Identity-based encryption with hierarchical key-insulation in the standard model

A key exposure problem is unavoidable since it seems human error can never be eliminated completely, and key-insulated encryption is one of the cryptographic solutions to the problem. At Asiacrypt\u2705, Hanaoka et al. introduced hierarchical key-insulation functionality, which is attractive functionality that enhances key exposure resistance, and proposed an identity-based hierarchical key-insulated encryption (hierarchical IKE) scheme in the random oracle model. In this paper, we first propose the hierarchical IKE scheme in the standard model (i.e., without random oracles). Our hierarchical IKE scheme is secure under the symmetric external Diffie–Hellman (SXDH) assumption, which is a static assumption. Particularly, in the non-hierarchical case, our construction is the first IKE scheme that achieves constant-size parameters including public parameters, secret keys, and ciphertexts. Furthermore, we also propose the first public-key-based key-insulated encryption (PK-KIE) in the hierarchical setting by using our technique

Identity-based Hierarchical Key-insulated Encryption without Random Oracles

Key-insulated encryption is one of the effective solutions to a key exposure problem. At Asiacrypt\u2705, Hanaoka et al. proposed an identity-based hierarchical key-insulated encryption (hierarchical IKE) scheme. Although their scheme is secure in the random oracle model, it has a ``hierarchical key-updating structure,\u27\u27 which is attractive functionality that enhances key exposure resistance. In this paper, we first propose the hierarchical IKE scheme without random oracles. Our hierarchical IKE scheme is secure under the symmetric external Diffie-Hellman (SXDH) assumption, which is known as the simple and static one. Particularly, in the non-hierarchical case, our construction is the first IKE scheme that achieves constant-size parameters including public parameters, secret keys, and ciphertexts. Furthermore, we also propose the first public-key-based key-insulated encryption (PK-KIE) in the hierarchical setting by using our technique

Certificateless Key Insulated Encryption: Cryptographic Primitive for Achieving Key-escrow free and Key-exposure Resilience

Certificateless encryption (CLE) alleviates the heavy certificate management in traditional public key encryption and the key escrow problem in the ID-based encryption simultaneously. Current CLE schemes assumed that the user’s secret key is absolutely secure. Unfortunately, this assumption is too strong in case the CLE is deployed in the hostile setting and the leakage of secret key is inevitable. In this paper, we present a new concept called an certificateless key insulated encryption scheme (CL-KIE). We argue that this is an important cryptographic primitive that can be used to achieve key-escrow free and key-exposure resilience. We also present an efficient CL-KIE scheme based on bilinear pairing. After that, the security of our scheme is proved under the Bilinear Diffie-Hellman assumption in the random oracle model. Certificateless encryption (CLE) alleviates the heavy certificate management in traditional public key encryption and the key escrow problem in the ID-based encryption simultaneously. Current CLE schemes assumed that the user’s secret key is absolutely secure. Unfortunately, this assumption is too strong in case the CLE is deployed in the hostile setting and the leakage of the secret key is inevitable. In this paper, we present a new concept called a certificateless key insulated encryption scheme (CL-KIE). We argue that this is an important cryptographic primitive that can be used to achieve key-escrow free and key-exposure resilience. We also present an efficient CL-KIE scheme based on bilinear pairing. After that, the security of our scheme is proved under the Bilinear DiffieHellman assumption in the random oracle model

Generic Constructions of Parallel Key-Insulated Encryption: Stronger Security Model and Novel Schemes

Exposure of a secret key is a significant threat in practice. As a notion of security against key exposure, Dodis et al. advocated key-insulated security, and proposed concrete key-insulated encryption (KIE) schemes in which secret keys are periodically updated by using a physically ``insulated\u27\u27 helper key. For significantly reducing possibility of exposure of the helper key, Hanaoka et al. further proposed the notion of parallel KIE (PKIE) in which multiple helper keys are used in alternate shifts. They also pointed out that in contrast to the case of the standard KIE, PKIE cannot be straightforwardly obtained from identity-based encryption (IBE). In this paper, we first discuss that previous security models for PKIE are somewhat weak, and thus re-formalize stronger security models for PKIE. Then we clarify that PKIE can be generically constructed (even in the strenghthened security models) by using a new primitive which we call one-time forward secure public key encryption (OTFS-PKE) and show that it is possible to construct OTFS-PKE from arbitrary IBE or hierarchical IBE (without degenerating into IBE). By using our method, we can obtain various new PKIE schemes which yield desirable properties. For example, we can construct first PKIE schemes from lattice or quadratic residuosity problems (without using bilinear maps), and PKIE with short ciphertexts and cheaper computational cost for both encryption and decryption. Interestingly, the resulting schemes can be viewed as the partial solutions to the open problem left by Libert, Quisquarter and Yung in PKC\u2707

Key Insulation and Intrusion Resilience Over a Public Channel

Key insulation (KI) and Intrusion resilience (IR) are methods to protect a user\u27s key against exposure by utilizing periodic communications with an auxiliary helper. But existing work assumes a secure channel between user and helper. If we want to realize KI or IR in practice we must realize this secure channel. This paper looks at the question of how to do this when the communication is over what we are more likely to have in practice, namely a public channel such as the Internet or a wireless network. We explain why this problem is not trivial, introduce models and definitions that capture the desired security in a public channel setting, and provide a complete (and surprising) answer to the question of when KI and IR are possible over a public channel. The information we provide is important to guide practitioners with regard to the usage of KI and IR and also to guide future research in this area

A Composable Look at Updatable Encryption

Updatable Encryption (UE), as originally defined by Boneh et al. in 2013, addresses the problem of key rotation on outsourced data while maintaining the communication complexity as low as possible. The security definitions for UE schemes have been constantly updated since then. However, the security notion that is best suited for a particular application remains unclear. To solve this problem in the ciphertext-independent setting, we use the Constructive Cryptography (CC) framework defined by Maurer et al. in 2011. We define and construct a resource that we call Updatable Server-Memory Resource USMR, and study the confidentiality guarantees it achieves when equipped with a UE protocol, that we also model in this framework. With this methodology, we are able to construct resources tailored for each security notion. In particular, we prove that IND-UE-RCCA is the right security notion for many practical UE schemes. As a consequence, we notably rectify a claim made by Boyd et al. , namely that their IND-UE security notion is better than the IND-ENC+UPD notions, in that it hides the age of ciphertexts. We show that this is only true when ciphertexts can leak at most one time per epoch. We stress that UE security is thought of in the context of adaptive adversaries, and UE schemes should thus bring post-compromise confidentiality guarantees to the client. To handle such adversaries, we use an extension of CC due to Jost et al. and give a clear, simple and composable description of the post-compromise security guarantees of UE schemes. We also model semi-honest adversaries in CC. Our adaption of the CC framework to UE is generic enough to model other interactive protocols in the outsourced storage setting

Provably Quantum-Secure Message Authentication Code

Die Gefahr von Quantencomputer gegen asymmetrische Kryptographie ist schon lange bekannt. Jedoch wurden die Auswirkungen auf die symmetrische Kryptographie als weniger einschlägig betrachtet. In den letzten Jahren sind mehrere effiziente Quantenangriffee gegen Nachrichtenauthentifizierungscode (message authentication code, MAC) entdeckt worden. Aus diesem Grund wurde beweisbare Sicherheit dieser Primitive im Quantenmodell erforscht. Einige existierende Algorithmen wurden als quantensicher bewiesen. Darüber hinaus wurden neuen Protokolle entworfen welche auch Quantenangriffen widerstehen können. In dieser Masterarbeit untersuchen wir den Einsatz von Noncen in der Konstruktion von quantensicheren Protokollen. In diesem Sinne hat eine vorherige Arbeit eine allgemeine Transformation für MACs eingeführt. Wir zeigen, dass diese Transformation im Allgemeinen nicht quantensicher ist. Dennoch behaupten wir, dass die Transformation in vielen spezifischen Fällen wirksam ist. Wir behandeln denn Fall von der CBC-MAC und zeigen das die transformierte Version quantensicher ist. Zudem formalisieren wir einige Entwurfstrategien für quantensichere Protokolle