feat: Make any SPK survive exactly one rotation
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@ -24,3 +24,9 @@ class NotEncryptedForDeviceException implements Exception {
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class NoDecryptionKeyException implements Exception {
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String errMsg() => 'No key available for decrypting the message';
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}
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/// Triggered by the Session Manager when the identifier of the used Signed Prekey
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/// is neither the current SPK's identifier nor the old one's.
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class UnknownSignedPrekeyException implements Exception {
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String errMsg() => 'Unknown Signed Prekey used.';
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}
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@ -156,27 +156,40 @@ class OmemoSessionManager {
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return OmemoKeyExchange()
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..pkId = kexResult.opkId
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..spkId = 0
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..spkId = bundle.spkId
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..ik = await device.ik.pk.getBytes()
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..ek = await kexResult.ek.pk.getBytes();
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}
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/// Build a new session with the user at [jid] with the device [deviceId] using data
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/// from the key exchange [kex].
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/// from the key exchange [kex]. In case [kex] contains an unknown Signed Prekey
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/// identifier an UnknownSignedPrekeyException will be thrown.
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Future<void> _addSessionFromKeyExchange(String jid, int deviceId, OmemoKeyExchange kex) async {
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// Pick the correct SPK
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final device = await getDevice();
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OmemoKeyPair? spk;
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if (kex.spkId == device.spkId) {
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spk = device.spk;
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} else if (kex.spkId == device.oldSpkId) {
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spk = device.oldSpk;
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} else {
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throw UnknownSignedPrekeyException();
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}
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assert(spk != null, 'The used SPK must be found');
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final kexResult = await x3dhFromInitialMessage(
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X3DHMessage(
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OmemoPublicKey.fromBytes(kex.ik!, KeyPairType.ed25519),
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OmemoPublicKey.fromBytes(kex.ek!, KeyPairType.x25519),
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kex.pkId!,
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),
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device.spk,
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spk!,
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device.opks.values.elementAt(kex.pkId!),
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device.ik,
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);
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final ratchet = await OmemoDoubleRatchet.acceptNewSession(
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device.spk,
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spk,
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OmemoPublicKey.fromBytes(kex.ik!, KeyPairType.ed25519),
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kexResult.sk,
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kexResult.ad,
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@ -249,4 +249,39 @@ void main() {
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expect(newDevice!.oldSpkId, oldDevice.spkId);
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expect(listsEqual(newDevice!.oldSpkSignature!, oldDevice.spkSignature), true);
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});
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test('Test accepting a session with an old SPK', () async {
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const aliceJid = 'alice@server.example';
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const bobJid = 'bob@other.server.example';
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// Alice and Bob generate their sessions
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final aliceSession = await OmemoSessionManager.generateNewIdentity(aliceJid, opkAmount: 1);
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final bobSession = await OmemoSessionManager.generateNewIdentity(bobJid, opkAmount: 1);
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// Alice encrypts a message for Bob
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const messagePlaintext = 'Hello Bob!';
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final aliceMessage = await aliceSession.encryptToJid(
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bobJid,
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messagePlaintext,
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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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expect(aliceMessage.encryptedKeys.length, 1);
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// Alice loses her Internet connection. Bob rotates his SPK.
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await bobSession.rotateSignedPrekey();
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// Alice regains her Internet connection and sends the message to Bob
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// ...
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// Bob decrypts it
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final bobMessage = await bobSession.decryptMessage(
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aliceMessage.ciphertext,
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aliceJid,
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(await aliceSession.getDevice()).id,
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aliceMessage.encryptedKeys,
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);
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expect(messagePlaintext, bobMessage);
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});
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}
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