# Controller-Pilot Data Link Communication Security
## 5 Potential Countermeasures
### Host Identity Protocol (HIP)  
The Host Identity Protocol (HIP) [24] is a new internetworking architecture developed at the Internet Engineering Task Force (IETF); it had its first stable version ready in 2007. HIP has been developed to be fully backwards compatible with today's infrastructure of the Internet Protocol version 4 (IPv4) and the growing version 6 (IPv6) infrastructure to make it possible to implement in an already existing environment without changes needed to be done to either applications or the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Encryption Algorithm** & **Public Key** & **Private Key** & **Encrypted Message** \\ \hline RSA & 1088 & 2048 & 1024 \\ ECC & 161 & 160 & 321 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Encrypted message sizes (in bits) for the RSA and elliptical curve cryptography (ECC) algorithms. routing infrastructure of the network, and upgraded HIP hosts are still able to communicate with non-HIP hosts. By implementing a new name space in the TCP/IP (Transmission Control Protocol/Internet Protocol) stack, a public key known as the Host Identifier, it disconnects the two roles of identifier and locator from each other in the IP address. The Host Identifier assumes the role of identifying the host and the IP address continues to work as a locator. This enables among other things a connection between two hosts to be kept open even if the two hosts are mobile and continuously change locations thereby enhancing connectivity and mobility [25]. Data packets will use the host identity as source address by having it converted into a Host Identity Tag (HIT), which looks like an IPv6 address using a predefined prefix, called Orchid. This HIT follows the packets through to the Internet Protocol Security (IPsec) Encapsulated PostScript (ESP) Security Association (SA), where it is yet again converted to an IP address and added to the header. This makes it possible for any non-HIP node in between the two communicating nodes to handle the packets like a normal IPsec ESP transport mode packet. When the packet is received at the other end it goes back through the IPsec module that, if it passes verification and integrity checks, discards the IP address and reapplies the HIT as the source address to the packet to assure upper layers of its validity. This ensures that the packet was created using the correct cryptographic protocol where the private key corresponding to the HIT was used and protects against IP spoofing. HIP can approach unwanted traffic in two separate ways, either directly by hiding or in a way it makes the recipients directly inaccessible by forcing the sender to gain the consent of the recipient before a connection is established. The second way is to increase the cost of sending data, or inversely minimize it. This is accomplished by introducing a four-way handshake between the sender and recipient before any other data can be sent. Hiding the hosts is achieved by creating an overlaying system that takes care of the handshake between the sender and receiver, and only lets the sender get in direct connection with the recipient after this handshake has been accepted by the recipient and allowed a direct path through a HIP firewall. The Host Identity Protocol has a potential to, if implemented for CPDLC protection, not only solve the first four requirements for the completely secure communication system, but also partially cover the availability through its way to handle unwanted traffic. It will function as a countermeasure to the two kinds of potential attacks: flooding and injection. A HIP-enabled host will discard any unauthorized data, but will not be protected against a jamming attack filling the medium with noise. Though the source of origin of a jamming attack is easily pinpointed using direction finding, it still poses a risk. The HIP appears to be a very effective in covering all aspects of secure communication.