Signal Protocol
What is Signal Protocol?
Signal ProtocolThe end-to-end-encryption protocol developed by Open Whisper Systems for the Signal messenger, combining the X3DH key agreement with the Double Ratchet algorithm.
The Signal Protocol is an open, peer-reviewed protocol for asynchronous end-to-end encrypted messaging, originally designed by Trevor Perrin and Moxie Marlinspike at Open Whisper Systems. A new conversation is bootstrapped with X3DH (Extended Triple Diffie-Hellman) over Curve25519, which establishes a shared secret even when one party is offline. Messages then advance through the Double Ratchet, which combines a Diffie-Hellman ratchet and a symmetric ratchet (HKDF + AES-256-GCM or ChaCha20-Poly1305) to provide forward secrecy and post-compromise security on every message. The protocol is also used by WhatsApp, Facebook Messenger Secret Conversations, Google Messages RCS encrypted chats, and Skype Private Conversations, making it one of the most widely deployed cryptographic protocols in the world.
● Examples
- 01
A WhatsApp chat negotiating X3DH to establish initial keys with an offline contact.
- 02
Signal rolling forward the Double Ratchet on every message to ensure forward secrecy.
● Frequently asked questions
What is Signal Protocol?
The end-to-end-encryption protocol developed by Open Whisper Systems for the Signal messenger, combining the X3DH key agreement with the Double Ratchet algorithm. It belongs to the Identity & Access category of cybersecurity.
What does Signal Protocol mean?
The end-to-end-encryption protocol developed by Open Whisper Systems for the Signal messenger, combining the X3DH key agreement with the Double Ratchet algorithm.
How does Signal Protocol work?
The Signal Protocol is an open, peer-reviewed protocol for asynchronous end-to-end encrypted messaging, originally designed by Trevor Perrin and Moxie Marlinspike at Open Whisper Systems. A new conversation is bootstrapped with X3DH (Extended Triple Diffie-Hellman) over Curve25519, which establishes a shared secret even when one party is offline. Messages then advance through the Double Ratchet, which combines a Diffie-Hellman ratchet and a symmetric ratchet (HKDF + AES-256-GCM or ChaCha20-Poly1305) to provide forward secrecy and post-compromise security on every message. The protocol is also used by WhatsApp, Facebook Messenger Secret Conversations, Google Messages RCS encrypted chats, and Skype Private Conversations, making it one of the most widely deployed cryptographic protocols in the world.
How do you defend against Signal Protocol?
Defences for Signal Protocol typically combine technical controls and operational practices, as detailed in the full definition above.
What are other names for Signal Protocol?
Common alternative names include: Open Whisper Signal protocol, X3DH + Double Ratchet.
● Related terms
- identity-access№ 380
End-to-End Encryption (E2EE)
An encryption model in which only the communicating endpoints hold the keys, so intermediate servers and network operators cannot read the plaintext.
- identity-access№ 986
Secure Messaging App
A communications app whose default mode applies end-to-end encryption, identity verification, and forward secrecy so that only the participants can read the messages.
- cryptography№ 249
Cryptography
The science of securing information through mathematical techniques that provide confidentiality, integrity, authenticity, and non-repudiation in the presence of adversaries.
- cryptography№ 379
Encryption
The cryptographic transformation of plaintext into ciphertext using an algorithm and key so that only authorized parties can recover the original data.
- cryptography№ 815
Perfect Forward Secrecy
A protocol property ensuring that the compromise of long-term keys does not allow decryption of past session traffic.
- cryptography№ 589
Key Rotation
The periodic replacement of cryptographic keys with new ones to limit the volume of data protected by any single key and contain the impact of compromise.