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. https://conversations.im/omemo/

Matrix Olm (libolm) is an independent Apache-licensed implementation of the Double Ratchet cryptographic ratchet in C & C++, which also includes the new Megolm group ratchet as used in Matrix. https://github.com/matrix-org/olm