate malicious nodes.  
(7) Self-stabilization  
Self-stabilization is to ensure that nodes should be able to recover from attacks independently in real time without intervention. If a node is self-stabilizing to malicious attacks, it can automatically recover its normal state, even if the attacker remained in the network.  
(8) Survivability  
This is the capability of the system to fulfill its mission in a timely manner, in the presence of accident, failure, intrusion or malicious attacks. It is to ensures that the network can restore and maintain essential services during and after malicious attacks, even if part of the network had been destroyed.