Matilda Backendal

Last Name

Backendal

First Name

Matilda

ORCID

0000-0002-8677-8301

Organisational unit

09653 - Paterson, Kenneth / Paterson, Kenneth

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Publications 1 - 9 of 9

End-to-End Encrypted Cloud Storage
Backendal, Matilda; Haller, Miro; Paterson, Kenny (2024)
IEEE Security & Privacy
End-to-end encryption is rapidly becoming the accepted security goal for personal data. In this article, we examine consumer cloud storage systems, focusing in particular on those systems that attempt to provide end-to-end security for customer data. We survey the security guarantees of current service providers and the issues they face, discuss open research questions, and highlight the challenges that impede the deployment of end-to-end secure cloud storage.
Puncturable Key Wrapping and Its Applications
Backendal, Matilda; Günther, Felix; Paterson, Kenneth G. (2022)
Advances in Cryptology – Asiacrypt 2022
We introduce puncturable key wrapping (PKW), a new cryptographic primitive that supports fine-grained forward security properties in symmetric key hierarchies. We develop syntax and security definitions, along with provably secure constructions for PKW from simpler components (AEAD schemes and puncturable PRFs). We show how PKW can be applied in two distinct scenarios. First, we show how to use PKW to achieve forward security for TLS 1.3 0-RTT session resumption, even when the server's long-term key for generating session tickets gets compromised. This extends and corrects a recent work of Aviram, Gellert, and Jager (Journal of Cryptology, 2021). Second, we show how to use PKW to build a protected file storage system with file shredding, wherein a client can outsource encrypted files to a potentially malicious or corrupted cloud server whilst achieving strong forward-security guarantees, relying only on local key updates.
MEGA: Malleable Encryption Goes Awry
Backendal, Matilda; Haller, Miro; Paterson, Kenneth G. (2023)
IEEE Symposium on Security and Privacy
MEGA is a leading cloud storage platform with more than 250 million users and 1000 Petabytes of stored data. MEGA claims to offer user-controlled, end-to-end security. This is achieved by having all data encryption and decryption operations done on MEGA clients, under the control of keys that are only available to those clients. This is intended to protect MEGA users from attacks by MEGA itself, or by adversaries who have taken control of MEGA's infrastructure. We provide a detailed analysis of MEGA's use of cryptography in such a malicious server setting. We present five distinct attacks against MEGA, which together allow for a full compromise of the confidentiality of user files. Additionally, the integrity of user data is damaged to the extent that an attacker can insert malicious files of their choice which pass all authenticity checks of the client. We built proof-of-concept versions of all the attacks. Four of the five attacks are eminently practical. They have all been responsibly disclosed to MEGA and remediation is underway. Taken together, our attacks highlight significant shortcomings in MEGA's cryptographic architecture. We present immediately deployable countermeasures, as well as longer-term recommendations. We also provide a broader discussion of the challenges of cryptographic deployment at massive scale under strong threat models.
When Messages Are Keys: Is HMAC a Dual-PRF?
Backendal, Matilda; Bellare, Mihir; Günther, Felix; et al. (2023)
Lecture Notes in Computer Science ~ Advances in Cryptology – CRYPTO 2023
In Internet security protocols including TLS 1.3, KEMTLS, MLS and Noise, HMAC is being assumed to be a dual-PRF, meaning a PRF not only when keyed conventionally (through its first input), but also when “swapped” and keyed (unconventionally) through its second (message) input. We give the first in-depth analysis of the dual-PRF assumption on HMAC. For the swap case, we note that security does not hold in general, but completely characterize when it does; we show that HMAC is swap-PRF secure if and only if keys are restricted to sets satisfying a condition called feasibility, that we give, and that holds in applications. The sufficiency is shown by proof and the necessity by attacks. For the conventional PRF case, we fill a gap in the literature by proving PRF security of HMAC for keys of arbitrary length. Our proofs are in the standard model, make assumptions only on the compression function underlying the hash function, and give good bounds in the multi-user setting. The positive results are strengthened through achieving a new notion of variable key-length PRF security that guarantees security even if different users use keys of different lengths, as happens in practice.
MEGA: Malleable Encryption Goes Awry
Backendal, Matilda; Haller, Miro; Paterson, Kenneth G. (2023)
2023 IEEE Symposium on Security and Privacy (SP)
MEGA is a leading cloud storage platform with more than 250 million users and 1000 Petabytes of stored data. MEGA claims to offer user-controlled, end-to-end security. This is achieved by having all data encryption and decryption operations done on MEGA clients, under the control of keys that are only available to those clients. This is intended to protect MEGA users from attacks by MEGA itself, or by adversaries who have taken control of MEGA’s infrastructure. We provide a detailed analysis of MEGA’s use of cryptography in such a malicious server setting. We present five distinct attacks against MEGA, which together allow for a full compromise of the confidentiality of user files. Additionally, the integrity of user data is damaged to the extent that an attacker can insert malicious files of their choice which pass all authenticity checks of the client. We built proof-of-concept versions of all the attacks. Four of the five attacks are eminently practical. They have all been responsibly disclosed to MEGA and remediation is underway. Taken together, our attacks highlight significant shortcomings in MEGA’s cryptographic architecture. We present immediately deployable countermeasures, as well as longer-term recommendations. We also provide a broader discussion of the challenges of cryptographic deployment at massive scale under strong threat models.
A Formal Treatment of End-to-End Encrypted Cloud Storage
Backendal, Matilda; Davis, Hannah; Günther, Felix; et al. (2024)
Lecture Notes in Computer Science ~ Advances in Cryptology – CRYPTO 2024
Users increasingly store their data in the cloud, thereby benefiting from easy access, sharing, and redundancy. To additionally guarantee security of the outsourced data even against a server compromise, some service providers have started to offer end-to-end encrypted (E2EE) cloud storage. With this cryptographic protection, only legitimate owners can read or modify the data. However, recent attacks on the largest E2EE providers have highlighted the lack of solid foundations for this emerging type of service. In this paper, we address this shortcoming by initiating the formal study of E2EE cloud storage. We give a formal syntax to capture the core functionality of a cloud storage system, capturing the real-world complexity of such a system’s constituent interactive protocols. We then define game-based security notions for confidentiality and integrity of a cloud storage system against a fully malicious server. We treat both selective and fully adaptive client compromises. Our notions are informed by recent attacks on E2EE cloud storage providers. In particular we show that our syntax is rich enough to capture the core functionality of MEGA and that recent attacks on it arise as violations of our security notions. Finally, we present an E2EE cloud storage system that provides all core functionalities and that is both efficient and provably secure with respect to our selective security notions. Along the way, we discuss challenges on the path towards bringing the security of cloud storage up to par with other end-to-end primitives, such as secure messaging and TLS.
Key Derivation Functions Without a Grain of Salt
Backendal, Matilda; Clermont, Sebastian; Fischlin, Marc; et al. (2025)
Lecture Notes in Computer Science ~ Advances in Cryptology – EUROCRYPT 2025: 44th Annual International Conference on the Theory and Applications of Cryptographic Techniques, Madrid, Spain, May 4–8, 2025, Proceedings, Part VIII
Key derivation functions (KDFs) are integral to many cryptographic protocols. Their functionality is to turn raw key material, such as a Diffie-Hellman secret, into a strong cryptographic key that is indistinguishable from random. This guarantee was formalized by Krawczyk together with the seminal introduction of HKDF (CRYPTO 2010), in a model where the KDF only takes a single key material input. Modern protocol designs, however, regularly need to combine multiple secrets, possibly even from different sources, with the guarantee that the derived key is secure as long as at least one of the inputs is good. This is particularly relevant in settings like hybrid key exchange for quantum-safe migration. Krawczyk's KDF formalism does not capture this goal, and there has been surprisingly little work on the security considerations for KDFs since then. In this work, we thus revisit the syntax and security model for KDFs to treat multiple, possibly correlated inputs. Our syntax is assertive: We do away with salts, which are needed in theory to extract from arbitrary sources in the standard model, but in practice, they are almost never used (or even available) and sometimes even misused, as we argue. We use our new model to analyze real-world multi-input KDFs—in Signal's X3DH protocol, ETSI's TS 103 744 standard, and MLS' combiner for pre-shared keys—as well as new constructions we introduce for specialized settings—e.g., a purely blockcipher-based one. We further discuss the importance of collision resistance for KDFs and finally apply our multi-input KDF model to show how hybrid KEM key exchange can be analyzed from a KDF perspective.
MFKDF: Multiple Factors Knocked Down Flat
Scarlata, Matteo; Backendal, Matilda; Haller, Miro (2024)
Proceedings of the 33rd USENIX Security Symposium
Nair and Song (USENIX 2023) introduce the concept of a Multi-Factor Key Derivation Function (MFKDF), along with constructions and a security analysis. MFKDF integrates dynamic authentication factors, such as HOTP and hardware tokens, into password-based key derivation. The aim is to improve the security of password-derived keys, which can then be used for encryption or as an alternative to multi-factor authentication. The authors claim an exponential security improvement compared to traditional password-based key derivation functions (PBKDF). We show that the MFKDF constructions proposed by Nair and Song fall short of the stated security goals. Underspecified cryptographic primitives and the lack of integrity of the MFKDF state lead to several attacks, ranging from full key recovery when an HOTP factor is compromised, to bypassing factors entirely or severely reducing their entropy. We reflect on the different threat models of key-derivation and authentication, and conclude that MFKDF is always weaker than plain PBKDF and multi-factor authentication in each setting.
Share with Care: Breaking E2EE in Nextcloud
Albrecht, Martin R.; Backendal, Matilda; Coppola, Daniele; et al. (2024)
2024 IEEE 9th European Symposium on Security and Privacy (EuroS&P)
Nextcloud is a leading cloud storage platform with more than 20 million users. Nextcloud offers an end-to-end encryption (E2EE) feature that is claimed to be able “to keep extremely sensitive data fully secure even in case of a full server breach”. They also claim that the Nextcloud server “has Zero Knowledge, that is, never has access to any of the data or keys in unencrypted form”. This is achieved by having encryption and decryption operations that are done using file keys that are only available to Nextcloud clients, with those file keys being protected by a key hierarchy that ultimately relies on long passphrases known exclusively to the users. We provide the first detailed documentation and security analysis of Nextcloud's E2EE feature. Nextcloud's strong security claims motivate conducting the analysis in the setting where the server itself is considered malicious. We present three distinct attacks against the E2EE security guarantees in this setting. Each one enables the confidentiality and integrity of all user files to be compromised. All three attacks are fully practical and we have built proof-of-concept implementations for each. The vulnerabilities make it trivial for a malicious Nextcloud server to access and manipulate users' data. We have responsibly disclosed the three vulnerabilities to N extcloud. The second and third vulnerabilities have been remediated. The first was addressed by temporarily disabling file sharing from the E2EE feature until a redesign of the feature can be made. We reflect on broader lessons that can be learned for designers of E2EE systems.

Publications 1 - 9 of 9

Matilda Backendal

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