Signal Protocol

Signal Protocol
Communication protocol
Purpose	End-to-end encrypted communications
Developer(s)	Signal Foundation
Based on	OTR, SCIMP
Influenced	OMEMO, Matrix
OSI layer	Application layer
Website	signal.org/docs

The Signal Protocol (formerly known as the TextSecure Protocol) is a non-federated cryptographic protocol that provides end-to-end encryption for voice and instant messaging conversations.^[2] The protocol was developed by Open Whisper Systems in 2013^[2] and was introduced in the open-source TextSecure app, which later became Signal. Several closed-source applications have implemented the protocol, such as WhatsApp, which is said to encrypt the conversations of "more than a billion people worldwide"^[3] or Google who provides end-to-end encryption by default to all RCS-based conversations between users of their Google Messages app for one-to-one conversations.^[4] Facebook Messenger also say they offer the protocol for optional Secret Conversations, as does Skype for its Private Conversations.

The protocol combines the Double Ratchet Algorithm, prekeys, and a triple Elliptic-curve Diffie–Hellman (3-DH) handshake,^[5] and uses Curve25519, AES-256, and HMAC-SHA256 as primitives.^[6]

^ Cite error: The named reference advanced-ratcheting was invoked but never defined (see the help page).
^ ^a ^b ^c Ermoshina, Ksenia; Musiani, Francesca; Halpin, Harry (September 2016). "Internet Science". In Bagnoli, Franco; et al. (eds.). Internet Science. INSCI 2016. Lecture Notes in Computer Science. Vol. 9934. Florence, Italy: Springer. pp. 244–254. doi:10.1007/978-3-319-45982-0_22. ISBN 978-3-319-45982-0.
^ "WhatsApp's Signal Protocol integration is now complete". Signal. Signal Blog. 2016. Archived from the original on 29 January 2021. Retrieved 5 April 2016.
^ Cite error: The named reference :0 was invoked but never defined (see the help page).
^ Unger et al. 2015, p. 241
^ Frosch et al. 2016

[advanced-ratcheting-1] Cite error: The named reference advanced-ratcheting was invoked but never defined (see the help page).

[Ermoshina-2016-2] Ermoshina, Ksenia; Musiani, Francesca; Halpin, Harry (September 2016). "Internet Science". In Bagnoli, Franco; et al. (eds.). Internet Science. INSCI 2016. Lecture Notes in Computer Science. Vol. 9934. Florence, Italy: Springer. pp. 244–254. doi:10.1007/978-3-319-45982-0_22. ISBN 978-3-319-45982-0.

[3] "WhatsApp's Signal Protocol integration is now complete". Signal. Signal Blog. 2016. Archived from the original on 29 January 2021. Retrieved 5 April 2016.

[:0-4] Cite error: The named reference :0 was invoked but never defined (see the help page).

[5] Unger et al. 2015, p. 241

[Frosch_2016-6] Frosch et al. 2016

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