The DARPA Quantum Network aims to strengthen QKD’s performance in these weaker areas. In some instances, this involves the introduction of newer QKD technologies; for example, we hope to achieve rapid delivery of keys by introducing a new, high-speed source of entangled photons. In other instances, we rely on an improved system architecture to achieve these goals; thus, we tackle distance- and location independence by introducing a network of trusted relays. Whereas most work to date has focused on the physical layer of quantum cryptography – e.g. the modulation, transmission, and detection of single photons – our own research effort aims to build QKD networks. As such, it is oriented to a large extent towards novel protocols and architectures for highly-secure communications across a heterogenous variety of under lying kinds of QKD links.
Figure 1. A Virtual Private Network (VPN) based on Quantum Key Distribution
Our security model is the cryptographic Virtual Private Network (VPN). Conventional VPNs use both public-key and symmetric cryptography to achieve confidentiality and authentication/integrity. Public-key mechanisms support key exchange or agreement, and authenticate the endpoints. Symmetric mechanisms (e.g. 3DES, SHA1) provide traffic confidentiality and integrity. Thus VPN systems can provide confidentiality and authentication / integrity without trusting the public network interconnecting the VPN sites. In our work, existing VPN key agreement primitives are augmented or completely replaced by keys provided by quantum cryptography. The remainder of the VPN construct is left unchanged; see Fig. 1. Thus our QKD-secured network isfully compatible with conventional Internet hosts, routers, firewalls, and so forth.
At time of writing, we are slightly over one year into a projected five-year effort to build the full DARPA Quantum Network. In our first year, we have built a complete quantum cryptographic link, and a QKD protocol engine and working suite of QKD protocols, and have integrated this cryptographic substrate into an IPsec-based Virtual Private Network. This entire system has been continuously operational since December 2002, and we are now in the process of characterizing its behavior and tuning it. In coming years, we plan to build a second link based on two photon entanglement, and to build various forms of end-to-end networks for QKD across a variety of kinds of links. We expect the majority of our links to be implemented in dark fiber but some may also be implemented in free space, either in the labor outdoors.