Paper 2025/1750 Original: eprint.iacr.org/2025/1750 ↗

Modeling Emails: On the Deniability of BCCs

Abstract

Emails are one of the main forms of digital communication. They were designed to provide many guarantees that have surprisingly not yet been formalized in cryptography. Yet many of the guarantees emails were designed to provide have not been formalized in cryptography. This paper models an important feature of email applications: the plausible deniability of including Bcc recipients. Concretely, - we define a basic (theoretical) email application capturing these guarantees in Constructive Cryptography (Maurer and Renner, ICS '11) - we introduce Email Encryption: a new cryptographic primitive that is tailor-made to construct our email application - we define game-based notions for Email Encryption schemes, proving that their combination is sufficient to construct our simple email application and - we give a generic (proof-of-concept) construction of an Email Encryption scheme that provides all these guarantees. Our work identifies and formalizes missing theoretical foundations for the security of emails providing the first step towards practical solutions.