First International Conference on Modern Communication & Computing Technologies (MCCT'14) (Short Paper / WiP) Quantum Cryptography and Its Implications Ghulam Abbas Lashari, Muhammad Irshad Nazeer Department of Computer Science, Sukkur IBA, airport road sukkur, Sindh-Pakistan {ghulam.abbas, irshad.nazeer} @iba-suk.edu.pk Abstract. Due to technological advancements in computing technology current cryptographic methods need improvements for reliable data transfer. There are so many cryptographic techniques which have been implemented to serve the error free dissemination of data: symmetric and asymmetric. These conventional techniques have many issues such as secure key exchange, authentication etc, these issues need to be resolved. Therefore, scientists have introduced another method for secure data transmission that is Quantum Cryptography. This paper describes the need of Quantum Cryptography in comparison with currently working classical cryptography. Meanwhile, it also explains the working principle of quantum cryptography, quantum key distribution protocols (BB84), and analysis of key distribution protocols. Keywords: Quantum Cryptography, Quantum Key Distribution (QKD), Qubits. 1 Introduction Cryptography is the way to transmit messages between two parties using an insecure channel so that no one can intercept it. During communication process the sender sends a message to receiver by applying suitable encryption techniques and receiver decrypts the message by applying decryption. The attacker can only intercept the message if he knows the key. So to maintain confidentiality and to avoid any kind of modification with data one should make secure key exchange Error! Reference source not found.. RSA is known as a public key cryptography algorithm which is considered as the most secure algorithm. The security of RSA depends on the difficulty of factoring large prime numbers. Richard Feynman suggested that binary information can be represented by photons and the properties of these particles such as polarizations can be exploited to encode and transmit information Error! Reference source not found.. Kristo explains that quantum computers are very fast as compared to classical computers Error! Reference source not found.. Quantum computers use Shor’s algorithm which can factor two large prime numbers to decrypt information encrypted by RSA [9]. So, there is a drastic need of a secure and efficient key distribution tech- 26-28 February, 2014, NawabShah, Pakistan First International Conference on Modern Communication & Computing Technologies (MCCT'14) (Short Paper / WiP) nique to meet the growing need of privacy. This paper describes Quantum Cryptography in comparison with currently working classical cryptography, quantum key distribution protocols, and analysis of key distribution protocols. 2 Quantum Cryptography In the quantum communication unit of quantum information is the photon or quantum bit (qubit).It has two values 0 or 1. Electromagnetic waves exhibit the phenomenon of polarization. A polarization filter is used which allows only light of a specified polarization direction to pass. A photon either will or will not pass through a polarization filter, but if it emerges it will be aligned with the filter regardless of its initial state. A photon detector can be used to determine the photon’s polarization to know the possibility whether it will pass through the filter [5].The foundation of quantum cryptography lies in the Heisenberg uncertainty principle which states that the position and momentum of a photon can’t be determined simultaneously. In particular, when we measure the polarization of a photon particle the choice of what direction to observe the effect all measurements [4]. According to no-cloning-theorem two identical copies of two unknown quantum states are very difficult. So, there is no chance for an attacker to measure the quantum state [3] [4]. Quantum channel and the public channel in quantum cryptography are used. The Quantum channel is used to transmit quantum bits whereas public channel is used to carry messages from sender to the receiver and to check error in quantum channel. BB84 protocol proposed by Bennett and Brassard in 1984 is the most widely accepted QKD protocol. The other protocols BBM92, B92 and E91 are newer and faster than BB84, but these all require highly accurate detectors. Still BB84 is very popular in QKD protocols. 2.1 Quantum Key Distribution Protocol (BB84) BB84 protocol uses the properties of polarized photons for secure key distribution. Error! Reference source not found. shows an example of key distribution using BB84 protocols. Fig. 1. , An example of the key establishment by the BB84 protocol [5] [3] 26-28 February, 2014, NawabShah, Pakistan First International Conference on Modern Communication & Computing Technologies (MCCT'14) (Short Paper / WiP) In figure 1(a) Alice and Bob want to share information by establishing a secret key. When Alice and Bob establish a new key they define the type of polarization by two words: rectilinear and diagonal. In rectilinear, photons with horizontal polarization 00 means bit 0 and photons with vertical polarization 900 means bit 1. In diagonal, photons with polarization -450 means bit 0 and photons with polarization 450 means bit 1. Bob can only measure photons with 00 and 900 with the help of the detector on the rectilinear basis and with -450 and 450 in diagonal basis. If there is a mismatch of polarization the photons will be lost as shown in figure1 (a). First of all Alice sends a string of bits to Bob encoded by polarized photons randomly on a quantum channel by rectilinear or diagonal basis. Bob gets those strings of bits by perfect detection, by randomly selecting rectilinear or diagonal basis. Bob informs to Alice that which basis he used for receiving those bits. Bob only discloses the basis he used but the information of the measurement is kept secret. Alice verifies whether Bob has chosen the right basis or not. The new key will be that set of bit for which Bob has chosen the correct basis [3] [1] [2]. In figure 1(a) the first photon is detected perfectly and is the first bit of the new key. In the second and third bit Bob has chosen wrong basis so these two bits will be rejected and the key will not include these bits. The fourth bit is detected correctly and can be the part of the key. The algorithm describes that the key should include only half bits sent by Alice [3]. To get the information transferred between Alice and Bob Eve must have information about polarization of photons using any of the basis rectilinear or diagonal. Eve chooses the basis randomly and if it is not the correct basis, the polarization will be changed. In figure1 (b) the first bit has a vertical polarization, but Eve obtained that bit using a diagonal basis and now the photon’s polarization is 450. . But when Bob measures this photon has horizontal polarization and the decoded bit is 0, even though Alice and Bob selected the same rectilinear basis, but bob got the wrong result, which is not the same as Alice sent. Now if Alice and Bob compare the part of the key they will be sure that the eavesdropper is trying to obtain the key. Eve can change the quantum state which is easily detected, but according to no-cloning theorem she can’t clone the unknown state of the photon, hence the BB84 provides secure key distribution [3]. 3 Challenges in QC and Possible Solutions Quantum cryptography is not so common and less people know about its issues. Various types of attacks have been studied in the domain of quantum communication: side-channel attacks, channel loss attacks, untrusted source attacks, man-in-themiddle attack, etc [2]. The two intercept/resend and beam-splitting attacks are very serious in quantum cryptography [9]. In 2007 experts in MIT discovered “energymomentum co-entanglement” attack on BB84. An attack on a commercial system was a time-shift attack in 2008 was discovered. In 2010, QKD system based on BB84 was completely broken. Random number generators should also be designed purely so that the attacker can’t detect the basis correctly. The problems in QKD systems occur due to the inefficiency of detectors [66] [5] [1] [2]. By comparing a portion of the key we compute the quantum bit error rate (QBER) using the following formula [3]. 26-28 February, 2014, NawabShah, Pakistan First International Conference on Modern Communication & Computing Technologies (MCCT'14) (Short Paper / WiP) (1) According to the estimated error, sender and receiver delete the compared part of the key and if the QBER is low enough, they apply key distillation process. If the error rate is greater than given threshold limit, key distillation process is used [3]. Sender and receiver should divide the key into blocks and apply parity on each block. After the parity check, both parties should reject one bit from each block to give partial knowledge to the attacker about the key. With the key distillation process, the key is reduced; Therefore users are sure that they have a secure key [3] [5]. Quantum privacy amplification in [1] [6], can make the existing quantum system more secure. 4 Security Analysis of QKD Protocols There should be a probabilistic function that shows the status of the key whether eavesdropper has seen or modified it. There comes a nice question that how many bits should be extracted to know the status of the key. To answer this question two J(k) and S(k) introduced by Hartley and Shannon can be used[3] [5]. (2), where the number of uncovered bits k, length of the key n and natural logarithm are used. Fig. 2. Measure of Security [3] Fig. 3. Measure of Entropy [3] 26-28 February, 2014, NawabShah, Pakistan First International Conference on Modern Communication & Computing Technologies (MCCT'14) (Short Paper / WiP) In figure2 J (k) shows that with the comparison of first 20 percent of bit security increases, but when we look at the last 30 percent of bits, there is small improvement. (3), where number of The second function is: uncovered bits k, length of the key n and natural logarithm are used. In figure3, it shows that we must uncover approximately 37 percent of the bits of key. The basic and advanced security graphs show the percentage number of bits of distributed key to uncover. The system is secure for personal and commercial use and for critical applications such as banks, police and military usage respectively. 5 Conclusion Due to vulnerability in classical cryptography, quantum based security mechanism is the best answer to attackers. In this paper Quantum key distribution protocols have been discussed and Error analysis has been made to get the secure key. Some challenges are associated with QKD protocols, but the work is going on to resolve those issues. Quantum systems also face some issue of inefficient photo detectors, random number generators and repeaters, etc. In the future, the main focus is on integration of quantum systems with existing communication standards. References 1. Krishnan, Aravind. "An overview of quantum wireless communication using quantum cryptography." Emerging Trends in Robotics and Communication Technologies (INTERACT), 2010 International Conference on. IEEE, 2010. 2. Stipcevic, Mario. "How secure is quantum cryptography?." MIPRO, 2012 Proceedings of the 35th International Convention. IEEE, 2012. 3. Niemiec, Marcin, and Andrzej R. Pach. "The measure of security in quantum cryptography." Global Communications Conference (GLOBECOM), 2012 IEEE. IEEE, 2012. 4. Imre, Sandor. "Quantum communications: explained for communication engineers." Communications Magazine, IEEE 51.8 (2013). 5. Niemiec, Marcin, and Andrzej R. Pach. "Management of security in quantum cryptography." Communications Magazine, IEEE 51.8 (2013). 6. Pinto A. N. , Silva N. A. , Almeida Á. J. , and Muga N. J.: Using Quantum Technologies to Improve Fiber Optic Communication Systems, IEEE Communications Magazine – (august 2013). 7. Humble, Travis S. "Quantum security for the physical layer." Communications Magazine, IEEE 51.8 (2013). 8. Vittorio, Salvatore. "Quantum cryptography: Privacy through uncertainty." released October (2002). 9. Kristo, Joshua. "Superposition And Encryption: The Implications Of Quantum Computing On Information Security." (2012). 26-28 February, 2014, NawabShah, Pakistan