The secure exchange of cryptographic keys remains a fundamental challenge in modern cryptography, and researchers continually seek methods to enhance control and security, even after initial distribution. Fuyuki Kitagawa from NTT Social Informatics Laboratories, Jiahui Liu from Fujitsu Research, and Shota Yamada from AIST, along with Takashi Yamakawa, now present a unified framework for ‘key leasing’, a system allowing verifiable key revocation. This work establishes a method where a classical, non-quantum server can lease and revoke cryptographic keys using only standard communication channels, representing a significant advancement over previous approaches. The team’s schemes, applicable to public-key encryption, pseudorandom functions, and digital signatures, not only improve upon existing encryption methods but also introduce the first pseudorandom functions and digital signatures with this classical-lessor key leasing property, bolstering security and control in a wide range of applications.
Researchers have developed a new framework for cryptographic key leasing, enabling temporary access to a key with the crucial ability to revoke it later in a verifiable manner. This work achieves a significant advancement by constructing schemes that rely entirely on classical communication, avoiding the need for complex quantum technologies. The team successfully built these key leasing schemes for three fundamental cryptographic applications: public-key encryption, pseudorandom functions, and digital signatures. This approach addresses a key limitation of traditional cryptography, allowing functionality to be revoked without requiring updates to public parameters.
Dual-Mode SFE from Oblivious Transfer Construction
Scientists have constructed a dual-mode Secure Function Evaluation (SFE) scheme using a special dual-mode Oblivious Transfer (OT) scheme as a fundamental building block. This approach reduces a complex cryptographic problem, SFE, to a simpler, well-studied problem, OT. Secure Function Evaluation allows two parties to compute a function on their private inputs without revealing those inputs to each other. Oblivious Transfer, a core cryptographic primitive, allows a sender to provide one of two inputs to a receiver without knowing which input the receiver selected. The team’s construction utilizes specific security properties and functionalities, with the “dual-mode” aspect relating to different modes of operation, such as honest versus malicious security.
Key components include the OT scheme, which generates a common reference string, and the SFE scheme, built upon the OT scheme. A garbled circuit, a technique for securely computing a function, transforms the OT-based construction into a complete SFE scheme. The security of the SFE scheme relies on the security of both the OT scheme and the garbled circuit scheme, demonstrating how to build a complex cryptographic primitive from a simpler one, leading to more efficient and secure cryptographic systems.
Classical Key Leasing with Verifiable Revocation
Scientists have achieved a breakthrough in cryptographic key leasing, developing a framework where a cryptographic key can be leased as a state and later revoked in a verifiable manner. This work addresses the challenge of revoking functionality without updating public parameters, a significant limitation of traditional cryptographic methods. The team constructed schemes for public-key encryption, pseudorandom functions, and digital signatures, all with the capability of secure key leasing using only classical communication channels. This advancement simplifies implementation by avoiding reliance on complex quantum technologies.
Notably, the developed public-key encryption scheme improves upon existing constructions, while the pseudorandom function and digital signature schemes represent the first of their kind with classical-lessor secure key leasing properties. Crucially, the team proved that all three schemes satisfy the strong security notion against verification key revealing attacks under the learning with errors assumption, ensuring the integrity of the revocation process. This research delivers a significant advancement by enabling a completely classical client or lessor, minimizing the quantum resources required for implementation, paving the way for applications in cloud computing, temporary software licensing, and secure delegation of cryptographic capabilities.
Classical Key Leasing for Cryptographic Applications
Researchers have presented a new framework for key leasing, a cryptographic technique allowing temporary access to a key with the ability to revoke it later in a verifiable manner. The team successfully constructed key leasing schemes using only classical communication, a significant advancement as it avoids reliance on more complex quantum technologies. These schemes were developed for three fundamental cryptographic applications: public-key encryption, pseudorandom functions, and digital signatures. This approach offers a practical solution for managing access to cryptographic keys in various applications. Notably, the developed public-key encryption scheme improves upon existing constructions, while the pseudorandom function and digital signature schemes represent the first of their kind with this classical-lessor key leasing property. The researchers also demonstrated the versatility of their approach by showing how it can be applied to a broader range of cryptographic primitives, provided those primitives support a specific type of parallel-extractable watermarking.