Cryptology ePrint Archive: Report 2020/224

Security under Message-Derived Keys: Signcryption in iMessage

Mihir Bellare and Igors Stepanovs

Abstract: At the core of Apple's iMessage is a signcryption scheme that involves symmetric encryption of a message under a key that is derived from the message itself. This motivates us to formalize a primitive we call Encryption under Message-Derived Keys (EMDK). We prove security of the EMDK scheme underlying iMessage. We use this to prove security of the signcryption scheme itself, with respect to definitions of signcryption we give that enhance prior ones to cover issues peculiar to messaging protocols. Our provable-security results are quantitative, and we discuss the practical implications for iMessage.

Category / Keywords: cryptographic protocols / Signcryption, Public-Key Encryption, Symmetric Encryption, Random Oracle Model, Messaging

Original Publication (with major differences): IACR-EUROCRYPT-2020

Date: received 19 Feb 2020, last revised 3 Mar 2020

Contact author: istepanovs at inf ethz ch,mihir@eng ucsd edu

Available format(s): PDF | BibTeX Citation

Version: 20200303:110117 (All versions of this report)

Short URL: ia.cr/2020/224