Crypto Wars in Secure Messaging: Covert Channels in Signal Despite Leaked Keys

End-to-end encryption (E2EE) is the foundation of modern secure messaging, with the Signal protocol as the de facto standard in applications such as Signal, WhatsApp, Facebook Messenger and Google Messages. At the same time, the deployment of E2EE has led to growing pressure from authorities to decrypt user traffic under law enforcement. This raises a critical question: if an adversary can routinely decrypt Signal messages (for example via a mandated access or a leaked key), can users still communicate securely and covertly?

We address this question through the lens of anamorphic encryption, which enables hidden communication within seemingly legitimate ciphertexts, even against an adversary who can decrypt them. We design two constructions that embed covert channels into the existing Signal Double Ratchet protocol. Concretely, we show how to embed covert messages (i) into Diffie-Hellman keys used in the asymmetric ratchet, or (ii) into authentication tags produced in the symmetric ratchet. Our techniques are compatible with existing Signal-style deployments and require no changes by the service provider.

We formalize security in threat models that capture adversaries with decryption capabilities granted through lawfulaccess mechanisms, and prove that the resulting protocol transcripts are indistinguishable from those of standard Signal. We implement our constructions in the official Signal library and Android client, and show that they incur low overhead and are practical in real-world settings. Our results show that covert communication channels can persist even when conventional E2EE guarantees are compromised.