Quantum Key Distribution Post Processing - A study on the Information Reconciliation Cascade Protocol

Hussan ul Maab, Ijaz Hussain, Zeeshan Alvi

Marco Ferrari, Alberto Tarable, Rudi Paolo Paganelli

Information Reconciliation (IR) is a crucial step in the process of distillation of unconditionally secure keys in Quantum key distribution (QKD). Novel IR rateless codes based on protograph low-density parity-check (LDPC) codes have been recently designed to replace Cascade. In this paper, a blind IR protocol is described that exploits all the features of rateless protograph LDPC codes.

Sunghoon Lee, Joo Youn Park, Jun Heo

Quantum key distribution (QKD) is a cryptographic system that generates an information-theoretically secure key shared by two legitimate parties. QKD consists of two parts: quantum and classical. The latter is referred to as classical post-processing (CPP). Information reconciliation is a part of CPP in which parties are given correlated variables and attempt to eliminate the discrepancies between them while disclosing a minimum amount of information. The elegant reconciliation protocol known as \emph{Cascade} was developed specifically for QKD in 1992 and has become the de-facto standard for all QKD implementations. However, the protocol is highly interactive. Thus, other protocols based on linear block codes such as Hamming codes, low-density parity-check (LDPC) codes, and polar codes have been researched. In particular, reconciliation using LDPC codes has been mainly studied because of its outstanding performance. Nevertheless, with small block size, the bit error rate performance of polar codes under successive-cancellation list (SCL) decoding with a cyclic redundancy check (CRC) is comparable to state-of-the-art turbo and LDPC codes. In this study, we demonstrate the use of polar codes to improve the performance of information reconciliation in a QKD system with small block size. The best decoder for polar codes, a CRC-aided SCL decoder, requires CRC-precoded messages. However, messages that are sifted keys in QKD are obtained arbitrarily as a result of a characteristic of the QKD protocol and cannot be CRC-precoded. We propose a method that allows arbitrarily obtained sifted keys to be CRC precoded by introducing a virtual string. Thus the best decoder can be used for reconciliation using polar codes and improves the efficiency of the protocol.

Gronberg, Jonsson

Thomas B. Pedersen, Mustafa Toyran

It is widely accepted in the quantum cryptography community that interactive information reconciliation protocols, such as cascade, are inefficient due to the communication overhead. Instead, non-interactive information reconciliation protocols based on i.e. LDPC codes or, more recently, polar codes have been proposed. In this work, we argue that interactive protocols should be taken into consideration in modern quantum key distribution systems. In particular, we demonstrate how to improve the performance of cascade by proper implementation and use. Our implementation of cascade reaches a throughput above 80 Mbps under realistic conditions. This is more than twice the throughput previously demonstrated in any information reconciliation protocol.

Ashutosh Bhatia, Sainath Bitragunta, Kamlesh Tiwari