Expand description
An implementation of the Olm double ratchet.
§Overview
The core component of the crate is the Account
, representing a single
Olm participant. An Olm Account
consists of a collection of key pairs,
though often documentation will shorten this to just “keys”. These are:
- An Ed25519 signing key pair representing the stable cryptographic identity of the participant (the participant’s “fingerprint”).
- A Curve25519 sender key pair (also sometimes called the identity key pair, somewhat confusingly).
- A number of one-time key pairs.
- A current and previous (if any) “fallback” key pair.
While the key in 1 is used for signing but not encryption, the keys in 2-4 participate in a triple Diffie-Hellman key exchange (3DH) with another Olm participant, thereby establishing an Olm session on each side of the communication channel. Ultimately, this session is used for deriving the concrete encryption keys for a particular message.
Olm sessions are represented by the Session
struct. Such a session is
created by calling Account::create_outbound_session
on one of the
participating accounts, passing it the Curve25519 sender key and one
Curve25519 one-time key of the other side. The protocol is asynchronous, so
the participant can start sending messages to the other side even before the
other side has created a session, producing so-called pre-key messages (see
PreKeyMessage
).
Once the other participant receives such a pre-key message, they can create
their own matching session by calling Account::create_inbound_session
and passing it the pre-key message they received and the Curve25519 sender
key of the other side. This completes the establishment of the Olm
communication channel.
use anyhow::Result;
use vodozemac::olm::{Account, InboundCreationResult, OlmMessage, SessionConfig};
fn main() -> Result<()> {
let alice = Account::new();
let mut bob = Account::new();
bob.generate_one_time_keys(1);
let bob_otk = *bob.one_time_keys().values().next().unwrap();
let mut alice_session = alice
.create_outbound_session(SessionConfig::version_2(), bob.curve25519_key(), bob_otk);
bob.mark_keys_as_published();
let message = "Keep it between us, OK?";
let alice_msg = alice_session.encrypt(message);
if let OlmMessage::PreKey(m) = alice_msg.clone() {
let result = bob.create_inbound_session(alice.curve25519_key(), &m)?;
let mut bob_session = result.session;
let what_bob_received = result.plaintext;
assert_eq!(alice_session.session_id(), bob_session.session_id());
assert_eq!(message.as_bytes(), what_bob_received);
let bob_reply = "Yes. Take this, it's dangerous out there!";
let bob_encrypted_reply = bob_session.encrypt(bob_reply).into();
let what_alice_received = alice_session
.decrypt(&bob_encrypted_reply)?;
assert_eq!(what_alice_received, bob_reply.as_bytes());
}
Ok(())
}
§Sending messages
To encrypt a message, just call Session::encrypt()
. This will
either produce an OlmMessage::PreKey
or OlmMessage::Normal
depending on whether the session is fully established. A session is fully
established once you receive (and decrypt) at least one message from the
other side.
Structs§
- An Olm
Account
manages all cryptographic keys used on a device. - A format suitable for serialization which implements
serde::Serialize
andserde::Deserialize
. Obtainable by callingAccount::pickle
. - Struct holding the two public identity keys of an
Account
. - Return type for the creation of inbound
Session
objects. - An encrypted Olm message.
- The result type for the one-time key generation operation.
- An encrypted Olm pre-key message.
- The public part of a [
RatchetKey
]. - An Olm session represents one end of an encrypted communication channel between two participants.
- A struct to configure how Olm sessions should work under the hood. Currently only the MAC truncation behaviour can be configured.
- The set of keys that were used to establish the Olm Session,
- A format suitable for serialization which implements
serde::Serialize
andserde::Deserialize
. Obtainable by callingSession::pickle
.
Enums§
- Error type for Olm-based decryption failures.
- An enum over the two supported message types.
- Enum over the different Olm message types.
- Error describing failure modes when creating a Olm
Session
from an incoming Olm message.