The Signal protocol, primarily known for its double ratchet algorithm, is used for end-to-end encryption of messages by the following chat applications and protocols. Many users are unaware that they are already benefiting from the security offered by the Signal protocol.

Chat Applications:

Signal app

WhatsApp

Skype: only available in the "Private Conversation"

Facebook Messenger: only available in the "Secret Conversations”

Simplex chat

Keychat

Chat Protocols:

XMPP

OMEMO is an XMPP Extension Protocol (XEP) for secure multi-client end-to-end encryption. It is an open standard based on a Double Ratchet and PEP, which can be freely used and implemented by anyone.