Sender Keys
In cryptography, Sender Keys is a key management protocol designed to provide end-to-end encryption in instant messaging for group communications. The protocol is designed to efficiently scale to large groups by taking advantage of server-side fan-out and avoiding computing a shared group key.[1][2][3] Sender Keys is commonly used by group messaging applications, including Signal, Matrix, WhatsApp, Session, and Facebook Messenger.[1][2][4][5][6][3]
The algorithm relies upon secure pairwise communication channels between peers that provide confidentiality and authentication. For example, an Authenticated Key Exchange algorithm such as Extended Triple Diffie-Hellman (X3DH) may be combined with the Double Ratchet Algorithm to construct such a channel in practice, as is the case with Whatsapp.[7][1]
Security properties
Security properties of Sender Keys include message confidentiality, message integrity, message authentication, forward secrecy, post-compromise security, scalability, and asynchronicity.[5][6]
Functioning
In Sender Keys, users within a group are assumed to maintain secure pairwise communication channels with each other user. Each user constructs a session that consists of a symmetric key and an asymmetric signing key pair; each user will then send their session's symmetric key and the signing key pair's public key to each other user through the pairwise secure channels.
To send a message, a user will "ratchet" their symmetric key forward by applying a cryptographic hash function, encrypt their message with the newly hashed symmetric key, and construct a digital signature protecting the encrypted message with the private signing key. The sender forwards the encrypted message to the server, who then fans it out to all receivers. Each receiver will check the signature with public signing key, hash their symmetric key to match the sender, and decrypt the message.
Users regenerate and re-transmit sessions periodically, or whenever a user leaves or joins the group.[1][2][3]
See also
References
- ↑ 1.0 1.1 1.2 1.3 WhatsApp Encryption Overview – Technical white paper (PDF) (Report). 8. WhatsApp LLC. 19 August 2024. p. 40. Retrieved 5 August 2025.
- ↑ 2.0 2.1 2.2 Messenger End-to-End Encryption Overview (PDF) (Report). 1. Meta LLC. 6 December 2023. p. 25. Retrieved 5 August 2025.
- ↑ 3.0 3.1 3.2 Marlinspike, Moxie (5 May 2014). "Private Group Messaging". Retrieved 11 August 2025.
- ↑ Jefferys, Kee (15 December 2020). "Session Protocol: Technical implementation details". Session Private Messenger Blog. Oxen Privacy Tech Foundation. Archived from the original on 19 June 2025. Retrieved 5 August 2025. Unknown parameter
|url-status=ignored (help) - ↑ 5.0 5.1 Balbas, David. WhatsUpp with Sender Keys? Analysis, Improvements and Security Proofs (PDF). AsiaCrypt 2023. Springer.
- ↑ 6.0 6.1 Albrecht, Martin. Device-Oriented Group Messaging: A Formal Cryptographic Analysis of Matrix' Core. 2024 IEEE Symposium on Security and Privacy (SP). IEEE.
- ↑ Albrecht, Martin. Formal Analysis of Multi-Device Group Messaging in WhatsApp (PDF). EuroCrypt 2025. Springer.
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