Paper 2020/224

Security under Message-Derived Keys: Signcryption in iMessage

Mihir Bellare and Igors Stepanovs


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.

Available format(s)
Cryptographic protocols
Publication info
A major revision of an IACR publication in EUROCRYPT 2020
SigncryptionPublic-Key EncryptionSymmetric EncryptionRandom Oracle ModelMessaging
Contact author(s)
istepanovs @ inf ethz ch
mihir @ eng ucsd edu
2020-12-09: last of 3 revisions
2020-02-21: received
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Creative Commons Attribution


      author = {Mihir Bellare and Igors Stepanovs},
      title = {Security under Message-Derived Keys: Signcryption in iMessage},
      howpublished = {Cryptology ePrint Archive, Paper 2020/224},
      year = {2020},
      note = {\url{}},
      url = {}
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