E2EE

End-to-end encryption (E2EE) is a method of secure communication in which only the communicating users can read the plaintext content of messages. In an E2EE system, messages are encrypted on the sender's device with keys that are only decryptable on the recipients' devices; servers that relay the messages only see ciphertext and metadata such as sender and recipient identifiers and timings, not the message content.

How it works: E2EE relies on public-key cryptography and often a per-session or per-message key derived through a ratcheting protocol. Each device holds private keys and derives a shared session key with each other party; forward secrecy prevents past messages from being decrypted if a key is later compromised. In group conversations, keys are rotated so that new participants cannot read past messages, while existing participants retain access to newer content.

Scope and limitations: E2EE protects the confidentiality of message contents in transit and on endpoints, but it does not hide metadata or protect data stored on backups or servers if those are not encrypted end-to-end. Device compromise, key loss, or compromised key verification can break security. Group chats introduce additional complexity in distributing and revoking keys. Some services offer optional cloud backups or device backup recovery that may weaken E2EE if not properly protected.

Notable implementations and considerations: The Signal Protocol underpins many consumer apps and is widely regarded for strong security properties. Other approaches include OMEMO for XMPP and PGP/S/MIME for email, each with different trade-offs for usability and interoperability.