keychat on Nostr: The Signal protocol, primarily known for its double ratchet algorithm, is used for ...

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

quoting note1ffg…ntlr

Many Nostr users know that the Signal protocol is a very secure end-to-end encryption protocol, recommended by Snowden. However, many people do not know why the Signal protocol is secure. In fact, the design of the Signal protocol has a historical basis and is very intuitive and simple.

End-to-end encryption protocols have gone through three key stages: the PGP (Pretty Good Privacy) protocol of the 1990s, the OTR (Off The Record) protocol of the 2000s, and the Signal protocol of the 201x.

PGP achieved end-to-end encryption through public key encryption. But if a private key leaks, all [past and future] messages can be decrypted. No [forward secrecy] and [backward secrecy].

[[In the PGP protocol, the identity key and the encryption key are the same; it serves as both the identity and is used to encrypt messages.]] [[As an identity key, it is a long-term key, and the longer it is used, the greater the likelihood of private key exposure.]]

The OTR protocol solves the problem of PGP. Its approach is simple: [[it separate the identity key from the encryption key and generates a unique encryption key for each conversation, which is deleted after use.]]

The Signal protocol inherited ideas from the OTR protocol. It mainly consists of two parts: the X3DH protocol, which is responsible for the initial key agreement, and the Double Ratchet Algorithm, which is responsible for deriving encryption keys. [[The Signal protocol derives a new encryption key for each message, which is deleted after use.]] It achieved better forward and backward secrecy.

BTW, the Signal protocol is not equivalent to the Signal app. WhatsApp also uses the Signal protocol for end-to-end encryption. The Signal protocol is only responsible for the end-to-end encryption of messages, meaning only the communicating parties can decrypt the content, not even the server administrators.