## Cryptology ePrint Archive: Report 2017/824

Improved Security Notions for Proxy Re-Encryption to Enforce Access Control

Ela Lee (née Berners-Lee)

Abstract: Proxy Re-Encryption (PRE) allows a ciphertext encrypted under Alice's public key to be transformed to an encryption under Bob's public key without revealing either the plaintext or the decryption keys. PRE schemes have clear applications to cryptographic access control by allowing outsourced data to be selectively shared to users via re-encryption to appropriate keys. One concern for this application is that the server should not be able to perform unauthorised re-encryptions. We argue that current security notions do not adequately address this concern. We revisit existing definitions for PRE, starting by challenging the concept of unidirectionality, which states that re-encryption tokens from $A$ to $B$ cannot be used to re-encrypt from $B$ to $A$. We strengthen this definition to reflect realistic scenarios in which adversaries may try to reverse a re-encryption by retaining information about prior ciphertexts and re-encryption tokens. We then strengthen the adversarial model to consider malicious adversaries that may collude with corrupt users and attempt to perform unauthorised re-encryptions; this models a malicious cloud service provider aiming to subvert the re-encryption process to leak sensitive data. Finally we revisit the notion of authenticated encryption for PRE. This currently assumes the same party who created the message also encrypted it, which is not necessarily the case in re-encryption. We thus introduce the notion of \emph{ciphertext origin authentication} to determine who encrypted the message (initiated a re-encryption) and show how to fufil this requirement in practice.

Category / Keywords: public-key cryptography / Proxy re-encryption, applied cryptography, unidirectional, multi-hop, malicious model, access control

Original Publication (with minor differences): to appear at Latincrypt 2017

Date: received 29 Aug 2017, last revised 18 May 2018

Contact author: Ela Berners-Lee 2010 at live rhul ac uk

Available format(s): PDF | BibTeX Citation

Short URL: ia.cr/2017/824

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