2022-08-09 13:48:26 +00:00
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import 'dart:convert';
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import 'package:omemo_dart/omemo_dart.dart';
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2022-06-30 12:00:18 +00:00
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2022-08-09 13:48:26 +00:00
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/// This example aims to demonstrate how omemo_dart is used. Since omemo_dart is not
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/// dependent on any XMPP library, you need to convert stanzas to the appropriate
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/// intermediary format and back.
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void main() async {
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const aliceJid = 'alice@some.server';
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const bobJid = 'bob@other.serve';
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// You are Alice and want to begin using OMEMO, so you first create a SessionManager
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final aliceSession = await OmemoSessionManager.generateNewIdentity(
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// The bare Jid of Alice as a String
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aliceJid,
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// The trust manager we want to use. In this case, we use the provided one that
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// implements "Blind Trust Before Verification". To make things simpler, we keep
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// no persistent data and can thus use the MemoryBTBVTrustManager. If we wanted to keep
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// the state, we would have to override BlindTrustBeforeVerificationTrustManager.
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MemoryBTBVTrustManager(),
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// Here we specify how many Onetime Prekeys we want to have. XEP-0384 recommends around
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// 100 OPKs, so let's generate 100. The parameter defaults to 100.
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//opkAmount: 100,
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);
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// Alice now wants to chat with Bob at his bare Jid "bob@other.server". To make things
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// simple, we just generate the identity bundle ourselves. In the real world, we would
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// request it using PEP and then convert the device bundle into a OmemoBundle object.
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final bobSession = await OmemoSessionManager.generateNewIdentity(
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bobJid,
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MemoryBTBVTrustManager(),
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// Just for illustrative purposes
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opkAmount: 1,
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);
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// Alice prepares to send the message to Bob, so she builds the message stanza and
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// collects all the children of the stanza that should be encrypted into a string.
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const aliceMessageStanzaBody = '''
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<body>Hello Bob, it's me, Alice!</body>
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<super-secret-element xmlns='super-secret-element' />
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''';
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// Since OMEMO 0.8.3 mandates usage of XEP-0420: Stanza Content Encryption, we have to
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// wrap our acual payload - aliceMessageStanzaBody - into an SCE envelope. Note that
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// the rpad element must contain a random string. See XEP-0420 for recommendations.
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// OMEMO makes the <time /> element optional, but let's use for this example.
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const envelope = '''
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<envelope xmlns='urn:xmpp:sce:1'>
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<content>
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$aliceMessageStanzaBody
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</content>
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<rpad>s0m3-r4nd0m-b9t3s</rpad>
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<from jid='$aliceJid' />
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<time stamp='1969-07-20T21:56:15-05:00' />
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</envelope>
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''';
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// Since Alice has no open session with Bob, we need to tell the session manager to build
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// it when sending the message.
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final message = await aliceSession.encryptToJid(
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// The bare receiver Jid
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bobJid,
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// The envelope we want to encrypt
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envelope,
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// Since this is the first time Alice contacts Bob from this device, we need to create
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// a new session. Let's also assume that Bob only has one device. We may, however,
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// add more bundles to newSessions, if we know of more.
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newSessions: [
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await (await bobSession.getDevice()).toBundle(),
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],
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);
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// Alice now builds the actual message stanza for Bob
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final payload = base64.encode(message.ciphertext!);
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final aliceDevice = await aliceSession.getDevice();
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// ignore: unused_local_variable
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final bobDevice = await bobSession.getDevice();
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// Since we know we have just one key for Bob, we take a shortcut. However, in the real
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// world, we have to serialise every EncryptedKey to a <key /> element and group them
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// per Jid.
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final key = message.encryptedKeys[0];
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// Note that the key's "kex" attribute refers to key.kex. It just means that the
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// encrypted key also contains the required data for Bob to build a session with Alice.
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// ignore: unused_local_variable
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final aliceStanza = '''
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<message from='$aliceJid/device1' to='$bobJid/device2'>
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<encrypted xmlns='urn:xmpp:omemo:2'>
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<header sid='${aliceDevice.id}'>
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<keys jid='$bobJid'>
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<key rid='${key.rid} kex='true'>
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${key.value}
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</key>
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</keys>
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</header>
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<payload>
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$payload
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</payload>
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</encrypted>
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</message>
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''';
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// Alice can now send this message to Bob using our preferred XMPP library.
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// ...
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// Bob now receives an OMEMO encrypted message from Alice and wants to decrypt it.
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// Since we have just one key, let's just deserialise the one key by hand.
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final keys = [
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EncryptedKey(bobJid, key.rid, key.value, true),
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];
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// Bob extracts the payload and attempts to decrypt it.
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// ignore: unused_local_variable
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final bobMessage = await bobSession.decryptMessage(
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// base64 decode the payload
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base64.decode(payload),
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// Specify the Jid of the sender
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aliceJid,
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// Specify the device identifier of the sender (the "sid" attribute of <header />)
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aliceDevice.id,
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// The deserialised keys
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keys,
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);
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// All Bob has to do now is replace the OMEMO wrapper element
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// <encrypted xmlns='urn:xmpp:omemo:2' />) with the content of the <content /> element
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// of the envelope we just decrypted.
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// Bob now has a session with Alice and can send encrypted message to her.
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// Since they both used the BlindTrustBeforeVerificationTrustManager, they currently
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// use blind trust, meaning that both Alice and Bob accept new devices without any
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// hesitation. If Alice, however, decides to verify one of Bob's devices and sets
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// it as verified using
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// ```
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// await aliceSession.trustManager.setDeviceTrust(bobJid, bobDevice.id, BTBVTrustState.verified)
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// ```
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// then Alice's OmemoSessionManager won't encrypt to new devices unless they are also
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// verified. To prevent user confusion, you should check if every device is trusted
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// before sending the message and ask the user for a trust decision.
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// If you want to make the BlindTrustBeforeVerificationTrustManager persistent, then
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// you need to subclass it and override the `Future<void> commitState()` and
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// `Future<void> loadState()` functions. commitState is called everytime the internal
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// state gets changed. loadState never gets automatically called but is more of a
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// function for the user to restore the trust manager. In those functions you have
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// access to `ratchetMap`, which maps a `RatchetMapKey` - essentially a tuple consisting
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// of a bare Jid and the device identifier - to the trust state, and `devices` which
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// maps a bare Jid to its device identifiers.
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// To make the entire OmemoSessionManager persistent, you have two options:
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// - use the provided `toJson()` and `fromJson()` functions. They, however, serialise
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// and deserialise *ALL* known sessions, so it might be slow.
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// - subscribe to the session manager's `eventStream`. There, events get triggered
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// everytime a ratchet changes, our own device changes or the internal ratchet map
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// gets changed. This give finer control over the the serialisation. The session
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2022-08-09 13:49:40 +00:00
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// manager can then be restored using its constructor. For a list of events, see
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// lib/src/omemo/events.dart.
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2022-06-30 12:00:18 +00:00
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}
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