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Cryptography

HKDF-Based Encryption Method with Very Long Keys for QKD and RKD

Ernst Piller

Published Oct 20, 2026• 0 citations
Publisher

Abstract

Established symmetric encryption schemes rely on short key lengths, typically between 128 and 256 bits. However, well-known physical key exchange schemes such as QKD (Quantum Key Distribution) and RKD (Radio Signal Key Distribution) generate megabits of new key material on a daily, weekly, or other basis. This article presents a new encryption scheme, called OTPH, which was specifically designed for these megabits of regularly generated new key material. In this scheme, the key material is treated as an extremely long key, and the key’s enormous entropy is fully exploited. OTPH first uses a bit-selection procedure that randomly selects bits from the circularly arranged key, thereby extending the key many times over. From this, a HKDF (Hash Key Derivation Function) derives many new keys, and the result is a bit sequence that serves as the key for encryption using an XOR operation. OTPH uses only the XOR operation and hash function, which are also used in QKD and RKD during postprocessing (albeit a different hash function). An initial cryptanalysis has also been performed for OTPH. OTPH operates in a manner entirely contrary to established methods. It requires a completely new perspective. OTPH will not compete with AES, ChaCha20 etc., but will instead be used primarily in the context of QKD and RKD.

DOI
10.3390/cryptography10050075

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Established symmetric encryption schemes rely on short key lengths, typically between 128 and 256 bits. However, well-known physical key exchange schemes such as QKD (Quantum Key Distribution) and RKD (Radio Signal Key Distribution) generate megabits of new key material on a daily, weekly, or other basis. This article presents a new encryption scheme, called OTPH, which was specifically designed for these megabits of regularly generated new key material. In this scheme, the key material is treated as an extremely long key, and the key’s enormous entropy is fully exploited. OTPH first uses a bit-selection procedure that randomly selects bits from the circularly arranged key, thereby extending the key many times over. From this, a HKDF (Hash Key Derivation Function) derives many new keys, and the result is a bit sequence that serves as the key for encryption using an XOR operation. OTPH uses only the XOR operation and hash function, which are also used in QKD and RKD during postprocessing (albeit a different hash function). An initial cryptanalysis has also been performed for OTPH. OTPH operates in a manner entirely contrary to established methods. It requires a completely new perspective. OTPH will not compete with AES, ChaCha20 etc., but will instead be used primarily in the context of QKD and RKD.
Abstract This chapter and https://cryptography.study/phys/modes introduce four encryption methods/modes and the one-time pad, all of which are based on the XOR operation. The new methods/modes limit the required key length and are therefore particularly interesting for QKD and RKD. Because hash functions are already used in QKD and RKD, no additional mathematical methods are required for data encryption. One of these four methods/modes also generates a MAC for the security objectives of integrity and authenticity. Because these methods/modes are very simple in structure—they contain only the XOR function for data encryption (like the 100% secure one-time pad) and a hash/MAC algorithm—cryptanalysis is also easily possible.
Abstract If the security assessments are not trusted, especially in cases of very high data security requirements, physical methods must be used for key generation and distribution, data encryption, and integrity and authenticity procedures. This ensures the confidentiality, integrity, and authenticity of the data against computationally powerful adversaries and mathematical attack methods that are still unknown (unpublished) today. This chapter provides an introduction to these topics, briefly compares mathematical methods, and discusses the generation and distribution of cryptographic keys, as well as QKD (Quantum Key Distribution), RKD (Radio signal Key Distribution), and MKD (Memeory Key Distribution).
In this section, I wish to put quantum key distribution (QKD) in the wider context of a cryptosystem. I shall discuss informally some aspects that are considered important. The questions I wish to answer here are: “What are the ingredients needed to make a QKD-based cryptosystem work? What services does it bring? What are its limitations?” As I shall detail below, QKD may be used to provide the users with confidential communications. This can be achieved when we combine QKD and the one-time pad. For the quantum modulation, QKD needs a source of truly random numbers. Also, QKD requires a classical authenticated channel to work, so authentication plays an essential role. As a consequence, QKD must start with a secret key, making it a secret-key encryption scheme. I will also discuss what happens if classical cryptography is introduced in the system. Finally, I will describe the implementation of a simple cryptosystem on top of QKD. A key distribution scheme The first function of QKD is to distribute a secret key between two parties. The use of this key is outside the scope of this first section – the need for a secret key is omnipresent in cryptography. As depicted in Fig. 5.1, QKD relies on a classical authenticated channel for sifting and secret-key distillation and on random bits for the modulation of quantum states. The key produced by QKD can be intended for encryption purposes – this will be discussed in Section 5.2 – but is also required by authentication. A part of the distributed key is used for authentication. When QKD is run for the first time, however, an initial secret key must be used instead.
Abstract This book is aimed at anyone interested in high data security in telecommunications and data storage, especially procurers, experts, and decision-makers. Anyone involved in procurement in this field makes decisions about algorithms, technologies, and providers, and thus also about infrastructure and security. The generation and distribution of keys for data encryption play a central role in this. Because security assessments for mathematical methods are based on assumptions, physical methods are becoming interesting in the high-security sector. They promise to link security more closely to the laws of nature. This raises the key question: Which technology is suitable for which application scenario, and what assumptions, costs, and operational risks are involved? This book provides answers and, for the first time, compares QKD (quantum key distribution), RKD (radio signal key distribution), and MKD (memory key distribution) in a common, comprehensible criteria grid: technology-neutral and practical. Secret key rates, ranges/attenuation, robustness, costs/infrastructure, standardization, and risks (implementation, integration, post-processing, side channels) are deliberately evaluated not as a ranking, but as a decision-making aid. QKD derives its security from the laws of quantum physics, but secret key rates decrease with increasing attenuation. Key management systems connect short QKD distances over longer distances, but only at the cost of additional attack surfaces (“trusted nodes”). Its use for mobile applications fails due to a lack of technical maturity. Very high financial costs and high maintenance requirements for QKD solutions make them not very suitable. RKD utilizes the reciprocal physical properties of a radio link and scores points for its low technical complexity, excellent suitability for mobile applications (e.g., vehicles or drones), and very low costs. However, RKD falls far short of the key rates achieved by QKD solutions and is still limited to shorter distances. In addition, there is no established infrastructure for distributing key material to more than two partners. MKD takes a completely different approach: each party produces key material, stores it on a data carrier, and transports it physically to the other party. Because MKD can securely transfer 16 TB of key material in a single transport, only MKD has the potential to continuously provide a one-time pad (OTP) and thus provably 100% secure data encryption. The price is organizational responsibility: secure generation, storage, transport, and documented chain of custody. The book examines data security in telecommunications and data storage, discusses three new encryption methods specifically designed for QKD and RKD, and addresses the question of when “OTP-like” security is more practical than theoretical purity. The result of approximately one year of source-critical research and the comparison of literature, manufacturer specifications, and practical observations with systematic cross-checking and our own R&D activities, this book helps to justify architecture and procurement decisions, locate risks (side channels, misconfigurations, logistical vulnerabilities), and separate “security gains” from “new attack surfaces.”
This research is inspired by the BB84 and Base64 algorithms and presents a fusion of both to introduce a new algorithmic technique for secure data transmission. It combines classical cryptographic principles with concepts from quantum key distribution. The proposed approach integrates Base64 encoding and BB84-inspired key exchange into a web-based cryptographic system. Messages or files from the sender (Alice) are encrypted using a randomly generated symmetric key, derived through HKDF and secured with AES-GCM encryption. The key is shared with the receiver (Bob) via a QR code or text file, functioning as a one-time password. The system also simulates quantum key exchange and basis alignment, serving as an educational tool for quantum-inspired secure communication.
Abstract MKD (Memory Key Distribution) takes a completely different approach to QKD and RKD. With MKD, non-deterministic random number generators generate the cryptographic keys, and the keys are transmitted using highly secure hardware tokens (usually smart cards) or special portable storage media (usually SSDs) with highly secure access protection for the data. Only highly secure storage media can be used for a one-time pad and thus for cryptography that is entirely free of mathematics (except for XOR).