Kirill Nikitin

Biographie

Hi, I am a sixth-year PhD student in the Decentralized and Distributed Systems lab at advised by Prof. Bryan Ford. My research interests revolve around the topics in Systems and Network Security, Privacy, Distributed Systems, and Smart Contracts. I am currently focused on improving security of software-update systems, reducing metadata leakage from encrypted data formats, and building a secure peer-to-peer collaboration platform.

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https://people.epfl.ch/kirill.nikitin?lang=fr

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Integrity and Metadata Protection in Data Retrieval

Kirill Nikitin

Secure retrieval of data requires integrity, confidentially, transparency, and metadata-privacy of the process. Existing protection mechanisms, however, provide only partially these properties: encryption schemes still expose cleartext metadata, protocols for private information retrieval neglect data integrity, and data-distribution architectures forego transparency. In this dissertation, by designing new cryptographic primitives and security architectures that provide a more comprehensive protection, we improve on the current security and privacy practices in data retrieval. First, we propose a new format for encrypted data; it protects both content and all encryption metadata, such as the application, the intended recipients, and the algorithms used. The format comes with a cryptographically-agile encoding scheme that facilitates efficient decryption of such ciphertexts without cleartext markers. Second, to address the lack of integrity in privacy-preserving data-retrieval protocols, we introduce the concept of single-server verifiable private information retrieval. In contrast to existing solutions where, in some deployment scenarios, a malicious server can violate client privacy by selectively tampering with the data, our approach ensures that an honest client either correctly obtains the data from the systemâs server or detects server misbehavior and aborts. Finally, we present a software-update framework that reinforces software-distribution processes. Building on the concepts of decentralization and verifiability, our framework eliminates single points of failure, enforces transparency, and ensures integrity and authenticity of software releases. By implementing and experimentally evaluating our primitives and framework, we demonstrate that better protection is practical and incurs only a modest additional cost.

EPFL2021

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Replicated state machines without replicated execution

Kirill Nikitin

This paper introduces a new approach to reduce end-to-end costs in large-scale replicated systems built under a Byzantine fault model. Specifically, our approach transforms a given replicated state machine (RSM) to another RSM where nodes incur lower costs by delegating state machine execution: an untrusted prover produces succinct cryptographic proofs of correct state transitions along with state changes, which nodes in the transformed RSM verify and apply respectively. To realize our approach, we build Piperine, a system that makes the proof machinery profitable in the context of RSMs. Specifically, Piperine reduces the costs of both proving and verifying the correctness of state machine execution while retaining liveness-a distinctive requirement in the context of RSMs. Our experimental evaluation demonstrates that, for a payment service, employing Piperine is more profitable than naive reexecution of transactions as long as there are more than 10^4 nodes. When we apply Piperine to ERC-20 transactions in Ethereum (a real-world RSM with up to 10^5 nodes), it reduces per-transaction costs by 5.4× and network costs by 2.7×.

IEEE2020

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Most encrypted data formats leak metadata via their plaintext headers, such as format version, encryption schemes used, number of recipients who can decrypt the data, and even the recipients’ identities. This leakage can pose security and privacy risks to users, e.g., by revealing the full membership of a group of collaborators from a single encrypted e-mail, or by enabling an eavesdropper to fingerprint the precise encryption software version and configuration the sender used. We propose that future encrypted data formats improve security and privacy hygiene by producing Padded Uniform Random Blobs or PURBs: ciphertexts indistinguishable from random bit strings to anyone without a decryption key. A PURB’s content leaks nothing at all, even the application that created it, and is padded such that even its length leaks as little as possible. Encoding and decoding ciphertexts with no cleartext markers presents efficiency challenges, however. We present cryptographically agile encodings enabling legitimate recipients to decrypt a PURB efficiently, even when encrypted for any number of recipients’ public keys and/or passwords, and when these public keys are from different cryptographic suites. PURBs employ Padmé, a novel padding scheme that limits information leakage via ciphertexts of maximum length M to a practical optimum of O(log log M) bits, comparable to padding to a power of two, but with lower overhead of at most 12% and decreasing with larger payloads.

Sciendo2019

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