Quantum Cryptography

Quantum encryption is a secure encryption method currently applicable on only Optical lines. This method aims to securely transmit cryptographic secret keys between parties. Optical communication offers certain advantages over existing wired communication and operates according to the principles of wireless communication. As in optical communication, there are certain factors that must be considered in wireless communication as well. These are:

• Unauthorized access to the communication and eavesdropping on the message.
• Alteration of the content of the data.
• The sending of false messages by presenting non-sent messages as if they were sent.

The resolution of these issues is demonstrated by implementing a reliable encryption system for key distribution. However, none of the existing classical encryption systems can guarantee the security of this key distribution or detect unauthorized access to the communication.

Quantum encryption differs from other known encryption methods in that it relies on physical laws rather than mathematical principles. The security of encryption is directly proportional to the security of the key. Therefore, it is essential to first understand the concept and operation of Quantum Key Distribution (QKD).

Quantum Key Distribution

The first developed quantum encryption protocol is the BB84 key distribution protocol, created in 1984 by IBM employee Charles Bennett and Gilles Brassard from the University of Montreal (Bennett and Brassard, 1984). Although other Quantum Key Distribution protocols such as B92 (Bennett, 1992) and SARG (Scrani, Acin, Ribordy, Gisin) exist theoretically, they have low practical applicability in daily life and require significantly more time for development compared to BB84. Due to the advantages of the BB84 protocol over other Quantum Key Distribution protocols, this thesis focuses on the BB84 protocol.

A photon is the term used for the particles that constitute light. To represent a bit, various properties such as frequency, phase, and polarization are available. Among these properties, polarization is used in quantum encryption.

Quantum key distribution operates in the following steps:

• First, the sender and receiver establish a coding system between themselves before commencing communication, as illustrated in Figure 1 below.

(Figure 1)

After the sender and receiver agree on the setup shown in Figure 1, the following steps occur sequentially:

• The sender selects a random bit sequence (unknown to the receiver).
• For each bit, the sender randomly chooses a polarization basis (+ or X) and encodes it using a correspondingly polarized photon (|, -, /, \), as shown in Figure 2, then sends the photon to the receiver.
• The receiver measures the polarization of each incoming photon but does not know the sender’s polarization basis; therefore, it randomly selects a polarization basis for measurement. Thus, the receiver measures the incoming photons randomly and obtains a bit sequence based on the information in Figure 1.
• The sender and receiver disclose their polarization bases to each other through a trusted channel (telephone, mail, etc.). When both parties used the same polarization basis, the sent and received bits match. These matching bits form the key for communication. As shown in Figure 2 below, the resulting 6-bit secret key is 010101.

(Figure 2)

Once the protocol is completed, the message is encrypted with the message key and decrypted and read by the receiver.