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diff --git a/users/tazjin/blog/posts/the-smu-problem.md b/users/tazjin/blog/posts/the-smu-problem.md new file mode 100644 index 0000000000..f411e31160 --- /dev/null +++ b/users/tazjin/blog/posts/the-smu-problem.md @@ -0,0 +1,151 @@ +After having tested countless messaging apps over the years, being +unsatisfied with most of them and finally getting stuck with +[Telegram](https://telegram.org/) I have developed a little theory about +messaging apps. + +SMU stands for *Security*, *Multi-Device* and *Usability*. Quite like +the [CAP-theorem](https://en.wikipedia.org/wiki/CAP_theorem) I believe +that you can - using current models - only solve two out of three things +on this list. Let me elaborate what I mean by the individual points: + +**Security**: This is mainly about encryption of messages, not so much +about hiding identities to third-parties. Commonly some kind of +asymmetric encryption scheme. Verification of keys used must be possible +for the user. + +**Multi-Device**: Messaging-app clients for multiple devices, with +devices being linked to the same identifier, receiving the same messages +and being independent of each other. A nice bonus is also an open +protocol (like Telegram\'s) that would let people write new clients. + +**Usability**: Usability is a bit of a broad term, but what I mean by it +here is handling contacts and identities. It should be easy to create +accounts, give contact information to people and have everything just +work in a somewhat automated fashion. + +Some categorisation of popular messaging apps: + +**SU**: Threema + +**MU**: Telegram, Google Hangouts, iMessage, Facebook Messenger + +**SM**: +[Signal](https://gist.github.com/TheBlueMatt/d2fcfb78d29faca117f5) + +*Side note: The most popular messaging app - WhatsApp - only scores a +single letter (U). This makes it completely uninteresting to me.* + +Let\'s talk about **SM** - which might contain the key to solving SMU. +Two approaches are interesting here. + +The single key model +-------------------- + +In Signal there is a single identity key which can be used to register a +device on the server. There exists a process for sharing this identity +key from a primary device to a secondary one, so that the secondary +device can register itself (see the link above for a description). + +This *almost* breaks M because there is still a dependence on a primary +device and newly onboarded devices can not be used to onboard further +devices. However, for lack of a better SM example I\'ll give it a pass. + +The other thing it obviously breaks is U as the process for setting it +up is annoying and having to rely on the primary device is a SPOF (there +might be a way to recover from a lost primary device, but I didn\'t find +any information so far). + +The multiple key model +---------------------- + +In iMessage every device that a user logs into creates a new key pair +and submits its public key to a per-account key pool. Senders fetch all +available public keys for a recipient and encrypt to all of the keys. + +Devices that join can catch up on history by receiving it from other +devices that use its public key. + +This *almost* solves all of SMU, but its compliance with S breaks due to +the fact that the key pool is not auditable, and controlled by a +third-party (Apple). How can you verify that they don\'t go and add +another key to your pool? + +A possible solution +------------------- + +Out of these two approaches I believe the multiple key one looks more +promising. If there was a third-party handling the key pool but in a way +that is verifiable, transparent and auditable that model could be used +to solve SMU. + +The technology I have been thinking about for this is some kind of +blockchain model and here\'s how I think it could work: + +1. Bob installs the app and begins onboarding. The first device + generates its keypair, submits the public key and an account + creation request. + +2. Bob\'s account is created on the messaging apps\' servers and a + unique identifier plus the fingerprint of the first device\'s public + key is written to the chain. + +3. Alice sends a message to Bob, her device asks the messaging service + for Bob\'s account\'s identity and public keys. Her device verifies + the public key fingerprint against the one in the blockchain before + encrypting to it and sending the message. + +4. Bob receives Alice\'s message on his first device. + +5. Bob logs in to his account on a second device. The device generates + a key pair and sends the public key to the service, the service + writes it to the blockchain using its identifier. + +6. The messaging service requests that Bob\'s first device signs the + second device\'s key and triggers a simple confirmation popup. + +7. Bob confirms the second device on his first device. It signs the key + and writes the signature to the chain. + +8. Alice sends another message, her device requests Bob\'s current keys + and receives the new key. It verifies that both the messaging + service and one of Bob\'s older devices have confirmed this key in + the chain. It encrypts the message to both keys and sends it on. + +9. Bob receives Alice\'s message on both devices. + +After this the second device can request conversation history from the +first one to synchronise old messages. + +Further devices added to an account can be confirmed by any of the +devices already in the account. + +The messaging service could not add new keys for an account on its own +because it does not control any of the private keys confirmed by the +chain. + +In case all devices were lost, the messaging service could associate the +account with a fresh identity in the block chain. Message history +synchronisation would of course be impossible. + +Feedback welcome +---------------- + +I would love to hear some input on this idea, especially if anyone knows +of an attempt to implement a similar model already. Possible attack +vectors would also be really interesting. + +Until something like this comes to fruition, I\'ll continue using +Telegram with GPG as the security layer when needed. + +**Update:** WhatsApp has launched an integration with the Signal guys +and added their protocol to the official WhatsApp app. This means +WhatsApp now firmly sits in the SU-category, but it still does not solve +this problem. + +**Update 2:** Facebook Messenger has also integrated with Signal, but +their secret chats do not support multi-device well (it is Signal +afterall). This means it scores either SU or MU depending on which mode +you use it in. + +An interesting service I have not yet evaluated properly is +[Matrix](http://matrix.org/). |