We put forward a symmetric encryption primitive tailored towards a specific application: outsourced storage. The setting assumes a memory-bounded computing device that inflates the amount of volatile or permanent memory available to it by letting other (untrusted) devices hold encryptions of information that they return on request. For instance, web servers typically hold for each of the client connections they manage a multitude of data, ranging from user preferences to technical information like database credentials. If the amount of data per session is considerable, busy servers sooner or later run out of memory. One admissible solution to this is to let the server encrypt the session data to itself and to let the client store the ciphertext, with the agreement that the client reproduce the ciphertext in each subsequent request (e.g., via a cookie) so that the session data can be recovered when required. In this article we develop the cryptographic mechanism that should be used to achieve confidential and authentic data storage in the encrypt-to-self setting, i.e., where encryptor and decryptor coincide and constitute the only entity holding keys. We argue that standard authenticated encryption represents only a suboptimal solution for preserving confidentiality, as much as message authentication codes are suboptimal for preserving authenticity. The crucial observation is that such schemes instantaneously give up on all security promises in the moment the key is compromised. In contrast, data protected with our new primitive remains fully integrity protected and unmalleable. In the course of this paper we develop a formal model for encrypt-to-self systems, show that it solves the outsourced storage problem, propose surprisingly efficient provably secure constructions, and report on our implementations. 9 We use the terms 'key leakage', 'user corruption', and 'state corruption' synonymously. 10 The above encrypt-then-hash (EtH) solution is secure in our models but requires two passes over the data. Our solutions are more efficient, getting along with just one pass.

A popular cryptographic option to implement Hierarchical Access Control in organizations is to combine a key assignment scheme with a symmetric encryption scheme. In brief, key assignment associates with each object in the hierarchy a unique symmetric key, and provides all higher-ranked “authorized” subjects with a method to recover it. This setup allows for encrypting the payloads associated with the objects so that they can be accessed by the authorized and remain inaccessible for the unauthorized. Both key assignment and symmetric encryption have been researched for roughly four decades now, and a plethora of efficient constructions have been the result. Surprisingly, a treatment of the joint primitive (key assignment combined with encryption, as used in practice) in the framework of provable security was conducted only very recently, leading to a publication in ToSC 2018(4). We first carefully revisit this publication. We then argue that there are actually two standard use cases for the combined primitive, which also require individual treatment. We correspondingly propose a fresh set of security models and provably secure constructions for each of them. Perhaps surprisingly, the two constructions call for different symmetric encryption primitives: While standard AEAD is the right tool for the one, we identify a less common tool called Encryptment as best fitting the other.

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Recent work by Pijnenburg and Poettering (ESORICS'20) explores the novel cryptographic Encrypt-to-Self primitive that is dedicated to use cases of symmetric encryption where encryptor and decryptor coincide. The primitive is envisioned to be useful whenever a memory-bounded computing device is required to encrypt some data with the aim of temporarily depositing it on an untrusted storage device. While the new primitive protects the confidentiality of payloads as much as classic authenticated encryption primitives would do, it provides considerably better authenticity guarantees: Specifically, while classic solutions would completely fail in a context involving user corruptions, if an encrypt-to-self scheme is used to protect the data, all ciphertexts and messages fully remain unforgeable. To instantiate their encrypt-to-self primitive, Pijnenburg et al. propose a mode of operation of the compression function of a hash function, with a carefully designed encoding function playing the central role in the serialization of the processed message and associated data. In the present work we revisit the design of this encoding function. Without questioning its adequacy for securely accomplishing the encrypt-to-self job, we improve on it from a technical/implementational perspective by proposing modifications that alleviate certain conditions that would inevitably require implementations to disrespect memory alignment restrictions imposed by the word-wise operation of modern CPUs, ultimately leading to performance penalties. Our main contributions are thus to propose an improved encoding function, to explain why it offers better performance, and to prove that it provides as much security as its predecessor. We finally report on our open-source implementation of the encrypt-to-self primitive based on the new encoding function. For the full version of this article, see arXiv:2009.02667. CCS CONCEPTS • Security and privacy → Symmetric cryptography and hash functions.