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+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/).