Quantum cryptography provides perfect secrecy if the key exchanged is used for one-time-pad encryption. However, this requires very long keys which are not practical in most scenarios. Further, the key exchange bit-rate for quantum key distribution is very low. Consequently, often quantum key distribution algorithms only exchange (128, 192 or 256 bit) AES keys and then AES is used for data encryption. While Grover's algorithm will force us to only use AES- 256 and does not pose a catastrophic failure to AES, AES does not provide provable security. In this paper, we discuss methods that provide provable security but at the same time the key lengths may be controlled to practical levels. Difficulty of breaking the cipher is directly proportional to the key length and depends on the specific algorithm employed. The key idea is that key lengths can be tweaked such that an acceptable and provable level of security is achieved and therefore the cipher is not reliant on any assumptions for security. The paper will discuss different ways of achieving the above goal.
Quantum cryptography provides perfect secrecy if the key exchanged is used for one-time-pad encryption. However, this requires very long keys which are not practical in most scenarios. Further, the key exchange bit-rate for quantum key distribution is very low. Consequently, often quantum key distribution algorithms only exchange (128, 192 or 256 bit) AES keys and then AES is used for data encryption. While Grover's algorithm will force us to only use AES- 256 and does not pose a catastrophic failure to AES, AES does not provide provable security. In this paper, we discuss methods that provide provable security but at the same time the key lengths may be controlled to practical levels. Difficulty of breaking the cipher is directly proportional to the key length and depends on the specific algorithm employed. The key idea is that key lengths can be tweaked such that an acceptable and provable level of security is achieved and therefore the cipher is not reliant on any assumptions for security. The paper will discuss different ways of achieving the above goal.
Secure data exchange over the network is ensured by the requirements of confidentiality, integrity and availability of information. To meet these requirements, sent data is encrypted on the sending side, and the received information is decrypted on the receiving side. Transmitted message is encrypted according to rules determined by the algorithm and key. Crypto security of encrypted information depends on the key length. Among the encryption algorithms, there are algorithms with a symmetric (private) key and with an asymmetric (public) key. Despite all the advantages of a private key, asymmetric keys are used to deliver it from one user to another (key distribution). However, high-performance quantum computers are capable to decrypt intercepted data. Therefore, modern cryptographic systems use quantum key distribution method. The idea of using quantum bits was proposed in 1970 by S. Wiesner. In 1984, Ch. Bennett and G. Brassard proposed the BB84 protocol. The use of «entangled » quanta for systems with quantum key distribution was offerred in 1991 by A. Eckert. All modern quantum cryptography systems are based on these above-mentioned protocols.
The practical implementation of quantum information technologies requires, for the most part, highly advanced and currently experimental procedures. One exception is quantum cryptography, or quantum key distribution, which has been successfully demonstrated in many laboratories and has reached an advanced level of development. It will probably become the first commercial application of quantum information. In quantum key distribution, Alice and Bob exploit a quantum channel to create a secret shared key comprising a random string of binary digits. This key can then be used to protect a subsequent communication between them. The principal idea is that the secrecy of the key distribution is ensured by the laws of quantum physics. Proving security for practical communication systems is a challenging problem and requires techniques that are beyond the scope of this book. At a fundamental level, however, the ideas are simple and may readily be understood with the knowledge we have already acquired. Quantum cryptography is the latest idea in the long history of secure (and not so secure) communications and, if it is to develop, it will have to compete with existing technologies. For this reason we begin with a brief survey of the history and current state of the art in secure communications before turning to the possibilities offered by quantum communications. The history of cryptography is a long and fascinating one. As a consequence of the success or, more spectacularly, the failure of ciphers, wars have been fought, battles decided, kingdoms won, and heads lost. In the information age, ciphers and cryptosystems have become part of everyday life; we use them to protect our computers, to shop over the Internet, and to access our money via an ATM (automated teller machine). One of the oldest and simplest of all ciphers is the transposition or Caesarean cipher (attributed to Julius Caesar), in which the letters are shifted by a known (and secret) number of places in the alphabet. If the shift is 1, for example, then A is enciphered as B, B→C, · · ·, Y→Z, Z→A. A shift of five places leads us to make the replacements A→F, B→G, · · ·, Y→D, Z→E.
The development of quantum computers presents a threat to the security of modern communication systems; with Shor’s algorithm, all currently used asymmetric cryptography schemes can be broken. Fortunately, secure communication is still possible in the post-quantum era using symmetric encryption algorithms, such as the Advanced Encryption Standard (AES) or the provably secure one-time pad (OTP). However, the main problem of symmetric cryptography, the key distribution, has to be solved. Quantum key distribution (QKD) uses the fundamental properties of quantum mechanics to distribute encryption keys in a provably secure manner. In this thesis, the practical applicability of QKD is examined. This is achieved by testing the performance, stability, and usability of one of the few commercially available QKD platforms, ID Quantique’s Clavis 2. The tested system was stable for multiple months in normal operation and did not require any intervention from the user. The maximum distance with the test platform was 54 km. However, at these distances, the secret key rate was only tens of bits per second. For distances below 25 km, secret key rates in the kilobits per second range were achieved. Thus, if high data rates or communication distances close to 50 km are desired, ciphers less secure than OTP have to be used. Additionally, two flaws were discovered in the tested system, one making the system unstable, and the other preventing the key distribution process from starting. Fortunately, neither issue reduces the security of the system, and both could be fixed by modifying the software. Our results show that QKD can be considered as a mature technology that is ready for practical applications and even for commercial use.
The peculiar properties of quantum mechanics enable possibilities not allowed by classical physics. In particular, two parties can generate a random, secret key at a distance, even though an eavesdropper can do anything permitted by the laws of physics. Measuring the quantum properties of the signals generating the key, would ultimately change them, and thus reveal the eavesdropper’s presence. This exchange of a random, secret key is known as quantum cryptography.\n\nQuantum cryptography can be, and has been proven unconditionally secure using perfect devices. However, when quantum cryptography is implemented, one must use components available with current technology. These are usually imperfect. Although the security of quantum cryptography has been proven for components with certain imperfections, the question remains: can quantum cryptography be implemented in a provable, unconditionally secure way, using components available with current technology? This thesis contains both a theoretical, and an experimental contribution to the answer of this question. On the experimental side, components used in, and complete quantum cryptography systems have been carefully examined for security loopholes. In particular, it turned out that two commercial quantum cryptography systems contained loopholes, which would allow an eavesdropper to capture the full secret key, without exposing her presence. Furthermore, this detector control attack could be implemented with current technology. The attack is applicable against a variety of quantum cryptography implementations and protocols.\n\nThe theoretical contribution consists of security proofs for quantum cryptography in a very general setting. Precisely, the security is proven with arbitrary individual imperfections in the source and detectors. These proofs should make it possible to use a wide array of imperfect devices in implementations of quantum cryptography.\n\nFinally, a secure detection scheme is proposed, immune to the detector control attack and compatible with those security proofs. Therefore, if this scheme is implemented correctly, it offers provable security.
With the rapid development of quantum computing, classical cryptography systems are increasingly vulnerable to security threats, thereby highlighting the urgency of constructing architectures that are resilient to quantum computing attacks. While Quantum Key Distribution (QKD) offers security with information-theoretic guarantees, its relatively low key generation rate necessitates integration with classical cryptographic techniques, particularly block ciphers such as AES and SM4, to facilitate practical applications. However, when a single QKD-key is employed to encrypt multiple data blocks, the reduction in cryptographic security strength has not yet been quantitatively analyzed. In this work, we focus on the security strength in the application scenario where QKD is combined with block ciphers. We propose a quantitative evaluation method for the security benefits of the QKD-key renewal period, aiming to provide a precise measure of the cryptographic security strength in such hybrid systems. Our method is based on concrete security paradigm of block cipher modes of operation. We demonstrate that under practical security level requirements, for files consisting of specific blocks, rekeying k times can provide an additional log2(k) to 2log2(k) bits of security. Our research offers a novel perspective on balancing the security and efficiency of QKD-based encryption.